http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
I've been really enjoying listening to it day after day during bench test burn-in.... until this evening it just plain QUIT !! lol
I didn't notice it for a while as I had a 2nd radio playing the same station so.. when it died I missed it.
As I went to turn them both off.. I noticed this was dead although the dial lamp was still lit as were the filaments of course.
I pulled it out of the cabinet and found no audio noise when I touched the grid cap of the 12Q7..... Just quiet silence.
I had another 35L6 output tube laying right there so I swapped that before testing anything else... but it had no effect.
I measured B+ and found it was almost nil at about 1 or 2 volts.
Humm.
I swapped the 35Z5 rectifier to no avail.
Strange... I wondered..if something is pulling down the B+ ? ...maybe??
A flaky internally shorted tube perhaps?
I swapped each tube one by one with no difference.
So.. time to isolate the cause.. seriously!
I un-soldered the 2nd section of the pi filter to isolate basically the entire radio's B+ load...
...so now only the first filter cap is connected.
Ahh ... okay.. now I have B+ at about 120vdc... ok... so far so good.. right?
So I re-connect that point and go unsolder everything from the load side after the 2nd filter cap....
Ok good .. still sort of nice B+ down at about 105v after the resistor at the 2nd filter cap ... ok... seems alright.
Okay.. so now I unsolder ALL the plate loads from everything and start to put them back on one by one ... figuring I'll find which one is too much.
However... this is the point at which I started REALLY scratching my head because ANY load at all would pull the B+ down to 45 volts or so!
Even if I had ALL loads off including the output tube.
If I only connected the plate of the 12Q7 the voltage would go down. I really puzzled with this a while. If I let that 12Q7 grid float.. the B+ would come back up.. to almost normal. But as soon as it was connected... down went that B+ again.
So I changed rectifiers again.... same thing.
Then I decided to put a silicone rectifier across the 35Z5 from pin 5 the plate to pin 8 the cathode as a test to see if it can help keep the B+ up.
It didn't work that way.. either... but when I clipped it on right from the AC line input point on pin 2 and then over to pin 8...
Okay... there was finally B+... solid and strong !
It started to work... but it didn't seem to make sense.
Now this radio has the pilot lamp across pins 2 and 3 as usual... but it ALSO has a 25 ohm resistor from pin 3 to the plate pin 5.
I guess it's for current limiting safety?
Most sets just have a jumper wire from pin 3 to pin 5.
Well the problem here was that the 25 ohm resistor has somehow drifted waaay high.. like about 10 megs or so...lol
I had replaced the old resistor with a nice new 1/2 watt film resistor only last week.... but seems it crapped out and drifted high!
That was the last thing I expected to fail. It wasn't discolored burned or anything indicating it had been stressed either.. just nice new-looking.
Odd.
Well.. I put another nice new 1 watt-er in there now.. and everything is back to good order... radio playing oh-so beautifully... lol
Right now I'm rewarding myself and I'm enjoying to my favorite local old big-band style radio station... and right now he's playing a truly great.. old Cole Porter's 1928 tune "Let's Misbehave" from the musical "Anything Goes"
http://www.youtube.com/watch?v=IuoDqJoiWwg
Now here's another couple of real fun you-tube interpretations:
http://www.youtube.com/watch?v=mEieFuIWPYc
http://www.youtube.com/watch?v=_xTSHv_-7hc
Thank you,
Vinny
:I finished that nice little Sonora KM-450 last week:
:http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
:
:I've been really enjoying listening to it day after day during bench test burn-in.... until this evening it just plain QUIT !! lol
:
:I didn't notice it for a while as I had a 2nd radio playing the same station so.. when it died I missed it.
:
:As I went to turn them both off.. I noticed this was dead although the dial lamp was still lit as were the filaments of course.
:
:I pulled it out of the cabinet and found no audio noise when I touched the grid cap of the 12Q7..... Just quiet silence.
:
:I had another 35L6 output tube laying right there so I swapped that before testing anything else... but it had no effect.
:
:I measured B+ and found it was almost nil at about 1 or 2 volts.
:
:Humm.
