Philco 90 Has Same Volume At All Volume Settings

4/12/2012 12:18:11 PMJohnny

I just finished overhauling this Philco 90 with the single 47 output tube. It has reception and wants to pull in stations across the dial. But the volume stays the same no matter where the volume control is positioned. I put a screwdriver on the CT of the VC and I get a very audible hum from the speaker. Audio circuit is good? I wonder if I have a bad volume control? The only anomally I can find in the set is the voltage at the plate of the Det. Amp. Paperwork is a little fuzzy but it looks like I should have 140Vdc there. I have 54Vdc.

Johnny

4/12/2012 1:19:35 PMWarren

The plate voltage does seem low. Check resistors from power supply that feed this plate. The volume control you can check with an ohm meter. It could have an open where it goes to ground.

4/12/2012 1:26:51 PMLou

Check the volume control especially if it is a wirewound type. Over the years these wear and portions of them become open.

Lou

:The plate voltage does seem low. Check resistors from power supply that feed this plate. The volume control you can check with an ohm meter. It could have an open where it goes to ground.
:

4/12/2012 2:12:48 PMJohnny

Thanks guys! The plate voltage is low by about 90 volts if I'm reading the fuzzy paperwork correctly for the #27 Det. Amplifier. S/B 140 Vdc and I have 54Vdc. I'll go back thru the circuit again but everything is new and so far wired correctly.

I've been suspicious about this volume control from the moment I bench tested it. The part number on it matches the parts list for a 500K pot. On the bench it read 250K across it and from the CT to each outer leg it was intermittent and did seem to have opens. If nothing else it sure wasn't linear. I don't have and couldn't find a 500K pot but Radio Shack had a 1 meg that should at least tell me if it's the pot or not. That oughta be a word... Potornot. Tryin to keep my sense of humor. Lot of hours into this set.

:Check the volume control especially if it is a wirewound type. Over the years these wear and portions of them become open.
:
:Lou
:
::The plate voltage does seem low. Check resistors from power supply that feed this plate. The volume control you can check with an ohm meter. It could have an open where it goes to ground.
::
:
:

4/12/2012 3:31:00 PMWarren

Here is a link that has the Philco 90 in the DjVu format.
It also has the voltage pin out. This schematic is clear.

http://www.audiophool.cjb.net/Philco.html

4/12/2012 4:07:06 PMJohnny

Thanks Warren. A lot clearer. And the plate voltage is supposed to be 140 volts. I have 54 and have traced it back but it doesn't make sense. There's a 51K resistor in series with a 70K resistor to B+ source. On the other side of the 51K coming off of the plate there's 140 volts. The resistor is good along with the 70K. And the cap to ground between the 2 resistors is good. And all of this doesn't explain why my volume is stuck at one level and the VC can't change it.

Johnny

:Here is a link that has the Philco 90 in the DjVu format.
:It also has the voltage pin out. This schematic is clear.
:
:http://www.audiophool.cjb.net/Philco.html
:

4/12/2012 5:49:22 PMWarren

Try grounding the grid of the Det. Amp tube. If the radio goes quiet the volume control is bad. The plate voltage may go down when you ground this grid.

4/12/2012 6:09:07 PMJohnny

Grounded the grid of the Det. Amp. tube and the volume stays the same. Didn't go quiet.

:Try grounding the grid of the Det. Amp tube. If the radio goes quiet the volume control is bad. The plate voltage may go down when you ground this grid.
:
:

4/12/2012 6:55:51 PMWarren

By grounding this grid, that would have killed any audio going into this tube. Somewhere you may have made an error when re-capping this set. Thus bypassing this stage. This is why the volume control has no effect.
There is audio going into the 1st AF tube grid that should have come from the plate of the Det Amp tube. Check your work in this area with the clear schematic.

4/12/2012 9:36:48 PMJohnny

It's late. I just enlarged two more schematics to 11X17 and in the morning I'm gonna go over the whole circuit again highlighting things as I go.

Johnny

:By grounding this grid, that would have killed any audio going into this tube. Somewhere you may have made an error when re-capping this set. Thus bypassing this stage. This is why the volume control has no effect.
:There is audio going into the 1st AF tube grid that should have come from the plate of the Det Amp tube. Check your work in this area with the clear schematic.
:

4/13/2012 3:32:18 PMJohnny

Well I've gone over the whole schematic and highlighted every circuit. For the third time now! I'm just not finding anything that's not right. I ground the grid of the Det. Amp. and nothing happens. I remove the Det. Amp. tube and the volume goes dead. I've swapped out tubes with known good tubes with no positive results or changes in volume control. I've checked the AVC action at the junction of #46 & #48 to ground and I do get a difference in the negative voltage reading. Kinda tells me the AVC is working.

I'm now at a total loss as to where to look next seeing how I've looked everywhere.

Johnny

4/13/2012 5:25:20 PMWarren

The chassis in not the most negative ground point. The center tap from the power transformer ( 12 ) is the most negative point. Resistor 58 has fore connections. At point 1 and 2 this resistor could be open? Or a poor chassis connection. This would cause no true reference to negative. By pulling the Det Amp tube and the loss of audio would seem that there is AF going into the volume control to the grid of the Det Amp.

