[Chas]

Has the area of the noise been identified as to being before or after the volume control. Determining that helps...

If the suspect area is the RF/Osc/IF and to eliminate the AVC, clamp the AVC at a given negative voltage with a battery or make a simple AVC voltage "generator" from a 9 volt battery and a linear potentiometer, 50-100k, across it. Attach positive to Ground (of which appears to be common with AC line) and the variable negative from wiper to AVC . Operate radio and set for comfortable volume level as well as AVC (which will control volume too). If radio continues to go up/down in volume it is the AF side, suspect leakage in AF transformer to frame or transformer. (That would exhibit a warm-up failure characteristic). If the radio is stable in volume disregarding aether fading, do test in daytime to be sure, then problem is in "possibly" the AVC, new defective bypass caps or defective resistors or leakage in the AVC circuits to ground. Note available AVC current is very small in order of pico amps so any meter except old school EICO VTVM which has a 25 meg input impedance cannot measure the AVC voltage correctly. That said, any leakage will effect volume.

To further confirm AVC circuits, turn off radio and unplug. Measure AVC for resistance to ground using a 20k ohms/volt meter AND lift R5 from the volume control. BTW a defective volume control can cause the problem, usually because it has been doused with incompatible cleaner. The resistance reading should be infinity if all is well. Any leakage is a problem and can be found by lifting branches of the AVC...

To make a socket cleaner use any dud tube with all the required pins, take very coarse sandpaper to pins like 80 grit, thoroughly roughen the pins all around, all pins. Insert this tube "tool" in/out of the socket repeatedly, some 25 times. The pins are a match for the contacting socket surfaces so any (soft) oxides will be abraded safely away without removing excessive base metal.

I would choose a metal tube, as a glass tube may loosen in the base or break in the hands.

Last blurbs:

Any connections via a rivet to the chassis are suspect! Use a large soldering bolt (much copper) and burnish connection at rivet (wire brush) and any terminal device involved, use flux and warm chassis with heat gun if need be and solder. Note: Absolute NO-NO! Don't do this if you are timid! Use a tiny dab of plumbers liquid acid flux with a cotton swab if there is cadmium or zinc corrosion present, rosin flux will not cut that oxide. The flux will puff-off when heat is correct and solder will flow like water into the joint. An alternate is to drill out the rivet and use hardware to make a sound connection. Real zinc chloride/hydrochloric acid liquid Plumbers flux is hard to find in some states. Do NOT use any grease/oil based version of this flux as that cannot be removed nor burns away and WILL corrode...

There is my experience and other ARF members of re-soldering chassis mechanical/electrical connections. These defective joints do NOT respond to DC measurement with an ohmmeter, so, are often ignored as a potential problem. Reminder: It will take a lot of heat to properly solder a chassis connection such that it does not look like "chicken poop", Spot warming the chassis to 120 F helps, a lot...

Do not ignore mechanical connections like the frame of the tuner to chassis...

GL

Chas

[RadioSvit]

Peter, Greg said he replaced the 12SQ7 three times by NOS tubes.

Oops. Sorry. Then there are already listed resistors and capacitors. I would observe the change in AVC voltage before and during silence.

[NostalgiaRadioTime]

That's excellent advice, Chas. I never thought of using an old tube as you suggested to clean the socket but I can see how that would work with the tube pins roughed up with the sandpaper. I'm suspecting it could possibly be the volume control. It's just very strange how it will play 30 minutes to an hour and then start acting up without even touching it. But perhaps the DeOxit I grabbed and squirted into it did it in. I grabbed the red can instead of the green. Is it just residual DeOxit that could be affecting the pot and if so can it be flushed out with a more standard contact cleaner that is fast drying/evaporating or is it just plain shot?

[Chas]

Is it just residual DeOxit that could be affecting the pot and if so can it be flushed out with a more standard contact cleaner that is fast drying/evaporating or is it just plain shot?

You could give it shot  

If it works then save all that (cumbersome) troubleshooting...

I am guessing the speaker frame has the output transformer... If the set was in a damp environment, transformer bobbin covered with fuzzy from mold there is a good chance it may be leaking from primary to the core.

If the speaker can be electrically/mechanically lifted from the chassis try that and see if there is B+ present. DO suppose there is a leak, as miss-handling the speaker with a leaking transformer can give a very unpleasant shock and possible muscle reflex damage to the cone.

