Recently I changed the volume control after the volume was stuck at one level and turning the knob caused no change in volume. It looked like the control was bad as the whole assembly was loose and falling apart.
I changed it out with a correct part and still have that same problem.
My voltages are good at each of the tube plates except for the 6Q7, which should be 175v, but is 293v. (I got the voltages from the schematic.) Also incorrect is the 6F6 (speaker socket) voltage which is 276v and should be 290v.
A resistor, R12 is part of that circuit and measures 120k on my digital meter. The parts list calls it 100M ohms. Does the "M" on the old schematics stand for Megohms? The same list has the smaller resistances, such as R11, (1 megohm) listed as "1 meg ohm". Why the difference in nomenclature?
Before I change that resistor out I am wondering if the Riders Manuals used "M" instead of "K" (kilohms) on these old schematics. The 120kohm reading that I got would be pretty close to tolerance if that were the case. If it is really supposed to be 100 Megohms, would that low a resistance account for my problem?
I dont have alot of experience with these. Thanks for your help.
Yes older schematics used M in place of K for 1000.
Plate of 6Q7 sounds high but it will read more with a modern meter. Older meters loaded down this voltage.
Could be a problem with the 6Q7? Is pin #8 near zero volts?
Volume control is a divider between signal and ground. It needs 3 wires to control volume.
http://www.nostalgiaair.org/PagesByModel/264/M0002264.pdf
Norm
:I replaced the caps about a year ago on this radio and have been using it for about a year with no problems.
:
:Recently I changed the volume control after the volume was stuck at one level and turning the knob caused no change in volume. It looked like the control was bad as the whole assembly was loose and falling apart.
:
:I changed it out with a correct part and still have that same problem.
:
:My voltages are good at each of the tube plates except for the 6Q7, which should be 175v, but is 293v. (I got the voltages from the schematic.) Also incorrect is the 6F6 (speaker socket) voltage which is 276v and should be 290v.
:
:A resistor, R12 is part of that circuit and measures 120k on my digital meter. The parts list calls it 100M ohms. Does the "M" on the old schematics stand for Megohms? The same list has the smaller resistances, such as R11, (1 megohm) listed as "1 meg ohm". Why the difference in nomenclature?
:
:Before I change that resistor out I am wondering if the Riders Manuals used "M" instead of "K" (kilohms) on these old schematics. The 120kohm reading that I got would be pretty close to tolerance if that were the case. If it is really supposed to be 100 Megohms, would that low a resistance account for my problem?
:
:I dont have alot of experience with these. Thanks for your help.
:
Thank you for helping.
:Hi Steve
:
: Yes older schematics used M in place of K for 1000.
:
: Plate of 6Q7 sounds high but it will read more with a modern meter. Older meters loaded down this voltage.
:
: Could be a problem with the 6Q7? Is pin #8 near zero volts?
:
: Volume control is a divider between signal and ground. It needs 3 wires to control volume.
:
:http://www.nostalgiaair.org/PagesByModel/264/M0002264.pdf
:
:Norm
:
:
:
:
::I replaced the caps about a year ago on this radio and have been using it for about a year with no problems.
::
::Recently I changed the volume control after the volume was stuck at one level and turning the knob caused no change in volume. It looked like the control was bad as the whole assembly was loose and falling apart.
::
::I changed it out with a correct part and still have that same problem.
::
::My voltages are good at each of the tube plates except for the 6Q7, which should be 175v, but is 293v. (I got the voltages from the schematic.) Also incorrect is the 6F6 (speaker socket) voltage which is 276v and should be 290v.
::
::A resistor, R12 is part of that circuit and measures 120k on my digital meter. The parts list calls it 100M ohms. Does the "M" on the old schematics stand for Megohms? The same list has the smaller resistances, such as R11, (1 megohm) listed as "1 meg ohm". Why the difference in nomenclature?
::
::Before I change that resistor out I am wondering if the Riders Manuals used "M" instead of "K" (kilohms) on these old schematics. The 120kohm reading that I got would be pretty close to tolerance if that were the case. If it is really supposed to be 100 Megohms, would that low a resistance account for my problem?