4/13/2012 8:14:00 PMJohnny

Yeah Warren the CT off the high voltage supply is the most neg. point in the radio. All of the resistors in #58 are new. The old one was fried. One thing I did notice on the Philco data sheet you gave me the link to is that it calls out most of the 24's and 27's as having a cathode voltage of around 20 volts. All of mine have 2.6 volts which come directly from connection 3 of th #58 resistor. Hmm! Also the voltage called out for the plate of the Det. Amp. is called out at 140 volts. I have 62 volts. 1st AF plate is called out at 45 volts. I have 172 volts. Almost as if there switched. But the set is wired correctly. Another Hmm!

Johnny

:The chassis in not the most negative ground point. The center tap from the power transformer ( 12 ) is the most negative point. Resistor 58 has fore connections. At point 1 and 2 this resistor could be open? Or a poor chassis connection. This would cause no true reference to negative. By pulling the Det Amp tube and the loss of audio would seem that there is AF going into the volume control to the grid of the Det Amp.
:

4/14/2012 4:54:39 PMMitch

:Yeah Warren the CT off the high voltage supply is the most neg. point in the radio. All of the resistors in #58 are new. The old one was fried. One thing I did notice on the Philco data sheet you gave me the link to is that it calls out most of the 24's and 27's as having a cathode voltage of around 20 volts. All of mine have 2.6 volts which come directly from connection 3 of th #58 resistor. Hmm! Also the voltage called out for the plate of the Det. Amp. is called out at 140 volts. I have 62 volts. 1st AF plate is called out at 45 volts. I have 172 volts. Almost as if there switched. But the set is wired correctly. Another Hmm!
:
:Johnny
:
:
::The chassis in not the most negative ground point. The center tap from the power transformer ( 12 ) is the most negative point. Resistor 58 has fore connections. At point 1 and 2 this resistor could be open? Or a poor chassis connection. This would cause no true reference to negative. By pulling the Det Amp tube and the loss of audio would seem that there is AF going into the volume control to the grid of the Det Amp.
::
:
:
Johnny,

I had a VC that i could not verify in the circuit as to if it had an open. Warren suggested that i disconnect it and use jumper wires to connect to a known good VC out of the circuit. The test proved to me that the VC itself in the radio was good. I finally found a bad speaker.

This test will atleast confirm if your VC is good or not.

Luck,
Mitch

4/14/2012 5:28:44 PMEdd

Sir Johnny . . . . .

Go to your second #27 tube "detector amp" and bypass its cathode to ground with a .47---1 ufd capacitor . . .what have you . . . in order to confirm that your "phantom " audio path is not being sneaked in thru the cathode circuitry.

Did the audio disappear or go heavily "bassy" ?

Then power down and measure your cathodes to ground resistance, on that common cathode buss shared by the RF, IF and #27 tube "detector amp" , then let us know what it is.

There is nothing mysterious with that first #27 detector, just treat it like it was being a 75 or 6SQ7 or 6AV6.

Its 1st grid is merely acting as a diode plate, and the conventional audio take off point is from the bottom of the secondary winding of the IF transformer.

That detected audio then passes thru the #17 99k, then the #18 .01 ufd cap and then into the high side of the audio tapered 500K volume control.

All is a standard audio flow path after that.

Now with your comment on the common cathodes of those tubes and the spec of them being in the 20V range and considering their common cathode resistor as being 60 ohms.
That number crunches out as a shared cathode current of ~ 333 ma . . . .or 6.66 watts . . .now, not really ?.

Your figure of ~2.6 Volts computes out at a more realistic shared current of 43 ma
at a dissipated 1.25 watt.

73's de Edd

How long a minute is . . . . . depends on . . . . . exactly WHICH side of the bathroom door YOU are on.

::Yeah Warren the CT off the high voltage supply is the most neg. point in the radio. All of the resistors in #58 are new. The old one was fried. One thing I did notice on the Philco data sheet you gave me the link to is that it calls out most of the 24's and 27's as having a cathode voltage of around 20 volts. All of mine have 2.6 volts which come directly from connection 3 of th #58 resistor. Hmm! Also the voltage called out for the plate of the Det. Amp. is called out at 140 volts. I have 62 volts. 1st AF plate is called out at 45 volts. I have 172 volts. Almost as if there switched. But the set is wired correctly. Another Hmm!
::
::Johnny
::
::
:::The chassis in not the most negative ground point. The center tap from the power transformer ( 12 ) is the most negative point. Resistor 58 has fore connections. At point 1 and 2 this resistor could be open? Or a poor chassis connection. This would cause no true reference to negative. By pulling the Det Amp tube and the loss of audio would seem that there is AF going into the volume control to the grid of the Det Amp.
:::
::
::
:Johnny,
:
:I had a VC that i could not verify in the circuit as to if it had an open. Warren suggested that i disconnect it and use jumper wires to connect to a known good VC out of the circuit. The test proved to me that the VC itself in the radio was good. I finally found a bad speaker.
:
:This test will atleast confirm if your VC is good or not.
:
:Luck,
:Mitch
: