This is a combo radio/phono from early '50's. I can't get the radio volume to go down to no sound at all. When I lower the vol. ctl. full counterclockwise I still get a low level radio sound on the radio. The phono sound does go to '0'(quiet) correctly when that is selected.
I disconnected C-11 from the volume center lug, and I still get some radio sound leakage (the sound actually goes up a little when I do ).
Since by disconnecting the volume control from the circuit, and still getting some quiet audio, there must be a sneak path for that low level signal to get to the audio circuit.
Here's the schematic...Any ideas?
http://www.nostalgiaair.org/Resources/893/M0014893.htm

Might be a lead dress problem on the detector tube socket or, less likely, an internal structure anomaly in the detector-first audio amp tube... I'm thinking that the tube's diode stage is crosstalking to the triode's grid. Check socket component lead dress and if no help, try subbing in another detector tube.

Thanks for the suggestions CV--I didn't have a spare 6AQ6 detector so I swapped it with the 6AQ6 phase inverter...No change, still leaking volume thru.
I messed the lead dress, and no change.
I went back and lifted the C-11 cap off the volume control o/p lug again, and the volume goes up considerably (when there shouldn't be any sound), loud enough to use in a small room.
Looks like they use a 10meg grid leak(?) bias resistor on the 6AQ6 detector grid..Maybe that set up isn't suppressing the signal enough and letting some thru?

One more thought... was the selenium rectifier replaced with a silicon diode, and no dropping resistor installed to compensate for the lessened forward voltage drop? (If the detector plate voltage is higher than the spec'ed 44V it could cause the tube to behave oddly.)

You also might try jumpering the grid leak with another 10M resistor. That should reduce the gain of the stage. However, it will also change its frequency response.

Hi Brian

Short pin #1 to pin #2 of 6AQ6, Det, AVC, AF tube. See if this stops audio?

Volume control may not go all the way to ground/B-?

AVC may not be operating? Has cap C2 been replaced?

If AVC doesn't operate signal level will be very high and can force it's way through the radio.

http://www.nostalgiaair.org/Resources/893/M0014893.htm

Norm

:This is a combo radio/phono from early '50's. I can't get the radio volume to go down to no sound at all. When I lower the vol. ctl. full counterclockwise I still get a low level radio sound on the radio. The phono sound does go to '0'(quiet) correctly when that is selected.
:I disconnected C-11 from the volume center lug, and I still get some radio sound leakage (the sound actually goes up a little when I do ).
:Since by disconnecting the volume control from the circuit, and still getting some quiet audio, there must be a sneak path for that low level signal to get to the audio circuit.
:Here's the schematic...Any ideas?
:http://www.nostalgiaair.org/Resources/893/M0014893.htm

CV---Original selenium rectifier is still there, voltage in spec. Tried different grid resistor combos, no change.
Norm---I shorted pin 1 (grid) to pin 2 (used as B- ground) on the 6AQ6 detector...could still hear some signal, although very faint. The volume control is grounded to B-.
Replaced C2 in the AVC, no change.

Shorting pin #1 to #2 would like turning volume all the way down. Maybe control doesn't get close enough to zero ohms. Not unusual to hear a little sound with volume down but shouldn't be listenable.

Norm

:CV---Original selenium rectifier is still there, voltage in spec. Tried different grid resistor combos, no change.
:Norm---I shorted pin 1 (grid) to pin 2 (used as B- ground) on the 6AQ6 detector...could still hear some signal, although very faint. The volume control is grounded to B-.
:Replaced C2 in the AVC, no change.

Norm---The volume control works fine playing the phono, no sound thru at all. For the record I'm getting about -3.5vdc at C-2 on the AVC on a decent strong station.
This (radio volume leakage) is just one of those small buggers of a problem that would be nice to figure out and fix. I could see if it was only audible close up to the speaker, but you can clearly hear the radio more than you should.
Maybe a design flaw?

:Norm---The volume control works fine playing the phono, no sound thru at all. For the record I'm getting about -3.5vdc at C-2 on the AVC on a decent strong station.
:This (radio volume leakage) is just one of those small buggers of a problem that would be nice to figure out and fix. I could see if it was only audible close up to the speaker, but you can clearly hear the radio more than you should.
:Maybe a design flaw?

Brian:
The design looks ok to me. This is way out, but why don't you try checking the resistance from the cathode pin of the detector/first audio tube (I am too lazy to look up the schematic again) and the low side of the volume control? Might be a high resistance that is keeping the control from completely grounding the grid of the tube.
Lewis

Grounding the grid of the tube as suggested by Norm should effectively have killed the audio to the output stage(s).