::
::I dont have alot of experience with these. Thanks for your help.
::
:
Sir Steve Beeeeeeeeee. . . .
HEYYY . . .you forgotted-ed to mention if the volume was hovering at its low or medium level, but, certainly not going full blast ?
I have taken the schematic of that set of yours and covered all of the audo portion that I think would be anyway involved and enhanced portions.
For sure, the 21st century parts values are now given , so that there will be no disparity in that respect.
Now, let me give you a schematic walk thru of the handling of the audio flow in the set:
We see the secondary of the 2nd IF transfrmer where its top end goes in for diode detection on the 6Q7.
There will be conduction of all of the + swinging nodes of the 455Kc signal while the conduction of negative swinging nodes will be ignored.
So that means that at the oposite side of that winding of the IF transformer REF [A] the - going nodes of the signal will be present there, and the whole diode detector scheme is needing a load resistor for voltage to deee-velop across, and that is fulfilled by R9.
Now this is a continuous collection of 455 1/2 sine wavelets'es-es, whose overall amplitude of the peaks, is varying in compliance with the audio portion that the transmitter sent. So there is now need to get back into that initial audio state, so that the signal can them processed on thru the sets audio portion to get it on up to speaker driving amplitude.
The first step will be to use that C7 100 pf bypass condenser to filter off the 455kc RF to ground and also, the very, very highest audio
frequencies, of which, you wouldn't even detect their presence.
BUTTTT, unless the above was done, the audio amp portion certainly would be working constant duty cycles, like a hound dog, in amping up ALLLLLLL of the MASSIVE numbers of those 455 "signalets", along with the detrimental effect on produced audio.
Now follow the yellow brick road down . . .errr, no, make that green arrow line . . .down thru to the R8 100K and then note that note that C10 100 mmfd down there also selectively extracts yet another portion of the very high audio spectrum and passes it off to ground.
Now in conjunction , we have the initial C7-R8 and C10 which make up what is called a pi section filter network, it effectively leaves us with a signal that is devoid of all of the initial 455 Kc RF signal as well as the high frequency audio spectrum above ~5 KC which is ALL that the station is transmitting of the very highest audio spectrum anyhow.
So that now leaves us with a clean audio signal which is replicating the audo sent by the station and it is then coupled via C11
.O5 capacitor into the high side of your volume control.
Now you are going to have that full available audio signal being across the volume controls high side [B] and its low side [C]
with you then opting for the degree of that signal that you will be wanting, via the volume controls rotor[D].
If you have the VC knob at its max CCW position, no volume, the rotor [D] will be mechanically positioned near terminal [C]
and nothing gets on up to the C12 .01 which is used to couple audio into Mr 6Q7's first grid, YET provide DC isolation for the
negative grid bias which is flowing upwards from R15 2Meg.
The whole volume control acts a variable- selective voltage divider, with the audio signal being the divideeeee. . .as the Kingfish on Amos 'n Andy used to say.
As you rotate the volume control CW the rotor walks on up, ever closer to terminal [B] which has the FULL available audio level present there.
Some where, you will stop, with that desired volume level presence, with it then passing on up thru C12 and into the 1st grid of the 'Q7 to be amplified.
Its amplified signal is developed across R12 plate load resistor and then is DC isolated and coupled over into the 1st grid circuitry of the 6F6 Audio Power Output amplifier tube via the C15 .01 capacitor.
This should cover the potential problem areas of your situation, and most likely it will be right at that volume control.
Now here is a different perspective of your set, and that is the volume control itself. You just mentioned its replacement like it was a breeze. Most others would typically be stymied by that EXTRA connection on the volume control.
That being a loudness / tone compensation tap. It is used in conjunction with the R10-R8-C9 network to enhance perceived bass response when the volume control rotor is positioned anywhere between that tap off point and the minimum volume position.
Now we KNOWS that all was fine with your set after the rework ~1 year ago, right up to the point where your volume control blew a bifurcated lineo-sidereal framostat and done went and "come apart" .
And your then effortless replacement with a like 1 meg control WITH its like tone/loudness compensation tap off.