:
:I swapped the 35Z5 rectifier to no avail.
:
:Strange... I wondered..if something is pulling down the B+ ? ...maybe??
:
:A flaky internally shorted tube perhaps?
:
:I swapped each tube one by one with no difference.
:
:So.. time to isolate the cause.. seriously!
:
:I un-soldered the 2nd section of the pi filter to isolate basically the entire radio's B+ load...
:...so now only the first filter cap is connected.
:
:Ahh ... okay.. now I have B+ at about 120vdc... ok... so far so good.. right?
:
:So I re-connect that point and go unsolder everything from the load side after the 2nd filter cap....
:Ok good .. still sort of nice B+ down at about 105v after the resistor at the 2nd filter cap ... ok... seems alright.
:
:Okay.. so now I unsolder ALL the plate loads from everything and start to put them back on one by one ... figuring I'll find which one is too much.
:However... this is the point at which I started REALLY scratching my head because ANY load at all would pull the B+ down to 45 volts or so!
:
:Even if I had ALL loads off including the output tube.
:If I only connected the plate of the 12Q7 the voltage would go down. I really puzzled with this a while. If I let that 12Q7 grid float.. the B+ would come back up.. to almost normal. But as soon as it was connected... down went that B+ again.
:
:So I changed rectifiers again.... same thing.
:
:Then I decided to put a silicone rectifier across the 35Z5 from pin 5 the plate to pin 8 the cathode as a test to see if it can help keep the B+ up.
:
:It didn't work that way.. either... but when I clipped it on right from the AC line input point on pin 2 and then over to pin 8...
:Okay... there was finally B+... solid and strong !
:It started to work... but it didn't seem to make sense.
:
:Now this radio has the pilot lamp across pins 2 and 3 as usual... but it ALSO has a 25 ohm resistor from pin 3 to the plate pin 5.
:I guess it's for current limiting safety?
:
:
:
:Most sets just have a jumper wire from pin 3 to pin 5.
:
:Well the problem here was that the 25 ohm resistor has somehow drifted waaay high.. like about 10 megs or so...lol
:
:I had replaced the old resistor with a nice new 1/2 watt film resistor only last week.... but seems it crapped out and drifted high!
:
:That was the last thing I expected to fail. It wasn't discolored burned or anything indicating it had been stressed either.. just nice new-looking.
:Odd.
:
:Well.. I put another nice new 1 watt-er in there now.. and everything is back to good order... radio playing oh-so beautifully... lol
:
:Right now I'm rewarding myself and I'm enjoying to my favorite local old big-band style radio station... and right now he's playing a truly great.. old Cole Porter's 1928 tune "Let's Misbehave" from the musical "Anything Goes"
:http://www.youtube.com/watch?v=IuoDqJoiWwg
:
:Now here's another couple of real fun you-tube interpretations:
:http://www.youtube.com/watch?v=mEieFuIWPYc
:http://www.youtube.com/watch?v=_xTSHv_-7hc
HCT stands for Heater Center Tap, which is used many times for pilot light connections on the 35Z5.
Doug
:Peter, sorry to bother you. I am new to antique radios (about 6 months), I am learning how to use my multimeter and read that you measured B+ voltgage, where did you put your probes to measure? and also for B-? you also mentioned about a faulty resistor across 2 & 3 Pins (I have philco with the same)but mentioned that there is usually a jumper wire across those two points, so why would a faulty resistor cause such a problem if ususally there is a jumper between those pins? I also noticed on the tube layout on nostalgia http://www.nostalgiaair.org/tubes/35z5.htm that the 3 pin is marked HCT as the 2 and 7 pins are marked H (heater). So what does the HCT designate?
:Thank you,
:Vinny
:
:
:
::I finished that nice little Sonora KM-450 last week:
::http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
::
::I've been really enjoying listening to it day after day during bench test burn-in.... until this evening it just plain QUIT !! lol
::
::I didn't notice it for a while as I had a 2nd radio playing the same station so.. when it died I missed it.
::
::As I went to turn them both off.. I noticed this was dead although the dial lamp was still lit as were the filaments of course.