Clifton

::Norm---The volume control works fine playing the phono, no sound thru at all. For the record I'm getting about -3.5vdc at C-2 on the AVC on a decent strong station.
::This (radio volume leakage) is just one of those small buggers of a problem that would be nice to figure out and fix. I could see if it was only audible close up to the speaker, but you can clearly hear the radio more than you should.
::Maybe a design flaw?
:
:Brian:
:The design looks ok to me. This is way out, but why don't you try checking the resistance from the cathode pin of the detector/first audio tube (I am too lazy to look up the schematic again) and the low side of the volume control? Might be a high resistance that is keeping the control from completely grounding the grid of the tube.
:Lewis

Thanks for the suggestions guys, but no change in the problem yet. The two biggest clues here are:
1---When I remove C-11 (from vol. ctl. to 6AQ6 det. grid) from the circuit, the radio signal stays and actually gets louder. So it must be coming in somehow.
2---When in phono mode, the volume works normal, no sound at fully ccw position.
Also...Yes, when in radio mode, and I short out the 6AQ6 det. grid (1) to cathode (2), the sound goes away.
http://www.nostalgiaair.org/Resources/893/M0014893.htm

Without fully understanding the switching, it appears to me that plate voltage to the IF amp tube might be cut off when the mode switch is set to "phono". So that would explain why no radio sound leakage occurs when in that setting.

Getting low on "probable causes". If the set uses phenolic wafer tube sockets, you might have contamination that would lead to interpin signal leakage at the detector/amp tube. This isn't the same thing as carbonization (also a phenolic-socket woe) and is hard to prove short of just replacing the socket. But if you've eliminated all other possibilities...

You're right about the voltage being cut off to the IF amp CV. I noticed that also at one point, but forgot about it (old timers). It gets hard to follow the switching on a 7 position switch, for me anyway.
I'll check into the leakage/socket angle, then that's it for me.

Some tube types which combine diodes with triodes or pentodes in one envelope are susceptible to the phenomenon of "play through", which could be what you are experiencing. For example, the 1S5 detector tube used in several models of Zenith Transoceanic is notorious for being both microphonic and for "playing through". It can be replaced by the 1U5 (as RCA did with their Strato-World clone of the TO) which is neither microphonic nor prone to play-through. Unfortunately, the two tube types have different pinouts so the 1U5 can't be directly subbed for the 1S5 in the TO.

If the tube really is the cause of your "play through" (not saying that it definitely is) you should be able to fix it by disconnecting the diode section and replacing it with a solid-state diode to ground at the IF can terminal. That should eliminate play-through, and would be a lot simpler than replacing the tube socket (which is likely to be disappointing, anyway, if the tube design itself is really at fault here).

Or, you can just live with the low level of sound coming out at the full CCW pot setting.

I suppose the triode grid resistor has been checked to determine it's value is within tolerance. I have experienced in the past of these high value resistors greatly increasing in value.

Clifton

:Some tube types which combine diodes with triodes or pentodes in one envelope are susceptible to the phenomenon of "play through", which could be what you are experiencing. For example, the 1S5 detector tube used in several models of Zenith Transoceanic is notorious for being both microphonic and for "playing through". It can be replaced by the 1U5 (as RCA did with their Strato-World clone of the TO) which is neither microphonic nor prone to play-through. Unfortunately, the two tube types have different pinouts so the 1U5 can't be directly subbed for the 1S5 in the TO.
:
:If the tube really is the cause of your "play through" (not saying that it definitely is) you should be able to fix it by disconnecting the diode section and replacing it with a solid-state diode to ground at the IF can terminal. That should eliminate play-through, and would be a lot simpler than replacing the tube socket (which is likely to be disappointing, anyway, if the tube design itself is really at fault here).
:
:Or, you can just live with the low level of sound coming out at the full CCW pot setting.

Clifton- see 3rd entry from Brian in this thread.

Clifton---I tried the s.s. diode substitute, and it did work. Now I know that it was 6AQ6's diode section that gave the play through problem. I guess earlier when I shorted out pin#1 to pin#2, per Norm, I should have realized what was going on. Anyway...This became more of a learning experience for me just to get to the bottom of the problem (as small, but annoying one), so thanks to everyone who pitched in here!!

Poor shielding inside the tube between the two sections. Now you know why the manufacturer went to the trouble of switching off the IF amp when the set is put in phono mode.