BUT now and heavy on the NOW, considering that you have the right control in there, what are the possibilities that you have erred in the terminal placement of the {wiring associated with the tone compensation tap} and the {rotor terminal associated wiring}.
An error of that nature THAT would result in a fixed audio output level, with the moving rotor then giving some WIERD tonal coloration to the audio at different rotor positionings.
My final thought was relevant to the development of erratic compressive clamp action within the Candohm / Muter resistor that you see on my lightly red higlighted R18 down at the bottom.The floating of the HV centertap in conjunction with the tap offs of that power resistor is used to acquire -3v,-5 and -22 volts bias levels for the audio tubes along with a counter referece bias for the AVC buss to give it a wider dynamic control range.
I had marked X's on the schema to mark the origin versus the tie in bias injection resistors up above.
Disregard for now , unless we have to come back to it, but also note that Sir Norman's info relevant to the cathode of the 'Q7 would not be at OV but at its designated bias level.
Standing back. . . .and that's waaaaay back . . . .for feed back. . .. (and ALSO seeing that you don't jump back onto my toes, when you gets your fingers into the B+)
73's de Edd
Ye Olde Tyme Belmont . . .audio profiled schema:  |
I am going to review/digest what you have an ddouble check my connections on the volume control to be extra sure thay are right.
The volume is not fixed at max or minimum level, but slightly lower than the midrange as I remember it to be.
I'll get back as soon as I get a chence to carefully go through your and Norms notes.
I glad there are so many informed and helpful people on this forum.
:
:
:
:
:Sir Steve Beeeeeeeeee. . . .
:
:
:
:HEYYY . . .you forgotted-ed to mention if the volume was hovering at its low or medium level, but, certainly not going full blast ?
:
:
:
:I have taken the schematic of that set of yours and covered all of the audo portion that I think would be anyway involved and enhanced portions.
:
:
:For sure, the 21st century parts values are now given , so that there will be no disparity in that respect.
:
:
:
:
:Now, let me give you a schematic walk thru of the handling of the audio flow in the set:
:
:
:
:
:We see the secondary of the 2nd IF transfrmer where its top end goes in for diode detection on the 6Q7.
:
:
:There will be conduction of all of the + swinging nodes of the 455Kc signal while the conduction of negative swinging nodes will be ignored.
:
:
:So that means that at the oposite side of that winding of the IF transformer REF [A] the - going nodes of the signal will be present there, and the whole diode detector scheme is needing a load resistor for voltage to deee-velop across, and that is fulfilled by R9.
:
:
:
:Now this is a continuous collection of 455 1/2 sine wavelets'es-es, whose overall amplitude of the peaks, is varying in compliance with the audio portion that the transmitter sent. So there is now need to get back into that initial audio state, so that the signal can them processed on thru the sets audio portion to get it on up to speaker driving amplitude.
:
:
:The first step will be to use that C7 100 pf bypass condenser to filter off the 455kc RF to ground and also, the very, very highest audio
:frequencies, of which, you wouldn't even detect their presence.
:
:
:BUTTTT, unless the above was done, the audio amp portion certainly would be working constant duty cycles, like a hound dog, in amping up ALLLLLLL of the MASSIVE numbers of those 455 "signalets", along with the detrimental effect on produced audio.
:
:
:Now follow the yellow brick road down . . .errr, no, make that green arrow line . . .down thru to the R8 100K and then note that note that C10 100 mmfd down there also selectively extracts yet another portion of the very high audio spectrum and passes it off to ground.
:
:
:Now in conjunction , we have the initial C7-R8 and C10 which make up what is called a pi section filter network, it effectively leaves us with a signal that is devoid of all of the initial 455 Kc RF signal as well as the high frequency audio spectrum above ~5 KC which is ALL that the station is transmitting of the very highest audio spectrum anyhow.
:
:
:So that now leaves us with a clean audio signal which is replicating the audo sent by the station and it is then coupled via C11
:.O5 capacitor into the high side of your volume control.
:
:
:Now you are going to have that full available audio signal being across the volume controls high side [B] and its low side [C]
:with you then opting for the degree of that signal that you will be wanting, via the volume controls rotor[D].