::
::I pulled it out of the cabinet and found no audio noise when I touched the grid cap of the 12Q7..... Just quiet silence.
::
::I had another 35L6 output tube laying right there so I swapped that before testing anything else... but it had no effect.
::
::I measured B+ and found it was almost nil at about 1 or 2 volts.
::
::Humm.
::
::I swapped the 35Z5 rectifier to no avail.
::
::Strange... I wondered..if something is pulling down the B+ ? ...maybe??
::
::A flaky internally shorted tube perhaps?
::
::I swapped each tube one by one with no difference.
::
::So.. time to isolate the cause.. seriously!
::
::I un-soldered the 2nd section of the pi filter to isolate basically the entire radio's B+ load...
::...so now only the first filter cap is connected.
::
::Ahh ... okay.. now I have B+ at about 120vdc... ok... so far so good.. right?
::
::So I re-connect that point and go unsolder everything from the load side after the 2nd filter cap....
::Ok good .. still sort of nice B+ down at about 105v after the resistor at the 2nd filter cap ... ok... seems alright.
::
::Okay.. so now I unsolder ALL the plate loads from everything and start to put them back on one by one ... figuring I'll find which one is too much.
::However... this is the point at which I started REALLY scratching my head because ANY load at all would pull the B+ down to 45 volts or so!
::
::Even if I had ALL loads off including the output tube.
::If I only connected the plate of the 12Q7 the voltage would go down. I really puzzled with this a while. If I let that 12Q7 grid float.. the B+ would come back up.. to almost normal. But as soon as it was connected... down went that B+ again.
::
::So I changed rectifiers again.... same thing.
::
::Then I decided to put a silicone rectifier across the 35Z5 from pin 5 the plate to pin 8 the cathode as a test to see if it can help keep the B+ up.
::
::It didn't work that way.. either... but when I clipped it on right from the AC line input point on pin 2 and then over to pin 8...
::Okay... there was finally B+... solid and strong !
::It started to work... but it didn't seem to make sense.
::
::Now this radio has the pilot lamp across pins 2 and 3 as usual... but it ALSO has a 25 ohm resistor from pin 3 to the plate pin 5.
::I guess it's for current limiting safety?
::
::
::
::Most sets just have a jumper wire from pin 3 to pin 5.
::
::Well the problem here was that the 25 ohm resistor has somehow drifted waaay high.. like about 10 megs or so...lol
::
::I had replaced the old resistor with a nice new 1/2 watt film resistor only last week.... but seems it crapped out and drifted high!
::
::That was the last thing I expected to fail. It wasn't discolored burned or anything indicating it had been stressed either.. just nice new-looking.
::Odd.
::
::Well.. I put another nice new 1 watt-er in there now.. and everything is back to good order... radio playing oh-so beautifully... lol
::
::Right now I'm rewarding myself and I'm enjoying to my favorite local old big-band style radio station... and right now he's playing a truly great.. old Cole Porter's 1928 tune "Let's Misbehave" from the musical "Anything Goes"
::http://www.youtube.com/watch?v=IuoDqJoiWwg
::
::Now here's another couple of real fun you-tube interpretations:
::http://www.youtube.com/watch?v=mEieFuIWPYc
::http://www.youtube.com/watch?v=_xTSHv_-7hc
Vinny
p.s been reading a lot from the site you sent me.
:Vinny,
:
:HCT stands for Heater Center Tap, which is used many times for pilot light connections on the 35Z5.
:
:Doug
:
::Peter, sorry to bother you. I am new to antique radios (about 6 months), I am learning how to use my multimeter and read that you measured B+ voltgage, where did you put your probes to measure? and also for B-? you also mentioned about a faulty resistor across 2 & 3 Pins (I have philco with the same)but mentioned that there is usually a jumper wire across those two points, so why would a faulty resistor cause such a problem if ususally there is a jumper between those pins? I also noticed on the tube layout on nostalgia http://www.nostalgiaair.org/tubes/35z5.htm that the 3 pin is marked HCT as the 2 and 7 pins are marked H (heater). So what does the HCT designate?
