Hello All,
I have replaced all 16 paper and filter capacitors in this radio. I can get very strong good receiption for about a minute and then it fades to very weak. If I measure the resistance from the movable part of the center trimmer capacitor on the tuning capacitor to the chassis and move the probe on that piece of metal, the reception instantly comes up to normal and then gradually drops down to very poor. I have to do this every couple of minutes to keep listening to the radio, which sounds very good. Could this be a cold solder connection on the lead to the tuning capacitor there? This is very odd. I almost never see these weird problems.
What am I doing here? How is just measuring making this happening? It works even on the volts measurement. I have measured every resistor and all are good except for the R4 which I replaced. These problems are always odd to me. This seems like a problem with a tube conducting. But usually there's an open resistor but all are good. What should I look at next?

Thanks,

Dave

No schematic on here, so I can't see what R4 is (maybe someone else can), but this sounds like either a bad AVC resistor or a bad oscillator. Make sure, too, that the AVC condenser you put in as a replacement is the correct value. If it's too small, the AVC can behave strangely. Usually there'll be feedback, and the signal will 'pop' out, but sometimes other things happen. Definitely sounds like a bad AVC resistor, though, or maybe a bad volume control if the AVC resistor connects to the volume control. If the AVC resistor or what it connects to is open, the grid(s) which it controls will build up with electrons and then the tube(s) will quit. The AVC resistor feeds the RF tube(s) with a varying negative voltage, but also leaks excess voltage to B-, that might otherwise build up on the grid(s) if it/they were left uncontrolled.

T.

Thomas,
RCA Model R71 is the other name for this radio. AVC, yes, that's it. That does describe the problem. For some reason, I didn't think of that. The AVC is attenuating the signal a bit too much. The service data at Nostalgiaair on this is almost illegable it's almost impossible to tell what the values are. I've seen service data like this (Blurry) right in a Riders Manual. As far as I can tell all capacitors are correct. AVC resistor? Which one is that? R1? R2? R3? Which capacitor is the one to make sure it's correct? I'll re-replace the capacitors and replace the AVC resistor if I have to, even though they may "seem" to be working.
Also, I'm getting low resistance readings on some of the resistors, but they're connected to the circuit. So I'm assuming that they are ok.
The only resistor with a problem was R4. That one was wide open. But that's not the AVC resistor. Volume control was replaced sometime in it's past with a 38K control. It must have been 40K at one time. It works smoothly and isn't open. 50K was the original value, as far as I can tell on the blurry service data.

Thanks,

Dave
Here's where the service data is:
http://www.nostalgiaair.org/Resources/707/M0015707.htm

:No schematic on here, so I can't see what R4 is (maybe someone else can), but this sounds like either a bad AVC resistor or a bad oscillator. Make sure, too, that the AVC condenser you put in as a replacement is the correct value. If it's too small, the AVC can behave strangely. Usually there'll be feedback, and the signal will 'pop' out, but sometimes other things happen. Definitely sounds like a bad AVC resistor, though, or maybe a bad volume control if the AVC resistor connects to the volume control. If the AVC resistor or what it connects to is open, the grid(s) which it controls will build up with electrons and then the tube(s) will quit. The AVC resistor feeds the RF tube(s) with a varying negative voltage, but also leaks excess voltage to B-, that might otherwise build up on the grid(s) if it/they were left uncontrolled.
:
:T.

R1, 2, and 3 are all AVC resistors. They distribute the AVC voltage to the antenna and IF circuits. R1 looks like either 300K or 900K (probably either will be alright....what would be bad is if the resistor was off the map). R2 looks like 900K, but could be something else. R3 appears to be 500K. R3 shunts some of the AVC voltage to B-. If it drifts, AVC will be stronger, and will attenuate more. If it is smaller in value, stations will be louder. If the other resistors drift dramatically, the tubes may saturate negatively, and may cease to conduct.

You also have a tube specifically dedicated to AVC; the '56 tube. If the tube is faulty, the RF tubes may or may not saturate. They probably won't, since the 500K resistor (R3) is there, but they might.

The volume of the receiver is controlled entirely by manipulating the AVC control tube. It's kind of like having a volume control in the RF circuit like earlier radios, but also has the addition of AVC. I was going to explain the operation of the circuit, but I'm kind of tired right now, and it's starting to not make sense, so I deleted it. I'll explain it later, if you haven't already figured it out.

T.

Thomas,
I will check R1, R2, and R3 again and see if I find a lot of drifting. But we still can't see what the resistor values are supposed to be. I'm sure someone has it somewhere.
If I only knew where I put my RCA Red Book. It fell apart and all I can find is the front and back cover of volume 1. I have the contents of volume 2 (B and C sections) but where volume 1 is, I have no idea. I'll probably find it when I know the resistor values.

Thanks,

Dave
:R1, 2, and 3 are all AVC resistors. They distribute the AVC voltage to the antenna and IF circuits. R1 looks like either 300K or 900K (probably either will be alright....what would be bad is if the resistor was off the map). R2 looks like 900K, but could be something else. R3 appears to be 500K. R3 shunts some of the AVC voltage to B-. If it drifts, AVC will be stronger, and will attenuate more. If it is smaller in value, stations will be louder. If the other resistors drift dramatically, the tubes may saturate negatively, and may cease to conduct.
:
:You also have a tube specifically dedicated to AVC; the '56 tube. If the tube is faulty, the RF tubes may or may not saturate. They probably won't, since the 500K resistor (R3) is there, but they might.
:
:The volume of the receiver is controlled entirely by manipulating the AVC control tube. It's kind of like having a volume control in the RF circuit like earlier radios, but also has the addition of AVC. I was going to explain the operation of the circuit, but I'm kind of tired right now, and it's starting to not make sense, so I deleted it. I'll explain it later, if you haven't already figured it out.
:
:T.

R-1, 2, 3, are all 500K

Dave

Thomas,
I found my RCA red book and on pages 304-B, 305-B and 306-B is the schematic for the R71. R1 is supposed to be 500K, R2 is suppsed to be 500K and R3 is supposed to be 500K. In other words, they are all 500K resistors. The red book doesn't have the parts view. I see that R1 is inside the antenna coil. R1 measures 676K, R2 measures 696K and R3 measures 732K. All are supposed to be 500K. Assuming that these are 10 % resistors, they could drift by as much as 50K. So 550K would be max. They have drifted up by about 200K. But maybe if they're all about 700K it could still work. I'll replace them with 500K resistors that are 500K and see what differencce it makes.