:
:
:If you have the VC knob at its max CCW position, no volume, the rotor [D] will be mechanically positioned near terminal [C]
:and nothing gets on up to the C12 .01 which is used to couple audio into Mr 6Q7's first grid, YET provide DC isolation for the
:negative grid bias which is flowing upwards from R15 2Meg.
:
:
:The whole volume control acts a variable- selective voltage divider, with the audio signal being the divideeeee. . .as the Kingfish on Amos 'n Andy used to say.
:
:
:As you rotate the volume control CW the rotor walks on up, ever closer to terminal [B] which has the FULL available audio level present there.
:
:
:Some where, you will stop, with that desired volume level presence, with it then passing on up thru C12 and into the 1st grid of the 'Q7 to be amplified.
:
:
:Its amplified signal is developed across R12 plate load resistor and then is DC isolated and coupled over into the 1st grid circuitry of the 6F6 Audio Power Output amplifier tube via the C15 .01 capacitor.
:
:
:This should cover the potential problem areas of your situation, and most likely it will be right at that volume control.
:
:
:Now here is a different perspective of your set, and that is the volume control itself. You just mentioned its replacement like it was a breeze. Most others would typically be stymied by that EXTRA connection on the volume control.
:
:
:That being a loudness / tone compensation tap. It is used in conjunction with the R10-R8-C9 network to enhance perceived bass response when the volume control rotor is positioned anywhere between that tap off point and the minimum volume position.
:
:
:Now we KNOWS that all was fine with your set after the rework ~1 year ago, right up to the point where your volume control blew a bifurcated lineo-sidereal framostat and done went and "come apart" .
:And your then effortless replacement with a like 1 meg control WITH its like tone/loudness compensation tap off.
:
:
:BUT now and heavy on the NOW, considering that you have the right control in there, what are the possibilities that you have erred in the terminal placement of the {wiring associated with the tone compensation tap} and the {rotor terminal associated wiring}.
:
:An error of that nature THAT would result in a fixed audio output level, with the moving rotor then giving some WIERD tonal coloration to the audio at different rotor positionings.
:
:
:
:My final thought was relevant to the development of erratic compressive clamp action within the Candohm / Muter resistor that you see on my lightly red higlighted R18 down at the bottom.The floating of the HV centertap in conjunction with the tap offs of that power resistor is used to acquire -3v,-5 and -22 volts bias levels for the audio tubes along with a counter referece bias for the AVC buss to give it a wider dynamic control range.
:
:
:I had marked X's on the schema to mark the origin versus the tie in bias injection resistors up above.
:
:
:Disregard for now , unless we have to come back to it, but also note that Sir Norman's info relevant to the cathode of the 'Q7 would not be at OV but at its designated bias level.
:
:
:
:
:
:Standing back. . . .and that's waaaaay back . . . .for feed back. . .. (and ALSO seeing that you don't jump back onto my toes, when you gets your fingers into the B+)
:
:
:
:
:
:
:73's de Edd
:
:
:
:
:
:
:
:
:
:
:Ye Olde Tyme Belmont . . .audio profiled schema: : : : : : : : : : |
:
:
:
:
This was after I checked all connections again and again, reviewed your comments and changed C8 with a new ceramic cap. Then I remembered the 6Q7 glass tube, which worked.
I dont have a tube tester and forgot about the spare. I am going to pick up a tester at the next swap meet if I can find one.
In my brief experience, the tubes are usually very durable so I didnt think of it. Guess not this time.
Steve
:Thanks Edd.
:
:I am going to review/digest what you have an ddouble check my connections on the volume control to be extra sure thay are right.
:
:The volume is not fixed at max or minimum level, but slightly lower than the midrange as I remember it to be.
:
:I'll get back as soon as I get a chence to carefully go through your and Norms notes.
:
:I glad there are so many informed and helpful people on this forum.
:
:
::
::
::
::
::Sir Steve Beeeeeeeeee. . . .
::
::
::
::HEYYY . . .you forgotted-ed to mention if the volume was hovering at its low or medium level, but, certainly not going full blast ?
::
::
::
::I have taken the schematic of that set of yours and covered all of the audo portion that I think would be anyway involved and enhanced portions.