::Thank you,
::Vinny
::
::
::
:::I finished that nice little Sonora KM-450 last week:
:::http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
:::
:::I've been really enjoying listening to it day after day during bench test burn-in.... until this evening it just plain QUIT !! lol
:::
:::I didn't notice it for a while as I had a 2nd radio playing the same station so.. when it died I missed it.
:::
:::As I went to turn them both off.. I noticed this was dead although the dial lamp was still lit as were the filaments of course.
:::
:::I pulled it out of the cabinet and found no audio noise when I touched the grid cap of the 12Q7..... Just quiet silence.
:::
:::I had another 35L6 output tube laying right there so I swapped that before testing anything else... but it had no effect.
:::
:::I measured B+ and found it was almost nil at about 1 or 2 volts.
:::
:::Humm.
:::
:::I swapped the 35Z5 rectifier to no avail.
:::
:::Strange... I wondered..if something is pulling down the B+ ? ...maybe??
:::
:::A flaky internally shorted tube perhaps?
:::
:::I swapped each tube one by one with no difference.
:::
:::So.. time to isolate the cause.. seriously!
:::
:::I un-soldered the 2nd section of the pi filter to isolate basically the entire radio's B+ load...
:::...so now only the first filter cap is connected.
:::
:::Ahh ... okay.. now I have B+ at about 120vdc... ok... so far so good.. right?
:::
:::So I re-connect that point and go unsolder everything from the load side after the 2nd filter cap....
:::Ok good .. still sort of nice B+ down at about 105v after the resistor at the 2nd filter cap ... ok... seems alright.
:::
:::Okay.. so now I unsolder ALL the plate loads from everything and start to put them back on one by one ... figuring I'll find which one is too much.
:::However... this is the point at which I started REALLY scratching my head because ANY load at all would pull the B+ down to 45 volts or so!
:::
:::Even if I had ALL loads off including the output tube.
:::If I only connected the plate of the 12Q7 the voltage would go down. I really puzzled with this a while. If I let that 12Q7 grid float.. the B+ would come back up.. to almost normal. But as soon as it was connected... down went that B+ again.
:::
:::So I changed rectifiers again.... same thing.
:::
:::Then I decided to put a silicone rectifier across the 35Z5 from pin 5 the plate to pin 8 the cathode as a test to see if it can help keep the B+ up.
:::
:::It didn't work that way.. either... but when I clipped it on right from the AC line input point on pin 2 and then over to pin 8...
:::Okay... there was finally B+... solid and strong !
:::It started to work... but it didn't seem to make sense.
:::
:::Now this radio has the pilot lamp across pins 2 and 3 as usual... but it ALSO has a 25 ohm resistor from pin 3 to the plate pin 5.
:::I guess it's for current limiting safety?
:::
:::
:::
:::Most sets just have a jumper wire from pin 3 to pin 5.
:::
:::Well the problem here was that the 25 ohm resistor has somehow drifted waaay high.. like about 10 megs or so...lol
:::
:::I had replaced the old resistor with a nice new 1/2 watt film resistor only last week.... but seems it crapped out and drifted high!
:::
:::That was the last thing I expected to fail. It wasn't discolored burned or anything indicating it had been stressed either.. just nice new-looking.
:::Odd.
:::
:::Well.. I put another nice new 1 watt-er in there now.. and everything is back to good order... radio playing oh-so beautifully... lol
:::
:::Right now I'm rewarding myself and I'm enjoying to my favorite local old big-band style radio station... and right now he's playing a truly great.. old Cole Porter's 1928 tune "Let's Misbehave" from the musical "Anything Goes"
:::http://www.youtube.com/watch?v=IuoDqJoiWwg
:::
:::Now here's another couple of real fun you-tube interpretations:
:::http://www.youtube.com/watch?v=mEieFuIWPYc
:::http://www.youtube.com/watch?v=_xTSHv_-7hc
The description of the failed 25 ohm resistor reminds me of the failed resistors we see in the base of tuning eyes. They look good, but are open. With these they have technically the right wattage rating, but being in an enclosed area they overheat and fail. Could your 25 ohm resistor be in a closed area?
