I just recapped a 97T, it works very well except that I am getting a deep hum when the tone control is in either extreme position. The hum is getting worse over the past few days and I have to adjust the tone to eliminate the hum by putting it somewhere in the center range. This position changes with the station tuned in.

Is this normal? Where should I start troubleshooting, if not the capacitors in the tone control circuit? Just wondering if there is another obvious culprit before I start changing parts that may be functional. I don't have a lot of test equipment, just a DVM.

Thanks for your help.

Steve

The clue you gave " when tuning in stations " Leads me to think it could be bad ground points. Check for weak grounds around the 6H6 detector tube. Could also be a gassy or shorting 6F5, or 6F6 tube. Sub those out if you have another. When in the phono position, does it still act the same way with a signal applied?

Steve

Try adding a .01 mf @ 600 volt cap across the AC line and AC line to chassis.

I missed "when tuning in stations". Sometimes the AC line causes modulation of the signal especially on strong stations.

Norm

:The clue you gave " when tuning in stations " Leads me to think it could be bad ground points. Check for weak grounds around the 6H6 detector tube. Could also be a gassy or shorting 6F5, or 6F6 tube. Sub those out if you have another. When in the phono position, does it still act the same way with a signal applied?
:

Thanks for your advice.

Unfortunately it has gotten worse. I was using the radio today when i heard a crackling noise. The transformer was very hot to the touch. I shut it down and tested the transformer leads with the rectifier out. The trans seems to have an open on the secondary side where it is supposed to be at 5v for the rectifier filament on pins 8 and 2 of the 5W4.

When I plug it back in, the rectifier does light up on top when I look at the glass tube. Perhaps this is the plate glowing? Could the filaments not be getting power on the rectifier and the tube still glow?

The transformer is not hot anymore when plugged in. The other tubes glow as well and there is no sound. I did try the rectifier tube in another radio and it works fine. I need to do more troubleshooting. The other trans windings show continuity and close to correct ohms.

I am not sure where to start troubleshooting. Either way I may need a transformer or a repair. Does anyone know a source for one of these in case?

I will not get to do more testing until the weekend.
Any advice is appreciated on how to start, I am still very new to this.

Thanks,
Steve

:Steve
:
: Try adding a .01 mf @ 600 volt cap across the AC line and AC line to chassis.
:
: I missed "when tuning in stations". Sometimes the AC line causes modulation of the signal especially on strong stations.
:
:Norm
:
::The clue you gave " when tuning in stations " Leads me to think it could be bad ground points. Check for weak grounds around the 6H6 detector tube. Could also be a gassy or shorting 6F5, or 6F6 tube. Sub those out if you have another. When in the phono position, does it still act the same way with a signal applied?
::
:

IF you are seeing a glow from the rectifier tube, that must be the filament still working. Check for B+ at the positive side of the filter capacitors, then on up to the plate of the audio output tube. I would think you have a wide open somewhere, This could be from a short in like the output tube, audio output transformer, field coil. Or even still could be the rectifier tube after it gets hot.

Hi

Something is drawing too much current on the B+ line. Were electrolytic filter caps replaced? They can become leaky and short. That would also account for hum.

Are any tubes gassy? Do you see blue glow.

Don't operate the radio very long drawing too much current. This will damage the power transformer.

Filament winding on your transformer is ok. It uses heavy wire and they seldom open.

Norm

:IF you are seeing a glow from the rectifier tube, that must be the filament still working. Check for B+ at the positive side of the filter capacitors, then on up to the plate of the audio output tube. I would think you have a wide open somewhere, This could be from a short in like the output tube, audio output transformer, field coil. Or even still could be the rectifier tube after it gets hot.
:

:Hi
:
: Something is drawing too much current on the B+ line. Were electrolytic filter caps replaced? They can become leaky and short. That would also account for hum.
:
: Are any tubes gassy? Do you see blue glow.
:
: Don't operate the radio very long drawing too much current. This will damage the power transformer.
:
: Filament winding on your transformer is ok. It uses heavy wire and they seldom open.
:
:Norm
:
:
:
::IF you are seeing a glow from the rectifier tube, that must be the filament still working. Check for B+ at the positive side of the filter capacitors, then on up to the plate of the audio output tube. I would think you have a wide open somewhere, This could be from a short in like the output tube, audio output transformer, field coil. Or even still could be the rectifier tube after it gets hot.
::
:

If a filter capacitor shorts out, it usually stays shorted. You say it plays good for about 10 minutes, then it goes bad. That sure sounds like the rectifier shorting after it gets good and hot. Sames goes for the 6F6 audio output tube, those run real hot anyway. I would sub out the rectifier first, try it again. If that's not it, then the audio output tube. Still could be an outside chance it's the audio output transformer shorting to ground, or the worst the power transformer shorting internally after it reaches a certain temperature.