Dave
:Thomas,
: I will check R1, R2, and R3 again and see if I find a lot of drifting. But we still can't see what the resistor values are supposed to be. I'm sure someone has it somewhere.
: If I only knew where I put my RCA Red Book. It fell apart and all I can find is the front and back cover of volume 1. I have the contents of volume 2 (B and C sections) but where volume 1 is, I have no idea. I'll probably find it when I know the resistor values.
:
:Thanks,
:
:Dave
::R1, 2, and 3 are all AVC resistors. They distribute the AVC voltage to the antenna and IF circuits. R1 looks like either 300K or 900K (probably either will be alright....what would be bad is if the resistor was off the map). R2 looks like 900K, but could be something else. R3 appears to be 500K. R3 shunts some of the AVC voltage to B-. If it drifts, AVC will be stronger, and will attenuate more. If it is smaller in value, stations will be louder. If the other resistors drift dramatically, the tubes may saturate negatively, and may cease to conduct.
::
::You also have a tube specifically dedicated to AVC; the '56 tube. If the tube is faulty, the RF tubes may or may not saturate. They probably won't, since the 500K resistor (R3) is there, but they might.
::
::The volume of the receiver is controlled entirely by manipulating the AVC control tube. It's kind of like having a volume control in the RF circuit like earlier radios, but also has the addition of AVC. I was going to explain the operation of the circuit, but I'm kind of tired right now, and it's starting to not make sense, so I deleted it. I'll explain it later, if you haven't already figured it out.
::
::T.

Thomas,
All I have is 470K and 560K. So I'm using the 470K resistors. I replacd R1 and R3, but I'll wait until tomorrow for R2. So far it made absolutely no difference. Maybe there will be a difference when I replace R2.
When I measure the resistance between the chassis and R1 when the radio is on, the signal is good. Trying to lengthen the antenna wire doesn't seem to have any effect at all
So I'll try replacing R2 tomorrow.

Thanks,

Dave

:Thomas,
: I found my RCA red book and on pages 304-B, 305-B and 306-B is the schematic for the R71. R1 is supposed to be 500K, R2 is suppsed to be 500K and R3 is supposed to be 500K. In other words, they are all 500K resistors. The red book doesn't have the parts view. I see that R1 is inside the antenna coil. R1 measures 676K, R2 measures 696K and R3 measures 732K. All are supposed to be 500K. Assuming that these are 10 % resistors, they could drift by as much as 50K. So 550K would be max. They have drifted up by about 200K. But maybe if they're all about 700K it could still work. I'll replace them with 500K resistors that are 500K and see what differencce it makes.
:
:Dave
::Thomas,
:: I will check R1, R2, and R3 again and see if I find a lot of drifting. But we still can't see what the resistor values are supposed to be. I'm sure someone has it somewhere.
:: If I only knew where I put my RCA Red Book. It fell apart and all I can find is the front and back cover of volume 1. I have the contents of volume 2 (B and C sections) but where volume 1 is, I have no idea. I'll probably find it when I know the resistor values.
::
::Thanks,
::
::Dave
:::R1, 2, and 3 are all AVC resistors. They distribute the AVC voltage to the antenna and IF circuits. R1 looks like either 300K or 900K (probably either will be alright....what would be bad is if the resistor was off the map). R2 looks like 900K, but could be something else. R3 appears to be 500K. R3 shunts some of the AVC voltage to B-. If it drifts, AVC will be stronger, and will attenuate more. If it is smaller in value, stations will be louder. If the other resistors drift dramatically, the tubes may saturate negatively, and may cease to conduct.
:::
:::You also have a tube specifically dedicated to AVC; the '56 tube. If the tube is faulty, the RF tubes may or may not saturate. They probably won't, since the 500K resistor (R3) is there, but they might.
:::
:::The volume of the receiver is controlled entirely by manipulating the AVC control tube. It's kind of like having a volume control in the RF circuit like earlier radios, but also has the addition of AVC. I was going to explain the operation of the circuit, but I'm kind of tired right now, and it's starting to not make sense, so I deleted it. I'll explain it later, if you haven't already figured it out.
:::
:::T.

Take a 470K and a couple of alligator clips and parallel all the resistors connected with the AVC circuit. It may take a few seconds to drain off the charge from the tube grids, but if it starts working, you have found your defective resistor.

Lewis

:Thomas,
: All I have is 470K and 560K. So I'm using the 470K resistors. I replacd R1 and R3, but I'll wait until tomorrow for R2. So far it made absolutely no difference. Maybe there will be a difference when I replace R2.
:When I measure the resistance between the chassis and R1 when the radio is on, the signal is good. Trying to lengthen the antenna wire doesn't seem to have any effect at all
: So I'll try replacing R2 tomorrow.
:
:Thanks,
:
:Dave
:
::Thomas,
:: I found my RCA red book and on pages 304-B, 305-B and 306-B is the schematic for the R71. R1 is supposed to be 500K, R2 is suppsed to be 500K and R3 is supposed to be 500K. In other words, they are all 500K resistors. The red book doesn't have the parts view. I see that R1 is inside the antenna coil. R1 measures 676K, R2 measures 696K and R3 measures 732K. All are supposed to be 500K. Assuming that these are 10 % resistors, they could drift by as much as 50K. So 550K would be max. They have drifted up by about 200K. But maybe if they're all about 700K it could still work. I'll replace them with 500K resistors that are 500K and see what differencce it makes.
::
::Dave
:::Thomas,
::: I will check R1, R2, and R3 again and see if I find a lot of drifting. But we still can't see what the resistor values are supposed to be. I'm sure someone has it somewhere.
::: If I only knew where I put my RCA Red Book. It fell apart and all I can find is the front and back cover of volume 1. I have the contents of volume 2 (B and C sections) but where volume 1 is, I have no idea. I'll probably find it when I know the resistor values.
:::
:::Thanks,
:::
:::Dave
::::R1, 2, and 3 are all AVC resistors. They distribute the AVC voltage to the antenna and IF circuits. R1 looks like either 300K or 900K (probably either will be alright....what would be bad is if the resistor was off the map). R2 looks like 900K, but could be something else. R3 appears to be 500K. R3 shunts some of the AVC voltage to B-. If it drifts, AVC will be stronger, and will attenuate more. If it is smaller in value, stations will be louder. If the other resistors drift dramatically, the tubes may saturate negatively, and may cease to conduct.
::::
::::You also have a tube specifically dedicated to AVC; the '56 tube. If the tube is faulty, the RF tubes may or may not saturate. They probably won't, since the 500K resistor (R3) is there, but they might.
::::
::::The volume of the receiver is controlled entirely by manipulating the AVC control tube. It's kind of like having a volume control in the RF circuit like earlier radios, but also has the addition of AVC. I was going to explain the operation of the circuit, but I'm kind of tired right now, and it's starting to not make sense, so I deleted it. I'll explain it later, if you haven't already figured it out.
::::
::::T.

Also, try connecting the AVC circuit directly to the chassis with alligator clips. If that brings in signals, then your trouble is in the AVC circuit, but might not be due to the resistors. It could be due to the AVC tube or any of the other components in that section of the circuit.

Otherwise, the trouble is elsewhere--possibly due to a faulty oscillator or IF tube....more likely an oscillator. I've been able to 'pop' in oscillators by tapping grids with a screwdriver. Then they fade out.

T.

Thomas,
I disconnected the tone control and I have full volume now. How the tone control could do that I have no idea. Now I hear an AC buzz at low volume. I think that it's being picked up somewhere. It's not 60 cycles, it sounds more like 120 cycles. I have replaced all the paper and filter capacitors. I think that all my connections are gooc but maybe not. That speaker cone doesn't sound very good either. At low volume its ok.