::
::
::For sure, the 21st century parts values are now given , so that there will be no disparity in that respect.
::
::
::
::
::Now, let me give you a schematic walk thru of the handling of the audio flow in the set:
::
::
::
::
::We see the secondary of the 2nd IF transfrmer where its top end goes in for diode detection on the 6Q7.
::
::
::There will be conduction of all of the + swinging nodes of the 455Kc signal while the conduction of negative swinging nodes will be ignored.
::
::
::So that means that at the oposite side of that winding of the IF transformer REF [A] the - going nodes of the signal will be present there, and the whole diode detector scheme is needing a load resistor for voltage to deee-velop across, and that is fulfilled by R9.
::
::
::
::Now this is a continuous collection of 455 1/2 sine wavelets'es-es, whose overall amplitude of the peaks, is varying in compliance with the audio portion that the transmitter sent. So there is now need to get back into that initial audio state, so that the signal can them processed on thru the sets audio portion to get it on up to speaker driving amplitude.
::
::
::The first step will be to use that C7 100 pf bypass condenser to filter off the 455kc RF to ground and also, the very, very highest audio
::frequencies, of which, you wouldn't even detect their presence.
::
::
::BUTTTT, unless the above was done, the audio amp portion certainly would be working constant duty cycles, like a hound dog, in amping up ALLLLLLL of the MASSIVE numbers of those 455 "signalets", along with the detrimental effect on produced audio.
::
::
::Now follow the yellow brick road down . . .errr, no, make that green arrow line . . .down thru to the R8 100K and then note that note that C10 100 mmfd down there also selectively extracts yet another portion of the very high audio spectrum and passes it off to ground.
::
::
::Now in conjunction , we have the initial C7-R8 and C10 which make up what is called a pi section filter network, it effectively leaves us with a signal that is devoid of all of the initial 455 Kc RF signal as well as the high frequency audio spectrum above ~5 KC which is ALL that the station is transmitting of the very highest audio spectrum anyhow.
::
::
::So that now leaves us with a clean audio signal which is replicating the audo sent by the station and it is then coupled via C11
::.O5 capacitor into the high side of your volume control.
::
::
::Now you are going to have that full available audio signal being across the volume controls high side [B] and its low side [C]
::with you then opting for the degree of that signal that you will be wanting, via the volume controls rotor[D].
::
::
::If you have the VC knob at its max CCW position, no volume, the rotor [D] will be mechanically positioned near terminal [C]
::and nothing gets on up to the C12 .01 which is used to couple audio into Mr 6Q7's first grid, YET provide DC isolation for the
::negative grid bias which is flowing upwards from R15 2Meg.
::
::
::The whole volume control acts a variable- selective voltage divider, with the audio signal being the divideeeee. . .as the Kingfish on Amos 'n Andy used to say.
::
::
::As you rotate the volume control CW the rotor walks on up, ever closer to terminal [B] which has the FULL available audio level present there.
::
::
::Some where, you will stop, with that desired volume level presence, with it then passing on up thru C12 and into the 1st grid of the 'Q7 to be amplified.
::
::
::Its amplified signal is developed across R12 plate load resistor and then is DC isolated and coupled over into the 1st grid circuitry of the 6F6 Audio Power Output amplifier tube via the C15 .01 capacitor.
::
::
::This should cover the potential problem areas of your situation, and most likely it will be right at that volume control.
::
::
::Now here is a different perspective of your set, and that is the volume control itself. You just mentioned its replacement like it was a breeze. Most others would typically be stymied by that EXTRA connection on the volume control.
::
::
::That being a loudness / tone compensation tap. It is used in conjunction with the R10-R8-C9 network to enhance perceived bass response when the volume control rotor is positioned anywhere between that tap off point and the minimum volume position.
::
::
::Now we KNOWS that all was fine with your set after the rework ~1 year ago, right up to the point where your volume control blew a bifurcated lineo-sidereal framostat and done went and "come apart" .
::And your then effortless replacement with a like 1 meg control WITH its like tone/loudness compensation tap off.