If the transformer doesn't get hot with the 5W4 pulled it should still be good.

Leaky filter caps can cause the transformer to heat in time. Usually the filter caps will also feel warm.

If pin #5 isn't 15 volts or so negative in relation to pin #8 on the 6F6 the tube will draw too much current and cause transformer overheating.

As Warren mentions try the rectifier and audio output tubes. These can cause the problem are are easy to check.

Norm

:If a filter capacitor shorts out, it usually stays shorted. You say it plays good for about 10 minutes, then it goes bad. That sure sounds like the rectifier shorting after it gets good and hot. Sames goes for the 6F6 audio output tube, those run real hot anyway. I would sub out the rectifier first, try it again. If that's not it, then the audio output tube. Still could be an outside chance it's the audio output transformer shorting to ground, or the worst the power transformer shorting internally after it reaches a certain temperature.
:

:If the transformer doesn't get hot with the 5W4 pulled it should still be good.
:
:Leaky filter caps can cause the transformer to heat in time. Usually the filter caps will also feel warm.
:
:If pin #5 isn't 15 volts or so negative in relation to pin #8 on the 6F6 the tube will draw too much current and cause transformer overheating.
:
:As Warren mentions try the rectifier and audio output tubes. These can cause the problem are are easy to check.
:
:Norm
:
::If a filter capacitor shorts out, it usually stays shorted. You say it plays good for about 10 minutes, then it goes bad. That sure sounds like the rectifier shorting after it gets good and hot. Sames goes for the 6F6 audio output tube, those run real hot anyway. I would sub out the rectifier first, try it again. If that's not it, then the audio output tube. Still could be an outside chance it's the audio output transformer shorting to ground, or the worst the power transformer shorting internally after it reaches a certain temperature.
::
:

::If the transformer doesn't get hot with the 5W4 pulled it should still be good.
::
::Leaky filter caps can cause the transformer to heat in time. Usually the filter caps will also feel warm.
::
::If pin #5 isn't 15 volts or so negative in relation to pin #8 on the 6F6 the tube will draw too much current and cause transformer overheating.
::
::As Warren mentions try the rectifier and audio output tubes. These can cause the problem are are easy to check.
::
::Norm
::
:::If a filter capacitor shorts out, it usually stays shorted. You say it plays good for about 10 minutes, then it goes bad. That sure sounds like the rectifier shorting after it gets good and hot. Sames goes for the 6F6 audio output tube, those run real hot anyway. I would sub out the rectifier first, try it again. If that's not it, then the audio output tube. Still could be an outside chance it's the audio output transformer shorting to ground, or the worst the power transformer shorting internally after it reaches a certain temperature.
:::
::
:
:I pulled the 5W4 this morning and turned the radio on. I monitored the temperature on the transformer by touching it every minute. It starts cool and slowly gets hot. In about 10 minutes, it got too hot to touch, but I shut it down before it started to "crackle" so I don't break anything.
:
:Should I try the same test with the 6f6 pulled? I notice that it never gets hot, unlike another radio I have where the 6f6 normally does get very hot. (This was with the 5w4 pulled).
:
:The filter caps are new, but I understand that there still could be a bad one. I think I will change them out this weekend if nothing else works.
:

With the rectifier pulled out, there is no B+ at all. The filter capacitors have no voltage on them as the power supply is dead. Sorry to say, but that's a good clue your Power Transformer has a short. At this point I would disconnect all of of the secondary windings. Make sure the wires are taped at the ends. Plug in the radio one last time. If the transformer gets hot now, it's got a short.

:With the rectifier pulled out, there is no B+ at all. The filter capacitors have no voltage on them as the power supply is dead. Sorry to say, but that's a good clue your Power Transformer has a short. At this point I would disconnect all of of the secondary windings. Make sure the wires are taped at the ends. Plug in the radio one last time. If the transformer gets hot now, it's got a short.
:
If this is the case, you can perform a post mortem on the transformer, and if you are really lucky, you might find an arc path from a connection to the bell. I've seen cases where all that was needed was to replace the lead coming out and re-insulate the connection and re-assemble. Good Luck!