Dave
:Also, try connecting the AVC circuit directly to the chassis with alligator clips. If that brings in signals, then your trouble is in the AVC circuit, but might not be due to the resistors. It could be due to the AVC tube or any of the other components in that section of the circuit.
:
:Otherwise, the trouble is elsewhere--possibly due to a faulty oscillator or IF tube....more likely an oscillator. I've been able to 'pop' in oscillators by tapping grids with a screwdriver. Then they fade out.
:
:T.

Thomas,
I have solved the buzz problem. I see I made a mistake. Also, when I couldn't read the capacitor value I used a 10 instead of a 33 mf capacitor. Also I forgot to connect the negative of the first filter capacitor to one side of the filter reactor. So that had no capacitor to help out. Now the antenna wires have no effect when I touch the end of either of them. But if I touch the antenna coil itself it does make a difference. So I'll check on that tomorrow,
Somehow the original thread posted twice. So I have to post answers twice.

Thanks for everyone's help,

Dave
:Thomas,
: I disconnected the tone control and I have full volume now. How the tone control could do that I have no idea. Now I hear an AC buzz at low volume. I think that it's being picked up somewhere. It's not 60 cycles, it sounds more like 120 cycles. I have replaced all the paper and filter capacitors. I think that all my connections are gooc but maybe not. That speaker cone doesn't sound very good either. At low volume its ok.
:
:Dave
::Also, try connecting the AVC circuit directly to the chassis with alligator clips. If that brings in signals, then your trouble is in the AVC circuit, but might not be due to the resistors. It could be due to the AVC tube or any of the other components in that section of the circuit.
::
::Otherwise, the trouble is elsewhere--possibly due to a faulty oscillator or IF tube....more likely an oscillator. I've been able to 'pop' in oscillators by tapping grids with a screwdriver. Then they fade out.
::
::T.

The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.

I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.

Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.

Thomas

Thomas,
The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
It's easy to replace, so maybe I should replace it.

Thanks,

Dave
:The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
:
:I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
:
:Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
:
:Thomas

Thomas,
I tried a brand new control and the exact same thing happens. When the tone control is hooked up most of the bass goes away. When the tone is turned all the way down the volume is very weak and only the loudest sounds get through. That's why it distorts. The instant it's disconnected, all is well, but there's no tone control.
So this proves that the original tone control works just fine. But I guess there are more resistor problems.

Thanks,

Dave

:Thomas,
:The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
:It's easy to replace, so maybe I should replace it.
:
:Thanks,
:
:Dave
::The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
::
::I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
::
::Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
::
::Thomas

Hello Again (Thomas, Edd, Norm),
I tried new resistors in the tone control circuit to no avail. If I connect the tone control (original or a new one) the sound distorts. If I disconnect it there's no such distortion. I have no idea what's going on. This is very frustrating.
Is there another place I can put a tone control that might work better?

Thanks,

Dave
:Thomas,
: I tried a brand new control and the exact same thing happens. When the tone control is hooked up most of the bass goes away. When the tone is turned all the way down the volume is very weak and only the loudest sounds get through. That's why it distorts. The instant it's disconnected, all is well, but there's no tone control.
: So this proves that the original tone control works just fine. But I guess there are more resistor problems.
:
:Thanks,
:
:Dave
:
::Thomas,
::The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
::It's easy to replace, so maybe I should replace it.
::
::Thanks,
::
::Dave
:::The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
:::
:::I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
:::
:::Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
:::
:::Thomas

Dave,

What voltage do you measure between the filament and the control grid of the 47 tube? I think it should be around minus 20 volts.

Radiodoc

:Hello Again (Thomas, Edd, Norm),
: I tried new resistors in the tone control circuit to no avail. If I connect the tone control (original or a new one) the sound distorts. If I disconnect it there's no such distortion. I have no idea what's going on. This is very frustrating.
: Is there another place I can put a tone control that might work better?
:
:Thanks,
:
:Dave
::Thomas,
:: I tried a brand new control and the exact same thing happens. When the tone control is hooked up most of the bass goes away. When the tone is turned all the way down the volume is very weak and only the loudest sounds get through. That's why it distorts. The instant it's disconnected, all is well, but there's no tone control.
:: So this proves that the original tone control works just fine. But I guess there are more resistor problems.
::
::Thanks,
::
::Dave
::
:::Thomas,
:::The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
:::It's easy to replace, so maybe I should replace it.
:::
:::Thanks,
:::
:::Dave
::::The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
::::
::::I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
::::
::::Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
::::
::::Thomas

Radiodoc,
I'll measure between the control grid and the chassis. I think that you mean pin 3 (G1). But I'll measure pin 4 the Grid, as well. That negative voltage never goes below about -12 volts with the volume all the way up. It's about -8 volts with the volume all the way down. This is with the tone control attached. Should this be more like -20 with the volume all the way up? What now? I replaced R20, and R21. This may be how it's supposed to be. But I do hear a bit more distortion when the tone control is connected, than when it's disconnected. There's more bass when the tone control is disconnected, than when it's connected.

The other grid is connected to the 275 volt line. Let me see if that's low too. 277.5 is the main B+, it's 126 volts on the other end of R4. That seems good to me.

Thanks,

:Dave,
:
:What voltage do you measure between the filament and the control grid of the 47 tube? I think it should be around minus 20 volts.
:
:Radiodoc
:
::Hello Again (Thomas, Edd, Norm),
:: I tried new resistors in the tone control circuit to no avail. If I connect the tone control (original or a new one) the sound distorts. If I disconnect it there's no such distortion. I have no idea what's going on. This is very frustrating.
:: Is there another place I can put a tone control that might work better?
::
::Thanks,
::
::Dave
:::Thomas,
::: I tried a brand new control and the exact same thing happens. When the tone control is hooked up most of the bass goes away. When the tone is turned all the way down the volume is very weak and only the loudest sounds get through. That's why it distorts. The instant it's disconnected, all is well, but there's no tone control.
::: So this proves that the original tone control works just fine. But I guess there are more resistor problems.
:::
:::Thanks,
:::
:::Dave
:::
::::Thomas,
::::The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
::::It's easy to replace, so maybe I should replace it.
::::
::::Thanks,
::::
::::Dave
:::::The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
:::::
:::::I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
:::::
:::::Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
:::::
:::::Thomas

Dave,

The voltage chart shows minus 20 volts on the control grid (G1) for minimum volume and maximum volume. I would tend to agree that the voltage should remain fairly constant at minus 20 volts.