::
::
::BUT now and heavy on the NOW, considering that you have the right control in there, what are the possibilities that you have erred in the terminal placement of the {wiring associated with the tone compensation tap} and the {rotor terminal associated wiring}.
::
::An error of that nature THAT would result in a fixed audio output level, with the moving rotor then giving some WIERD tonal coloration to the audio at different rotor positionings.
::
::
::
::My final thought was relevant to the development of erratic compressive clamp action within the Candohm / Muter resistor that you see on my lightly red higlighted R18 down at the bottom.The floating of the HV centertap in conjunction with the tap offs of that power resistor is used to acquire -3v,-5 and -22 volts bias levels for the audio tubes along with a counter referece bias for the AVC buss to give it a wider dynamic control range.
::
::
::I had marked X's on the schema to mark the origin versus the tie in bias injection resistors up above.
::
::
::Disregard for now , unless we have to come back to it, but also note that Sir Norman's info relevant to the cathode of the 'Q7 would not be at OV but at its designated bias level.
::
::
::
::
::
::Standing back. . . .and that's waaaaay back . . . .for feed back. . .. (and ALSO seeing that you don't jump back onto my toes, when you gets your fingers into the B+)
::
::
::
::
::
::
::73's de Edd
::
::
::
::
::
::
::
::
::
::
::Ye Olde Tyme Belmont . . .audio profiled schema: :: :: :: :: :: :: :: :: :: |
::
::
::
::
:
Sir Steve Beeeeeeeeee. . . .
HEYYY . . .I had to look back schematically to even see WHICH cap C8 was. And with it shunting the volume control in its tone filter utilization it never even gets subjected to more than a few volts.
But, old paper capacitor types can just go sour on their own. Your selection choice of a ceramic was excellent, I find them to be
amongst the most trouble free capacitors. The reliability of late 50-60 vintage Zeniths, which heavily utilized them , help attest for their degree of reliability.
Your mention of the tube fault reminds me of times when I actually have seen cathode coating material flake off and then fall down and flash across internal tube elements, when it landed down on the lower mica disc framework supporter. Who knows if that there is not, sometimes, a "spot welding" action between elements with that arcing action. Or at least, a contaminated trail across the mica.
You might do a pin by pin voltage test on the working tube and then sub in the old unit and see if it is causing a variance in readings when it is in circuit, mainly considering the 1st grid and cathode voltages.
Additionally, since mica is a natural occuring product, the additional possiblity of contaminants being between the individual layers that forms up a thicker sheet. They could carbonify and make a conductive path with time.
Additionally a spot weld of a tube connection could open up or be intermittent.
I usually can seek those out with subjective mechanical testing with my Ruby Red eraser tipped pencil used as a "soft sledge hammer"
73's de Edd
:
:
:
:
:Sir Steve Beeeeeeeeee. . . .
:
:
:
:HEYYY . . .you forgotted-ed to mention if the volume was hovering at its low or medium level, but, certainly not going full blast ?
:
:
:
:I have taken the schematic of that set of yours and covered all of the audo portion that I think would be anyway involved and enhanced portions.
:
:
:For sure, the 21st century parts values are now given , so that there will be no disparity in that respect.
:
:
:
:
:Now, let me give you a schematic walk thru of the handling of the audio flow in the set:
:
:
:
:
:We see the secondary of the 2nd IF transfrmer where its top end goes in for diode detection on the 6Q7.
:
:
:There will be conduction of all of the + swinging nodes of the 455Kc signal while the conduction of negative swinging nodes will be ignored.
:
:
:So that means that at the oposite side of that winding of the IF transformer REF [A] the - going nodes of the signal will be present there, and the whole diode detector scheme is needing a load resistor for voltage to deee-velop across, and that is fulfilled by R9.
:
:
:
:Now this is a continuous collection of 455 1/2 sine wavelets'es-es, whose overall amplitude of the peaks, is varying in compliance with the audio portion that the transmitter sent. So there is now need to get back into that initial audio state, so that the signal can them processed on thru the sets audio portion to get it on up to speaker driving amplitude.
:
:
:The first step will be to use that C7 100 pf bypass condenser to filter off the 455kc RF to ground and also, the very, very highest audio
:frequencies, of which, you wouldn't even detect their presence.