Best Regards,
Bill Grimm
Richard
:
:I finished that nice little Sonora KM-450 last week:
:http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
:
:I've been really enjoying listening to it day after day during bench test burn-in.... until this evening it just plain QUIT !! lol
:
:I didn't notice it for a while as I had a 2nd radio playing the same station so.. when it died I missed it.
:
:As I went to turn them both off.. I noticed this was dead although the dial lamp was still lit as were the filaments of course.
:
:I pulled it out of the cabinet and found no audio noise when I touched the grid cap of the 12Q7..... Just quiet silence.
:
:I had another 35L6 output tube laying right there so I swapped that before testing anything else... but it had no effect.
:
:I measured B+ and found it was almost nil at about 1 or 2 volts.
:
:Humm.
:
:I swapped the 35Z5 rectifier to no avail.
:
:Strange... I wondered..if something is pulling down the B+ ? ...maybe??
:
:A flaky internally shorted tube perhaps?
:
:I swapped each tube one by one with no difference.
:
:So.. time to isolate the cause.. seriously!
:
:I un-soldered the 2nd section of the pi filter to isolate basically the entire radio's B+ load...
:...so now only the first filter cap is connected.
:
:Ahh ... okay.. now I have B+ at about 120vdc... ok... so far so good.. right?
:
:So I re-connect that point and go unsolder everything from the load side after the 2nd filter cap....
:Ok good .. still sort of nice B+ down at about 105v after the resistor at the 2nd filter cap ... ok... seems alright.
:
:Okay.. so now I unsolder ALL the plate loads from everything and start to put them back on one by one ... figuring I'll find which one is too much.
:However... this is the point at which I started REALLY scratching my head because ANY load at all would pull the B+ down to 45 volts or so!
:
:Even if I had ALL loads off including the output tube.
:If I only connected the plate of the 12Q7 the voltage would go down. I really puzzled with this a while. If I let that 12Q7 grid float.. the B+ would come back up.. to almost normal. But as soon as it was connected... down went that B+ again.
:
:So I changed rectifiers again.... same thing.
:
:Then I decided to put a silicone rectifier across the 35Z5 from pin 5 the plate to pin 8 the cathode as a test to see if it can help keep the B+ up.
:
:It didn't work that way.. either... but when I clipped it on right from the AC line input point on pin 2 and then over to pin 8...
:Okay... there was finally B+... solid and strong !
:It started to work... but it didn't seem to make sense.
:
:Now this radio has the pilot lamp across pins 2 and 3 as usual... but it ALSO has a 25 ohm resistor from pin 3 to the plate pin 5.
:I guess it's for current limiting safety?
:
:
:
:Most sets just have a jumper wire from pin 3 to pin 5.
:
:Well the problem here was that the 25 ohm resistor has somehow drifted waaay high.. like about 10 megs or so...lol
:
:I had replaced the old resistor with a nice new 1/2 watt film resistor only last week.... but seems it crapped out and drifted high!
:
:That was the last thing I expected to fail. It wasn't discolored burned or anything indicating it had been stressed either.. just nice new-looking.
:Odd.
:
:Well.. I put another nice new 1 watt-er in there now.. and everything is back to good order... radio playing oh-so beautifully... lol
:
:Right now I'm rewarding myself and I'm enjoying to my favorite local old big-band style radio station... and right now he's playing a truly great.. old Cole Porter's 1928 tune "Let's Misbehave" from the musical "Anything Goes"
:http://www.youtube.com/watch?v=IuoDqJoiWwg
:
:Now here's another couple of real fun you-tube interpretations:
:http://www.youtube.com/watch?v=mEieFuIWPYc
:http://www.youtube.com/watch?v=_xTSHv_-7hc
I'm not Peter, but the other section of the tuning cap would normally connect to the oscillator circuit. Some of these caps have solder lugs on both sides or top and bottom of both sections. If there is a solder lug on the other side of the cap with the loose wire, it may connect to the oscillator circuits, and in such case the loose wire should be removed.
marv
::Hi Peter. This is not related to your current post but I thought I'd use the opportunity to ask you a question about my Zenith 6-S-223. I believe you said you had done a couple of these. Let me try to make some sense here. On top of the tuning capacitor there is a wire that goes to the control grid cap of one of the tubes. On the top of the other section there is a wire that is only long enough to reach the top of the other half of the tuning cap but it is connected to nothing. Does this wire actually belong there? Do the contacts on top of the two sections of the tuning capacitor connect to each other? Thanks Peter!