Radiodoc
*************

:Radiodoc,
: I'll measure between the control grid and the chassis. I think that you mean pin 3 (G1). But I'll measure pin 4 the Grid, as well. That negative voltage never goes below about -12 volts with the volume all the way up. It's about -8 volts with the volume all the way down. This is with the tone control attached. Should this be more like -20 with the volume all the way up? What now? I replaced R20, and R21. This may be how it's supposed to be. But I do hear a bit more distortion when the tone control is connected, than when it's disconnected. There's more bass when the tone control is disconnected, than when it's connected.
:
: The other grid is connected to the 275 volt line. Let me see if that's low too. 277.5 is the main B+, it's 126 volts on the other end of R4. That seems good to me.
:
:Thanks,
:
::Dave,
::
::What voltage do you measure between the filament and the control grid of the 47 tube? I think it should be around minus 20 volts.
::
::Radiodoc
::
:::Hello Again (Thomas, Edd, Norm),
::: I tried new resistors in the tone control circuit to no avail. If I connect the tone control (original or a new one) the sound distorts. If I disconnect it there's no such distortion. I have no idea what's going on. This is very frustrating.
::: Is there another place I can put a tone control that might work better?
:::
:::Thanks,
:::
:::Dave
::::Thomas,
:::: I tried a brand new control and the exact same thing happens. When the tone control is hooked up most of the bass goes away. When the tone is turned all the way down the volume is very weak and only the loudest sounds get through. That's why it distorts. The instant it's disconnected, all is well, but there's no tone control.
:::: So this proves that the original tone control works just fine. But I guess there are more resistor problems.
::::
::::Thanks,
::::
::::Dave
::::
:::::Thomas,
:::::The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
:::::It's easy to replace, so maybe I should replace it.
:::::
:::::Thanks,
:::::
:::::Dave
::::::The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
::::::
::::::I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
::::::
::::::Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
::::::
::::::Thomas

I have lost out on this thread a bit, ever since you found your good... LEGIBLE... RCA schema. Plus others may have touched upon it. But does the volume still have its oddity that you mentioned , or is that aspect now fixed. But you have now found a second quirk in the respect that if the tone control is at one end,...max bass...the sound distorts.
The circuit just basically being a 50 K variable resistor that varies the degree that C26 ..025 capacitor gets shunted across driver transformer T2's secondary dampening its treble response.
Witrh your having both / different replacement pots and cap in the circuit and the same effect, that nullified my though of the pots resistance element having a leakage path to ground at some point around its perophery.
Have you tried the monitoring of the 1st grid voltage of the AF output tube and initially taken a reading with the tone control at max resistance ..in the non afffectual position....and then rotated the tone control to the problem and and see if the voltage varies ???

Then there is the consideration of the output stage grid bias being acquired down in the power supply
where the voltage is established by the effect of the speaker field coil being above actual ground level and that tier of established voltage level is fed over to a voltage divider bridge of R20 and R21 of which R21 is definitely 100K but you can confirm, with your good RCA copy, if R20 is of a value of 500k ??? or not. (My reference is not that clear.)
That voltage divider junction is the derivative of your output stages grid biasing...and assuming that C32 and its .5 ufd might have received a changing in your capacitor sweep. and hopefully NOT... OPEN / OPENING, as it both bypasses the bias line and holds the bias level relatively stable under voltage swing excursions of the changing drive input levels.

The hopeless situation...... in case you don't find success and time is of essence... yes, the tone control could be shifted to the plate circuitry where there currently is a high freq clipping RC network.
Your pot would replace the fixed resistor and the cap value might have to go upwards to .05 ufd in accordance to the degree of hi freq bypassing deemed appropriate to get the desired max tone
effect.
Another secondary aspect to consider is the fact that the cap wil be receiving the max P/P excursions in the ptate supply and keeping DC isolatiion from the pot, but the internal insulation integrity of the pot to ground would have to be greater than the B+ of the set applied at that point. (Most 3/4-1 inch pots are 500VDC rated)
(Not so.... on the newer very micro sized pots...with their tab mounts,not even using a bushing and control nut... as used on solid state design)

73's de Edd

--------------------------------------------------------------B/T---------------------------------------------------------------------------

:Hello Again (Thomas, Edd, Norm),
: I tried new resistors in the tone control circuit to no avail. If I connect the tone control (original or a new one) the sound distorts. If I disconnect it there's no such distortion. I have no idea what's going on. This is very frustrating.
: Is there another place I can put a tone control that might work better?
:
:Thanks,
:
:Dave
::Thomas,
:: I tried a brand new control and the exact same thing happens. When the tone control is hooked up most of the bass goes away. When the tone is turned all the way down the volume is very weak and only the loudest sounds get through. That's why it distorts. The instant it's disconnected, all is well, but there's no tone control.
:: So this proves that the original tone control works just fine. But I guess there are more resistor problems.
::
::Thanks,
::
::Dave
::
:::Thomas,
:::The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
:::It's easy to replace, so maybe I should replace it.
:::
:::Thanks,
:::
:::Dave
::::The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
::::
::::I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
::::
::::Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
::::
::::Thomas

Edd,
When the tone control is connected, there is a loss of bass response. When I disconnect it, it comes back. There is no position of the tone control when connected, that brings back the low frequencies. You are right, it is supposed to be a treble control. It should have no effect on bass response. That's why this is so odd. The sound distorts when the tone control is attached. It doesn't distort when it's disconnected. No position of the tone control has any effect (affect) on the distortion. Only when removed fromt the circuit, does the bass return and the distortion go away.
Yes, I also, still have the other oddity when the tone control is connected. The sound is not as strong but when I measure using my ohm meter fromt he chassis to the middle trimmer capacitor, and move the probe around making a staticy sound, and then remove the probe, the sound is good and strong for a while, and the distortion goes away. This doesn't happen when the tone control is disconnected.
I have substituted with another tone control but it makes absolutely no difference. All paper and filter capacitors have been replaced. There are no old capacitors left in the radio at all. I replaced all 16 of them. I replaced R20 and R21, so they are the exact values they are supposed to be, it's a little better but still distorts when the tone control is connected. The voltage never goes below about -12 volts. But it probably goes lower (I just checked -19 volts) when I do that trick with the ohm meter when it plays louder. I can do the same trick touching the top pin of the RF amp with the ohm meter probe, when it's attached to the antenna coil. That also works briefly.
If I can't get anywhere with this tone control arrangement, I'll move it. The original control is fine. This is an interesting problem.