:
:
:BUTTTT, unless the above was done, the audio amp portion certainly would be working constant duty cycles, like a hound dog, in amping up ALLLLLLL of the MASSIVE numbers of those 455 "signalets", along with the detrimental effect on produced audio.
:
:
:Now follow the yellow brick road down . . .errr, no, make that green arrow line . . .down thru to the R8 100K and then note that note that C10 100 mmfd down there also selectively extracts yet another portion of the very high audio spectrum and passes it off to ground.
:
:
:Now in conjunction , we have the initial C7-R8 and C10 which make up what is called a pi section filter network, it effectively leaves us with a signal that is devoid of all of the initial 455 Kc RF signal as well as the high frequency audio spectrum above ~5 KC which is ALL that the station is transmitting of the very highest audio spectrum anyhow.
:
:
:So that now leaves us with a clean audio signal which is replicating the audo sent by the station and it is then coupled via C11
:.O5 capacitor into the high side of your volume control.
:
:
:Now you are going to have that full available audio signal being across the volume controls high side [B] and its low side [C]
:with you then opting for the degree of that signal that you will be wanting, via the volume controls rotor[D].
:
:
:If you have the VC knob at its max CCW position, no volume, the rotor [D] will be mechanically positioned near terminal [C]
:and nothing gets on up to the C12 .01 which is used to couple audio into Mr 6Q7's first grid, YET provide DC isolation for the
:negative grid bias which is flowing upwards from R15 2Meg.
:
:
:The whole volume control acts a variable- selective voltage divider, with the audio signal being the divideeeee. . .as the Kingfish on Amos 'n Andy used to say.
:
:
:As you rotate the volume control CW the rotor walks on up, ever closer to terminal [B] which has the FULL available audio level present there.
:
:
:Some where, you will stop, with that desired volume level presence, with it then passing on up thru C12 and into the 1st grid of the 'Q7 to be amplified.
:
:
:Its amplified signal is developed across R12 plate load resistor and then is DC isolated and coupled over into the 1st grid circuitry of the 6F6 Audio Power Output amplifier tube via the C15 .01 capacitor.
:
:
:This should cover the potential problem areas of your situation, and most likely it will be right at that volume control.
:
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:Now here is a different perspective of your set, and that is the volume control itself. You just mentioned its replacement like it was a breeze. Most others would typically be stymied by that EXTRA connection on the volume control.
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:That being a loudness / tone compensation tap. It is used in conjunction with the R10-R8-C9 network to enhance perceived bass response when the volume control rotor is positioned anywhere between that tap off point and the minimum volume position.
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:Now we KNOWS that all was fine with your set after the rework ~1 year ago, right up to the point where your volume control blew a bifurcated lineo-sidereal framostat and done went and "come apart" .
:And your then effortless replacement with a like 1 meg control WITH its like tone/loudness compensation tap off.
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:BUT now and heavy on the NOW, considering that you have the right control in there, what are the possibilities that you have erred in the terminal placement of the {wiring associated with the tone compensation tap} and the {rotor terminal associated wiring}.
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:An error of that nature THAT would result in a fixed audio output level, with the moving rotor then giving some WIERD tonal coloration to the audio at different rotor positionings.
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:My final thought was relevant to the development of erratic compressive clamp action within the Candohm / Muter resistor that you see on my lightly red higlighted R18 down at the bottom.The floating of the HV centertap in conjunction with the tap offs of that power resistor is used to acquire -3v,-5 and -22 volts bias levels for the audio tubes along with a counter referece bias for the AVC buss to give it a wider dynamic control range.
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:I had marked X's on the schema to mark the origin versus the tie in bias injection resistors up above.
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:Disregard for now , unless we have to come back to it, but also note that Sir Norman's info relevant to the cathode of the 'Q7 would not be at OV but at its designated bias level.
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:Standing back. . . .and that's waaaaay back . . . .for feed back. . .. (and ALSO seeing that you don't jump back onto my toes, when you gets your fingers into the B+)
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:73's de Edd
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:Ye Olde Tyme Belmont . . .audio profiled schema: : : : : : : : : : |
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