::
::I finished that nice little Sonora KM-450 last week:
::http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
::
::I've been really enjoying listening to it day after day during bench test burn-in.... until this evening it just plain QUIT !! lol
::
::I didn't notice it for a while as I had a 2nd radio playing the same station so.. when it died I missed it.
::
::As I went to turn them both off.. I noticed this was dead although the dial lamp was still lit as were the filaments of course.
::
::I pulled it out of the cabinet and found no audio noise when I touched the grid cap of the 12Q7..... Just quiet silence.
::
::I had another 35L6 output tube laying right there so I swapped that before testing anything else... but it had no effect.
::
::I measured B+ and found it was almost nil at about 1 or 2 volts.
::
::Humm.
::
::I swapped the 35Z5 rectifier to no avail.
::
::Strange... I wondered..if something is pulling down the B+ ? ...maybe??
::
::A flaky internally shorted tube perhaps?
::
::I swapped each tube one by one with no difference.
::
::So.. time to isolate the cause.. seriously!
::
::I un-soldered the 2nd section of the pi filter to isolate basically the entire radio's B+ load...
::...so now only the first filter cap is connected.
::
::Ahh ... okay.. now I have B+ at about 120vdc... ok... so far so good.. right?
::
::So I re-connect that point and go unsolder everything from the load side after the 2nd filter cap....
::Ok good .. still sort of nice B+ down at about 105v after the resistor at the 2nd filter cap ... ok... seems alright.
::
::Okay.. so now I unsolder ALL the plate loads from everything and start to put them back on one by one ... figuring I'll find which one is too much.
::However... this is the point at which I started REALLY scratching my head because ANY load at all would pull the B+ down to 45 volts or so!
::
::Even if I had ALL loads off including the output tube.
::If I only connected the plate of the 12Q7 the voltage would go down. I really puzzled with this a while. If I let that 12Q7 grid float.. the B+ would come back up.. to almost normal. But as soon as it was connected... down went that B+ again.
::
::So I changed rectifiers again.... same thing.
::
::Then I decided to put a silicone rectifier across the 35Z5 from pin 5 the plate to pin 8 the cathode as a test to see if it can help keep the B+ up.
::
::It didn't work that way.. either... but when I clipped it on right from the AC line input point on pin 2 and then over to pin 8...
::Okay... there was finally B+... solid and strong !
::It started to work... but it didn't seem to make sense.
::
::Now this radio has the pilot lamp across pins 2 and 3 as usual... but it ALSO has a 25 ohm resistor from pin 3 to the plate pin 5.
::I guess it's for current limiting safety?
::
::
::
::Most sets just have a jumper wire from pin 3 to pin 5.
::
::Well the problem here was that the 25 ohm resistor has somehow drifted waaay high.. like about 10 megs or so...lol
::
::I had replaced the old resistor with a nice new 1/2 watt film resistor only last week.... but seems it crapped out and drifted high!
::
::That was the last thing I expected to fail. It wasn't discolored burned or anything indicating it had been stressed either.. just nice new-looking.
::Odd.
::
::Well.. I put another nice new 1 watt-er in there now.. and everything is back to good order... radio playing oh-so beautifully... lol
::
::Right now I'm rewarding myself and I'm enjoying to my favorite local old big-band style radio station... and right now he's playing a truly great.. old Cole Porter's 1928 tune "Let's Misbehave" from the musical "Anything Goes"
::http://www.youtube.com/watch?v=IuoDqJoiWwg
::
::Now here's another couple of real fun you-tube interpretations:
::http://www.youtube.com/watch?v=mEieFuIWPYc
::http://www.youtube.com/watch?v=_xTSHv_-7hc
Johnny
:
:Johnny,
:I'm not Peter, but the other section of the tuning cap would normally connect to the oscillator circuit. Some of these caps have solder lugs on both sides or top and bottom of both sections. If there is a solder lug on the other side of the cap with the loose wire, it may connect to the oscillator circuits, and in such case the loose wire should be removed.