Thanks,

Dave

:I have lost out on this thread a bit, ever since you found your good... LEGIBLE... RCA schema. Plus others may have touched upon it. But does the volume still have its oddity that you mentioned , or is that aspect now fixed. But you have now found a second quirk in the respect that if the tone control is at one end,...max bass...the sound distorts.
:The circuit just basically being a 50 K variable resistor that varies the degree that C26 ..025 capacitor gets shunted across driver transformer T2's secondary dampening its treble response.
:Witrh your having both / different replacement pots and cap in the circuit and the same effect, that nullified my though of the pots resistance element having a leakage path to ground at some point around its perophery.
:Have you tried the monitoring of the 1st grid voltage of the AF output tube and initially taken a reading with the tone control at max resistance ..in the non afffectual position....and then rotated the tone control to the problem and and see if the voltage varies ???
:
:Then there is the consideration of the output stage grid bias being acquired down in the power supply
:where the voltage is established by the effect of the speaker field coil being above actual ground level and that tier of established voltage level is fed over to a voltage divider bridge of R20 and R21 of which R21 is definitely 100K but you can confirm, with your good RCA copy, if R20 is of a value of 500k ??? or not. (My reference is not that clear.)
:That voltage divider junction is the derivative of your output stages grid biasing...and assuming that C32 and its .5 ufd might have received a changing in your capacitor sweep. and hopefully NOT... OPEN / OPENING, as it both bypasses the bias line and holds the bias level relatively stable under voltage swing excursions of the changing drive input levels.
:
:The hopeless situation...... in case you don't find success and time is of essence... yes, the tone control could be shifted to the plate circuitry where there currently is a high freq clipping RC network.
:Your pot would replace the fixed resistor and the cap value might have to go upwards to .05 ufd in accordance to the degree of hi freq bypassing deemed appropriate to get the desired max tone
:effect.
:Another secondary aspect to consider is the fact that the cap wil be receiving the max P/P excursions in the ptate supply and keeping DC isolatiion from the pot, but the internal insulation integrity of the pot to ground would have to be greater than the B+ of the set applied at that point. (Most 3/4-1 inch pots are 500VDC rated)
:(Not so.... on the newer very micro sized pots...with their tab mounts,not even using a bushing and control nut... as used on solid state design)
:
:73's de Edd
:
:--------------------------------------------------------------B/T---------------------------------------------------------------------------
:
::Hello Again (Thomas, Edd, Norm),
:: I tried new resistors in the tone control circuit to no avail. If I connect the tone control (original or a new one) the sound distorts. If I disconnect it there's no such distortion. I have no idea what's going on. This is very frustrating.
:: Is there another place I can put a tone control that might work better?
::
::Thanks,
::
::Dave
:::Thomas,
::: I tried a brand new control and the exact same thing happens. When the tone control is hooked up most of the bass goes away. When the tone is turned all the way down the volume is very weak and only the loudest sounds get through. That's why it distorts. The instant it's disconnected, all is well, but there's no tone control.
::: So this proves that the original tone control works just fine. But I guess there are more resistor problems.
:::
:::Thanks,
:::
:::Dave
:::
::::Thomas,
::::The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
::::It's easy to replace, so maybe I should replace it.
::::
::::Thanks,
::::
::::Dave
:::::The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
:::::
:::::I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
:::::
:::::Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
:::::
:::::Thomas

Guys,

Is it possible the 47 is gassy causing the weirdness? The G1 grid would be a good place to pick off the audio and feed to either a signal tracer input or audio amp of some sort and see what it sounds like even with the tone control installed.

Radiodoc
***********

:I have lost out on this thread a bit, ever since you found your good... LEGIBLE... RCA schema. Plus others may have touched upon it. But does the volume still have its oddity that you mentioned , or is that aspect now fixed. But you have now found a second quirk in the respect that if the tone control is at one end,...max bass...the sound distorts.
:The circuit just basically being a 50 K variable resistor that varies the degree that C26 ..025 capacitor gets shunted across driver transformer T2's secondary dampening its treble response.
:Witrh your having both / different replacement pots and cap in the circuit and the same effect, that nullified my though of the pots resistance element having a leakage path to ground at some point around its perophery.
:Have you tried the monitoring of the 1st grid voltage of the AF output tube and initially taken a reading with the tone control at max resistance ..in the non afffectual position....and then rotated the tone control to the problem and and see if the voltage varies ???
:
:Then there is the consideration of the output stage grid bias being acquired down in the power supply
:where the voltage is established by the effect of the speaker field coil being above actual ground level and that tier of established voltage level is fed over to a voltage divider bridge of R20 and R21 of which R21 is definitely 100K but you can confirm, with your good RCA copy, if R20 is of a value of 500k ??? or not. (My reference is not that clear.)
:That voltage divider junction is the derivative of your output stages grid biasing...and assuming that C32 and its .5 ufd might have received a changing in your capacitor sweep. and hopefully NOT... OPEN / OPENING, as it both bypasses the bias line and holds the bias level relatively stable under voltage swing excursions of the changing drive input levels.
:
:The hopeless situation...... in case you don't find success and time is of essence... yes, the tone control could be shifted to the plate circuitry where there currently is a high freq clipping RC network.
:Your pot would replace the fixed resistor and the cap value might have to go upwards to .05 ufd in accordance to the degree of hi freq bypassing deemed appropriate to get the desired max tone
:effect.
:Another secondary aspect to consider is the fact that the cap wil be receiving the max P/P excursions in the ptate supply and keeping DC isolatiion from the pot, but the internal insulation integrity of the pot to ground would have to be greater than the B+ of the set applied at that point. (Most 3/4-1 inch pots are 500VDC rated)
:(Not so.... on the newer very micro sized pots...with their tab mounts,not even using a bushing and control nut... as used on solid state design)
:
:73's de Edd
:
:--------------------------------------------------------------B/T---------------------------------------------------------------------------
:
::Hello Again (Thomas, Edd, Norm),
:: I tried new resistors in the tone control circuit to no avail. If I connect the tone control (original or a new one) the sound distorts. If I disconnect it there's no such distortion. I have no idea what's going on. This is very frustrating.
:: Is there another place I can put a tone control that might work better?
::
::Thanks,
::
::Dave
:::Thomas,
::: I tried a brand new control and the exact same thing happens. When the tone control is hooked up most of the bass goes away. When the tone is turned all the way down the volume is very weak and only the loudest sounds get through. That's why it distorts. The instant it's disconnected, all is well, but there's no tone control.
::: So this proves that the original tone control works just fine. But I guess there are more resistor problems.
:::
:::Thanks,
:::
:::Dave
:::
::::Thomas,
::::The tone capacitor is brand new and was brand new when I experienced the trouble. All the paper and filter capacitors in the can have been completely discconected and they have all been replaced. There are no old capacitors connected to anything. Maybe somehow the one side of the line that's connected to the switch is interfering with the tone control's operation. It feels like a wire wound control. I can feel bumps as I turn it up and down. Maybe it should feel smooth?
::::It's easy to replace, so maybe I should replace it.
::::
::::Thanks,
::::
::::Dave
:::::The tone condenser could have had a leak of sorts that was broken up by a strong voltage change due to the crackling that would have been produced by you dragging your test probe over the tuner screw (or terminal....I forgot). I don't know why it would have come back, though. Doesn't sound like a logical explanation.
:::::
:::::I can't really come up with anything logical about the volume slowly dying. If the tone condenser was connected to some more negative bias point than what it's connected to, which is the same as the secondary of the audio transformer, it could slowly make the grid go more negative, which would fade out the audio. The condenser is connected to the same place as the transformer winding, though, so it isn't going to feed the grid anything other than what it's getting already.
:::::
:::::Whatever it was, though, it was being affected by the voltage surge caused by touching the tuning condenser in the front end. By the time the crackle gets through those amplifier stages, it's quite a bit stronger.
:::::
:::::Thomas

Regarding the tone control, the probable reason why it cuts off bass is because the condenser is too large. It should be more like .002 to .005 MFD. .025 MFD passes really low audio frequencies really well at those impedances. With the tone control at maximum resistance, the bass notes will still get through, since bass goes through resistors better than treble, and the .025 MFD condenser will pass the bass readily to B-. When the tone control is at minimum resistance, most of the audio will be bypassed by the condenser, due to its large size.