:
:marv
:
:::Hi Peter. This is not related to your current post but I thought I'd use the opportunity to ask you a question about my Zenith 6-S-223. I believe you said you had done a couple of these. Let me try to make some sense here. On top of the tuning capacitor there is a wire that goes to the control grid cap of one of the tubes. On the top of the other section there is a wire that is only long enough to reach the top of the other half of the tuning cap but it is connected to nothing. Does this wire actually belong there? Do the contacts on top of the two sections of the tuning capacitor connect to each other? Thanks Peter!
:::
:::I finished that nice little Sonora KM-450 last week:
:::http://www.nostalgiaair.org/PagesByModel/328/M0006328.pdf
:::
:::I've been really enjoying listening to it day after day during bench test burn-in.... until this evening it just plain QUIT !! lol
:::
:::I didn't notice it for a while as I had a 2nd radio playing the same station so.. when it died I missed it.
:::
:::As I went to turn them both off.. I noticed this was dead although the dial lamp was still lit as were the filaments of course.
:::
:::I pulled it out of the cabinet and found no audio noise when I touched the grid cap of the 12Q7..... Just quiet silence.
:::
:::I had another 35L6 output tube laying right there so I swapped that before testing anything else... but it had no effect.
:::
:::I measured B+ and found it was almost nil at about 1 or 2 volts.
:::
:::Humm.
:::
:::I swapped the 35Z5 rectifier to no avail.
:::
:::Strange... I wondered..if something is pulling down the B+ ? ...maybe??
:::
:::A flaky internally shorted tube perhaps?
:::
:::I swapped each tube one by one with no difference.
:::
:::So.. time to isolate the cause.. seriously!
:::
:::I un-soldered the 2nd section of the pi filter to isolate basically the entire radio's B+ load...
:::...so now only the first filter cap is connected.
:::
:::Ahh ... okay.. now I have B+ at about 120vdc... ok... so far so good.. right?
:::
:::So I re-connect that point and go unsolder everything from the load side after the 2nd filter cap....
:::Ok good .. still sort of nice B+ down at about 105v after the resistor at the 2nd filter cap ... ok... seems alright.
:::
:::Okay.. so now I unsolder ALL the plate loads from everything and start to put them back on one by one ... figuring I'll find which one is too much.
:::However... this is the point at which I started REALLY scratching my head because ANY load at all would pull the B+ down to 45 volts or so!
:::
:::Even if I had ALL loads off including the output tube.
:::If I only connected the plate of the 12Q7 the voltage would go down. I really puzzled with this a while. If I let that 12Q7 grid float.. the B+ would come back up.. to almost normal. But as soon as it was connected... down went that B+ again.
:::
:::So I changed rectifiers again.... same thing.
:::
:::Then I decided to put a silicone rectifier across the 35Z5 from pin 5 the plate to pin 8 the cathode as a test to see if it can help keep the B+ up.
:::
:::It didn't work that way.. either... but when I clipped it on right from the AC line input point on pin 2 and then over to pin 8...
:::Okay... there was finally B+... solid and strong !
:::It started to work... but it didn't seem to make sense.
:::
:::Now this radio has the pilot lamp across pins 2 and 3 as usual... but it ALSO has a 25 ohm resistor from pin 3 to the plate pin 5.
:::I guess it's for current limiting safety?
:::
:::
:::
:::Most sets just have a jumper wire from pin 3 to pin 5.
:::
:::Well the problem here was that the 25 ohm resistor has somehow drifted waaay high.. like about 10 megs or so...lol
:::
:::I had replaced the old resistor with a nice new 1/2 watt film resistor only last week.... but seems it crapped out and drifted high!
:::
:::That was the last thing I expected to fail. It wasn't discolored burned or anything indicating it had been stressed either.. just nice new-looking.
:::Odd.
:::
:::Well.. I put another nice new 1 watt-er in there now.. and everything is back to good order... radio playing oh-so beautifully... lol
:::
:::Right now I'm rewarding myself and I'm enjoying to my favorite local old big-band style radio station... and right now he's playing a truly great.. old Cole Porter's 1928 tune "Let's Misbehave" from the musical "Anything Goes"
:::http://www.youtube.com/watch?v=IuoDqJoiWwg
:::
:::Now here's another couple of real fun you-tube interpretations:
:::http://www.youtube.com/watch?v=mEieFuIWPYc
:::http://www.youtube.com/watch?v=_xTSHv_-7hc
:Johnny
::
::Johnny,
::I'm not Peter, but the other section of the tuning cap would normally connect to the oscillator circuit. Some of these caps have solder lugs on both sides or top and bottom of both sections. If there is a solder lug on the other side of the cap with the loose wire, it may connect to the oscillator circuits, and in such case the loose wire should be removed.
::
::marv
::
::::Hi Peter. This is not related to your current post but I thought I'd use the opportunity to ask you a question about my Zenith 6-S-223. I believe you said you had done a couple of these. Let me try to make some sense here. On top of the tuning capacitor there is a wire that goes to the control grid cap of one of the tubes. On the top of the other section there is a wire that is only long enough to reach the top of the other half of the tuning cap but it is connected to nothing. Does this wire actually belong there? Do the contacts on top of the two sections of the tuning capacitor connect to each other? Thanks Peter!
Johnny:
On my 222 here the rear most condenser wire goes directly to the RF tube grid cap. And the front condenser does have a short wire on it too.. and that wire has it's loose end wrapped around the the other wire about 3 turns... sort of like a gimmick cap connection.
::Johnny
:::
:::Johnny,
:::I'm not Peter, but the other section of the tuning cap would normally connect to the oscillator circuit. Some of these caps have solder lugs on both sides or top and bottom of both sections. If there is a solder lug on the other side of the cap with the loose wire, it may connect to the oscillator circuits, and in such case the loose wire should be removed.
:::
:::marv
:::
:::::Hi Peter. This is not related to your current post but I thought I'd use the opportunity to ask you a question about my Zenith 6-S-223. I believe you said you had done a couple of these. Let me try to make some sense here. On top of the tuning capacitor there is a wire that goes to the control grid cap of one of the tubes. On the top of the other section there is a wire that is only long enough to reach the top of the other half of the tuning cap but it is connected to nothing. Does this wire actually belong there? Do the contacts on top of the two sections of the tuning capacitor connect to each other? Thanks Peter!
:
:
:
:Johnny:
:On my 222 here the rear most condenser wire goes directly to the RF tube grid cap. And the front condenser does have a short wire on it too.. and that wire has it's loose end wrapped around the the other wire about 3 turns... sort of like a gimmick cap connection.
:
:Thanks Peter! That makes sense because the short wire on my 223 looks original and the solder joint looks original. But does this wire being wrapped around the other wire really serve any purpose?
Johnny
:Johnny
I don't know why it was designed that way... but when I tried removing it it changed my alignment a bit.
So maybe I could have re-aligned it without it.. but I assumed that since it was there from the factory ..I left it.
Johnny
:
:::Thanks Peter! That makes sense because the short wire on my 223 looks original and the solder joint looks original. But does this wire being wrapped around the other wire really serve any purpose?
::Johnny
:
:I don't know why it was designed that way... but when I tried removing it it changed my alignment a bit.
:
:So maybe I could have re-aligned it without it.. but I assumed that since it was there from the factory ..I left it.
Richard
Well that's what I get for shooting from the hip!!! After finally looking at the schema, that wire shows up as a coil wrapped around one tuning cap wire at the grid from the other tuning cap section. This is a first for me, but far be it from me to question those old time Zenith engineers.
marv
:It is a gimmick capacitor as Peter mentioned. Without it shortwave operation will likely be unstable. Provides a small amount of negative feedback. Ran into a problem with a Zenith 8S154 that had this wire removed. Radio used a 6A8 converter tube that would oscillate and create a large AVC voltage losing all reception on shortwave.
:
:Richard