The schematic lists a .025 MFD condenser for the tone control, but I think that it's too large.

Also, if the value turns out to be correct for the circuit it's used in, perhaps the condenser is developing a leak, even though it is new. A leak will also cut off bass. I don't know why the condenser would become leaky in this circuit, since it's a grid circuit, but if it's one of those metalized film units, a strong 'pop' from your tapping the tuning condenser could melt away a small leak. I don't know why it would come back, though.

Perhaps you should try other condensers of the same value, and then some of lower values around .005 to .002 MFD.

T.

Thomas,
That's a good idea because I can kind of disable the tone control without disconnecting it just by lowering the capacitance. I really don't think this radio needs a tone control at all. I'll re-replace the tone capacitor a little later this afternoon and let everyone know if it helped.
I think it will.

Thanks,

Dave
:Regarding the tone control, the probable reason why it cuts off bass is because the condenser is too large. It should be more like .002 to .005 MFD. .025 MFD passes really low audio frequencies really well at those impedances. With the tone control at maximum resistance, the bass notes will still get through, since bass goes through resistors better than treble, and the .025 MFD condenser will pass the bass readily to B-. When the tone control is at minimum resistance, most of the audio will be bypassed by the condenser, due to its large size.
:
:The schematic lists a .025 MFD condenser for the tone control, but I think that it's too large.
:
:Also, if the value turns out to be correct for the circuit it's used in, perhaps the condenser is developing a leak, even though it is new. A leak will also cut off bass. I don't know why the condenser would become leaky in this circuit, since it's a grid circuit, but if it's one of those metalized film units, a strong 'pop' from your tapping the tuning condenser could melt away a small leak. I don't know why it would come back, though.
:
:Perhaps you should try other condensers of the same value, and then some of lower values around .005 to .002 MFD.
:
:T.
:
:

Thomas,
I replaced the capacitor. The sound doesn't muffle as much when the tone control is turned down all the way. That part is better but I still have to use the ohmmeter trick to get the signal stronger. So this didn't solve the problem at all. It did solve one problem. .025 was a bit too much for today's AM broadcast sound. Maybe the RF tube is weak?

Thanks,

Dave
:Thomas,
: That's a good idea because I can kind of disable the tone control without disconnecting it just by lowering the capacitance. I really don't think this radio needs a tone control at all. I'll re-replace the tone capacitor a little later this afternoon and let everyone know if it helped.
:I think it will.
:
:Thanks,
:
:Dave
::Regarding the tone control, the probable reason why it cuts off bass is because the condenser is too large. It should be more like .002 to .005 MFD. .025 MFD passes really low audio frequencies really well at those impedances. With the tone control at maximum resistance, the bass notes will still get through, since bass goes through resistors better than treble, and the .025 MFD condenser will pass the bass readily to B-. When the tone control is at minimum resistance, most of the audio will be bypassed by the condenser, due to its large size.
::
::The schematic lists a .025 MFD condenser for the tone control, but I think that it's too large.
::
::Also, if the value turns out to be correct for the circuit it's used in, perhaps the condenser is developing a leak, even though it is new. A leak will also cut off bass. I don't know why the condenser would become leaky in this circuit, since it's a grid circuit, but if it's one of those metalized film units, a strong 'pop' from your tapping the tuning condenser could melt away a small leak. I don't know why it would come back, though.
::
::Perhaps you should try other condensers of the same value, and then some of lower values around .005 to .002 MFD.
::
::T.
::
::

Thomas,
The instant the wire is cut leading to the tone control, sound is perfectly clear and full and I don't have to do the ohm meter trick at all.
This makes absolutely no sense whatsoever. That grid voltage with the tone circuit disconnected is: -11 volts. So adding the tone control does limit the volume but it has nothing to do with the problem. I tried the ohm meter trick with it at what I thought was a full volume, and it gets even louder. Maybe the RF amp is weak or something like that. Nothing I can do can fix the problem. The only thing I can do is leave the tone control disconnected. I just wasted about 5 hours on this mysterious problem. All the resistors in the AVC circuit have been replaced. The resistors in the voltage divider for the -20 volts (which it never gets) have been replaced. -16 is about the most I can get. But I have to use the ohm meter trick to get it to do that. Most of the time it's around -9 volts no matter what I do.
With the tone control attached the volume is so low the radio is unusable. Nothing is touching the wires on the tone control except what's supposed to be connected to it. One end of a capacitor is connected to one end of the tone control and nothing else. The old capacitor was completely removed. The other end of the tone control is attached to the grid pin on the 47 and that's it. Noting else is attached. It makes not sense. When I disconnect the tone control I get full volume and the grid voltate goes up to about -11 volts from -9 volts. I have tried other 56s and 47s, but it continues to work exactly the same way. What do I do next? I am ready to give up.

Thanks,

Dave
:Thomas,
: I replaced the capacitor. The sound doesn't muffle as much when the tone control is turned down all the way. That part is better but I still have to use the ohmmeter trick to get the signal stronger. So this didn't solve the problem at all. It did solve one problem. .025 was a bit too much for today's AM broadcast sound. Maybe the RF tube is weak?
:
:Thanks,
:
:Dave
::Thomas,
:: That's a good idea because I can kind of disable the tone control without disconnecting it just by lowering the capacitance. I really don't think this radio needs a tone control at all. I'll re-replace the tone capacitor a little later this afternoon and let everyone know if it helped.
::I think it will.
::
::Thanks,
::
::Dave
:::Regarding the tone control, the probable reason why it cuts off bass is because the condenser is too large. It should be more like .002 to .005 MFD. .025 MFD passes really low audio frequencies really well at those impedances. With the tone control at maximum resistance, the bass notes will still get through, since bass goes through resistors better than treble, and the .025 MFD condenser will pass the bass readily to B-. When the tone control is at minimum resistance, most of the audio will be bypassed by the condenser, due to its large size.
:::
:::The schematic lists a .025 MFD condenser for the tone control, but I think that it's too large.
:::
:::Also, if the value turns out to be correct for the circuit it's used in, perhaps the condenser is developing a leak, even though it is new. A leak will also cut off bass. I don't know why the condenser would become leaky in this circuit, since it's a grid circuit, but if it's one of those metalized film units, a strong 'pop' from your tapping the tuning condenser could melt away a small leak. I don't know why it would come back, though.
:::
:::Perhaps you should try other condensers of the same value, and then some of lower values around .005 to .002 MFD.
:::
:::T.
:::
:::

What is the resistance of the secondary of the audio driver transformer...and is it stable at that value when you whack on the transformer?

73's de Edd

---------------------------------------------------------------------------------------------B/T----------------------------------------------------------------------------------------------------------------
:Thomas,
: The instant the wire is cut leading to the tone control, sound is perfectly clear and full and I don't have to do the ohm meter trick at all.
: This makes absolutely no sense whatsoever. That grid voltage with the tone circuit disconnected is: -11 volts. So adding the tone control does limit the volume but it has nothing to do with the problem. I tried the ohm meter trick with it at what I thought was a full volume, and it gets even louder. Maybe the RF amp is weak or something like that. Nothing I can do can fix the problem. The only thing I can do is leave the tone control disconnected. I just wasted about 5 hours on this mysterious problem. All the resistors in the AVC circuit have been replaced. The resistors in the voltage divider for the -20 volts (which it never gets) have been replaced. -16 is about the most I can get. But I have to use the ohm meter trick to get it to do that. Most of the time it's around -9 volts no matter what I do.
: With the tone control attached the volume is so low the radio is unusable. Nothing is touching the wires on the tone control except what's supposed to be connected to it. One end of a capacitor is connected to one end of the tone control and nothing else. The old capacitor was completely removed. The other end of the tone control is attached to the grid pin on the 47 and that's it. Noting else is attached. It makes not sense. When I disconnect the tone control I get full volume and the grid voltate goes up to about -11 volts from -9 volts. I have tried other 56s and 47s, but it continues to work exactly the same way. What do I do next? I am ready to give up.
:
:
:Thanks,
:
:Dave
::Thomas,
:: I replaced the capacitor. The sound doesn't muffle as much when the tone control is turned down all the way. That part is better but I still have to use the ohmmeter trick to get the signal stronger. So this didn't solve the problem at all. It did solve one problem. .025 was a bit too much for today's AM broadcast sound. Maybe the RF tube is weak?
::
::Thanks,
::
::Dave
:::Thomas,
::: That's a good idea because I can kind of disable the tone control without disconnecting it just by lowering the capacitance. I really don't think this radio needs a tone control at all. I'll re-replace the tone capacitor a little later this afternoon and let everyone know if it helped.
:::I think it will.
:::
:::Thanks,
:::
:::Dave
::::Regarding the tone control, the probable reason why it cuts off bass is because the condenser is too large. It should be more like .002 to .005 MFD. .025 MFD passes really low audio frequencies really well at those impedances. With the tone control at maximum resistance, the bass notes will still get through, since bass goes through resistors better than treble, and the .025 MFD condenser will pass the bass readily to B-. When the tone control is at minimum resistance, most of the audio will be bypassed by the condenser, due to its large size.
::::
::::The schematic lists a .025 MFD condenser for the tone control, but I think that it's too large.
::::
::::Also, if the value turns out to be correct for the circuit it's used in, perhaps the condenser is developing a leak, even though it is new. A leak will also cut off bass. I don't know why the condenser would become leaky in this circuit, since it's a grid circuit, but if it's one of those metalized film units, a strong 'pop' from your tapping the tuning condenser could melt away a small leak. I don't know why it would come back, though.
::::
::::Perhaps you should try other condensers of the same value, and then some of lower values around .005 to .002 MFD.
::::
::::T.
::::
::::

Edd,
I don't know where the transformer is situated. When I find it, I will tap it. Is it in one of the cans on the back of the chassis? Yes it's on the top rear of the chassis. I can pound on it and it makes no difference, There's nothing loose inside at all. When the tone control is disconnected, it sounds fine even when pounding on the transformer. I'm leaving it this way. I don't want to spend years on this set. The buzzy speaker cone is going to have to stay that way too. If I pull on one of the voice coil wires it stops some of the buzzing. But I can't see anything wrong. I have tried loosening the screw in the middle and pulling on the cone at a lot of different angles and then tightening the screw and it makes absolutely no difference. It's a buzzy speaker. I think I have put more than enough time into this radio for now. I must move on to something else.

Thanks,

Dave
:What is the resistance of the secondary of the audio driver transformer...and is it stable at that value when you whack on the transformer?
:
:73's de Edd
:
:---------------------------------------------------------------------------------------------B/T----------------------------------------------------------------------------------------------------------------
::Thomas,
:: The instant the wire is cut leading to the tone control, sound is perfectly clear and full and I don't have to do the ohm meter trick at all.
:: This makes absolutely no sense whatsoever. That grid voltage with the tone circuit disconnected is: -11 volts. So adding the tone control does limit the volume but it has nothing to do with the problem. I tried the ohm meter trick with it at what I thought was a full volume, and it gets even louder. Maybe the RF amp is weak or something like that. Nothing I can do can fix the problem. The only thing I can do is leave the tone control disconnected. I just wasted about 5 hours on this mysterious problem. All the resistors in the AVC circuit have been replaced. The resistors in the voltage divider for the -20 volts (which it never gets) have been replaced. -16 is about the most I can get. But I have to use the ohm meter trick to get it to do that. Most of the time it's around -9 volts no matter what I do.
:: With the tone control attached the volume is so low the radio is unusable. Nothing is touching the wires on the tone control except what's supposed to be connected to it. One end of a capacitor is connected to one end of the tone control and nothing else. The old capacitor was completely removed. The other end of the tone control is attached to the grid pin on the 47 and that's it. Noting else is attached. It makes not sense. When I disconnect the tone control I get full volume and the grid voltate goes up to about -11 volts from -9 volts. I have tried other 56s and 47s, but it continues to work exactly the same way. What do I do next? I am ready to give up.
::
::
::Thanks,
::
::Dave
:::Thomas,
::: I replaced the capacitor. The sound doesn't muffle as much when the tone control is turned down all the way. That part is better but I still have to use the ohmmeter trick to get the signal stronger. So this didn't solve the problem at all. It did solve one problem. .025 was a bit too much for today's AM broadcast sound. Maybe the RF tube is weak?
:::
:::Thanks,
:::
:::Dave
::::Thomas,
:::: That's a good idea because I can kind of disable the tone control without disconnecting it just by lowering the capacitance. I really don't think this radio needs a tone control at all. I'll re-replace the tone capacitor a little later this afternoon and let everyone know if it helped.
::::I think it will.
::::
::::Thanks,
::::
::::Dave
:::::Regarding the tone control, the probable reason why it cuts off bass is because the condenser is too large. It should be more like .002 to .005 MFD. .025 MFD passes really low audio frequencies really well at those impedances. With the tone control at maximum resistance, the bass notes will still get through, since bass goes through resistors better than treble, and the .025 MFD condenser will pass the bass readily to B-. When the tone control is at minimum resistance, most of the audio will be bypassed by the condenser, due to its large size.
:::::
:::::The schematic lists a .025 MFD condenser for the tone control, but I think that it's too large.
:::::
:::::Also, if the value turns out to be correct for the circuit it's used in, perhaps the condenser is developing a leak, even though it is new. A leak will also cut off bass. I don't know why the condenser would become leaky in this circuit, since it's a grid circuit, but if it's one of those metalized film units, a strong 'pop' from your tapping the tuning condenser could melt away a small leak. I don't know why it would come back, though.
:::::
:::::Perhaps you should try other condensers of the same value, and then some of lower values around .005 to .002 MFD.
:::::
:::::T.
:::::
:::::

Edd has something there. The audio may work even if the transformer secondary is open. It'll work by capacitance. This may also be why the tone condenser loads down the audio so badly. Furthermore, your touching the tuning condenser with your ohm meter may be causing enough of a surge for something to arc at the secondary.

An open secondary might explain why your bias voltage isn't as low as it should be. If the secondary is open, bias at the grid will only be whatever builds up on the grid.

Another problem could be that there's a leak from the primary to the secondary.

As for the buzzy speaker, make sure, too, that the voice coil isn't loose. If the glue has broken, it'll buzz. If you think that the cone is warped, you can shim the outside of the coil and then moisten the cone. Leave it shimmed until it dries. You should be able to center the coil with the centering screw, though. Put some strips of picture film between the outside of the coil and the pole piece (since you can't get at the inside of the coil). Loosen the screw and center the cone. Actually, put the shims in after the screw is loose. Then tighten the screw. Moisten the cone, too, if you wish. Once the cone dries, remove the shims.

T.

Thomas,
Ok. I don't think that these are very expensive but it will make the radio look different on top.

I have to move on to something else. This something else is under a different topic. Stromberg-Carlson 435. I'll post there a little later.
I am having a problem with that too. But I don't think that is as severe.
Thanks,

Dave
:Edd has something there. The audio may work even if the transformer secondary is open. It'll work by capacitance. This may also be why the tone condenser loads down the audio so badly. Furthermore, your touching the tuning condenser with your ohm meter may be causing enough of a surge for something to arc at the secondary.
:
:An open secondary might explain why your bias voltage isn't as low as it should be. If the secondary is open, bias at the grid will only be whatever builds up on the grid.
:
:Another problem could be that there's a leak from the primary to the secondary.
:
:As for the buzzy speaker, make sure, too, that the voice coil isn't loose. If the glue has broken, it'll buzz. If you think that the cone is warped, you can shim the outside of the coil and then moisten the cone. Leave it shimmed until it dries. You should be able to center the coil with the centering screw, though. Put some strips of picture film between the outside of the coil and the pole piece (since you can't get at the inside of the coil). Loosen the screw and center the cone. Actually, put the shims in after the screw is loose. Then tighten the screw. Moisten the cone, too, if you wish. Once the cone dries, remove the shims.
:
:T.

Thomas, Edd,
So this is the input transformer for the output stage? This is called P-2 on the schematic. I had a similar problem in a Crosley 124. That model had a lot of defective interstage transformers. I think that this is unusual in an RCA. But anyway, I need to be sure that this is the case before ordering a replacement transformer. Also, is there a way to do away with the imput transformer? Yes, I'd be modifying the circuit but it would probably cost less than a transformer.

Thanks,

Dave
:Edd has something there. The audio may work even if the transformer secondary is open. It'll work by capacitance. This may also be why the tone condenser loads down the audio so badly. Furthermore, your touching the tuning condenser with your ohm meter may be causing enough of a surge for something to arc at the secondary.
:
:An open secondary might explain why your bias voltage isn't as low as it should be. If the secondary is open, bias at the grid will only be whatever builds up on the grid.
:
:Another problem could be that there's a leak from the primary to the secondary.
:
:As for the buzzy speaker, make sure, too, that the voice coil isn't loose. If the glue has broken, it'll buzz. If you think that the cone is warped, you can shim the outside of the coil and then moisten the cone. Leave it shimmed until it dries. You should be able to center the coil with the centering screw, though. Put some strips of picture film between the outside of the coil and the pole piece (since you can't get at the inside of the coil). Loosen the screw and center the cone. Actually, put the shims in after the screw is loose. Then tighten the screw. Moisten the cone, too, if you wish. Once the cone dries, remove the shims.
:
:T.

You can do away with the interstage transformer by simply converting it to a resistance/capacitance coupled circuit. Start with basic values and change to your liking. 500K for the plate current and grid bias will be fine. .05 MFD for the bypass will work as well. Increase the plate resistance as much as possible to increase gain without increasing distortion (1 Meg should probably be the limit). Decrease the bypass capacitance as necessary to reduce the excessive bass that will result. It's probably best to leave the grid resistance at 500K. You can try higher values if you wish. I know that for the 6L6 they recommend not going over 500K. I'm not sure about the '47. For my 2A3 amplifier, the recommended value is 4.7 Meg, even with cathode bias. 2A3 is a triode, though, so it has different requirements. I've run 6V6 tubes quite successfully with 1 Meg grid resistors. You're probably not running enough current in that radio to worry about drawing grid current.

Are you sure that the transformer has a problem?

Thomas

Thomas,
How can I tell for sure that my input transformer is having problems?

Thanks,

Dave
:You can do away with the interstage transformer by simply converting it to a resistance/capacitance coupled circuit. Start with basic values and change to your liking. 500K for the plate current and grid bias will be fine. .05 MFD for the bypass will work as well. Increase the plate resistance as much as possible to increase gain without increasing distortion (1 Meg should probably be the limit). Decrease the bypass capacitance as necessary to reduce the excessive bass that will result. It's probably best to leave the grid resistance at 500K. You can try higher values if you wish. I know that for the 6L6 they recommend not going over 500K. I'm not sure about the '47. For my 2A3 amplifier, the recommended value is 4.7 Meg, even with cathode bias. 2A3 is a triode, though, so it has different requirements. I've run 6V6 tubes quite successfully with 1 Meg grid resistors. You're probably not running enough current in that radio to worry about drawing grid current.
:
:Are you sure that the transformer has a problem?
:
:Thomas

Dave

Check with an ohm meter. It will either show resistance, less than 10K, if good or be open if bad.

Norm

:Thomas,
: How can I tell for sure that my input transformer is having problems?
:
:Thanks,
:
:Dave
::You can do away with the interstage transformer by simply converting it to a resistance/capacitance coupled circuit. Start with basic values and change to your liking. 500K for the plate current and grid bias will be fine. .05 MFD for the bypass will work as well. Increase the plate resistance as much as possible to increase gain without increasing distortion (1 Meg should probably be the limit). Decrease the bypass capacitance as necessary to reduce the excessive bass that will result. It's probably best to leave the grid resistance at 500K. You can try higher values if you wish. I know that for the 6L6 they recommend not going over 500K. I'm not sure about the '47. For my 2A3 amplifier, the recommended value is 4.7 Meg, even with cathode bias. 2A3 is a triode, though, so it has different requirements. I've run 6V6 tubes quite successfully with 1 Meg grid resistors. You're probably not running enough current in that radio to worry about drawing grid current.
::
::Are you sure that the transformer has a problem?
::
::Thomas

Not that this is likely, but also check the secondary of the transformer with the set in operation. Disconnect it from everything and see if there's a leak between the primary and secondary. If such a leak developed (again, unlikely), it'd throw off bias voltages.

T.

:Not that this is likely, but also check the secondary of the transformer with the set in operation. Disconnect it from everything and see if there's a leak between the primary and secondary. If such a leak developed (again, unlikely), it'd throw off bias voltages.
:
:T.

And to chassis too.

Radiodoc
/o/o/o/o/o/