GL

Chas

[MrFixr55]

Intermittents- the nightmare f any mechanic, technician, engineer, etc.!

One interesting observation, you have 2 of these and when you replaced the e-cap and 2 wax caps the same issue occurs.  Did the other radio play correctly until the caps were replaced?

If you have an o-scope or a DVM that can read AC ripple, measure the junction of C10 and R4  Does the ripple increase as the plate voltage drop?  Does the same thing happen on the other side of R4 (the side that connects to R3 and the 2nd Lytic?  Either the caps are funky (like a bad batch) or something is loading it down.  

Another interesting point is that the 36V supply is not just to the RF amp plate and screen, it is to the IF amp screen.

If the other radio played well till you changed its lytic, reinstall the originals one at t time (if you have them).  Whatever fixes it, do to the first radio.

One other trick would be to remove the 12SK7 and bridge the heater connections with an 80 Ohm 5W resistor, then connect a wire antenna to the stator of the convertor section of the tuning cap, essentially making it a 5 tube radio. If the issue still exists, the RF Amp is not the issue.

But If I read all the posts correctly and you had another working radio that exhibited the same issues after you replaced the caps, then Occam's razor indicates that a batch of bad caps may be the issue.

Finally, are the caps connected to the exact same points (not schematic-wise but physically. Is the common negative of the 2 caps connected to the exact same point on the negative bus?


Good luck!!

[NostalgiaRadioTime]

I think Chas stumbled on to something I hadn't even thought about, because I use the stuff so often and never encountered this before. I think this could possibly be DeOxit residue in the volume control and/or the tube sockets. It's a good cleaner, but does not evaporate and takes a long time to dry. That may explain why the set is now playing fine at longer intervals before it starts acting up. The DeOxit may be starting to dry in the control, but still slowly seeping into places where it's causing problems. I took some fast drying WD-40 Specialist contact cleaner and flushed it out, spraying it into the control and letting it run out, then following with a gentle blast of compressed air (not from an air compressor but from one of those aerosol cans...making sure it was just air and not a blast of liquid these things sometimes surprise you with). I did the same thing with the tube sockets. I'm going to let it dry overnight and see if I have any improvement.

Unfortunately, I did the same thing to the second chassis I have, and I suspect that the DeOxit is causing problems there too. So I did the same flushing procedure to this chassis as well. I didn't try the set prior to changing out the e-caps because there was some dark residue all over the bottom of the chassis around the capacitor can connections, seeming to indicate there was some smoke or possibly even a flame going on there! It didn't look safe so I changed them out. But those, and the leaky .1 wax cap (C10) with the bulged ends is all I changed. I was careful in installing them to make sure I had solid connections. I even took a magnifying glass to look over each solder joint! But yes the negative of the two electrolytics are connected to the same common chassis ground point.

I appreciate greatly all of the suggestions and will definitely explore them further if the flushing doesn't clear up the problem. If it does turn out to be DeOxit residue causing all of this grief, well I just learned a valuable lesson to stick with the stuff that evaporates whenever possible. At least use that first and if the controls are still dirty, then use DeOxit but make I give it plenty enough time to dry!

You are so right...intermittents ARE a nightmare! I'll let you know what I discover tomorrow.

[morzh]

Greg

It should be different Deoxit in sockets and the pot, and they behave differently. Hope the D5 was not put in the pot, and F5 is useless in the sockets. If it is the proper one, I never had problems with F5 inside the pot, except where, in the beginning of me learning about this hobby, I mixed it with some Lithium grease. That did create a problem at least with one of them, though not the pot. It oozed to the power switch, and now that one is intermittent.
But F5 inside the pot eventually will dry up and even when wet, it does not do much bad, even if used soon after the application.

[NostalgiaRadioTime]

Unfortunately, Mike, I believe it was the D5 that I squirted into the pot. I had both cans sitting side by side, and was going to use that in the sockets and the F5 in the pot, but grabbed the wrong one by mistake. Easy enough to do but it could have been the cause of the volume fluctuations I've been experiencing.

I haven't had a chance to see if the flushing with the WD-40 fast drying contact cleaner did any good but will get down to the workshop later today.

[morzh]

My #2 bullet was "volume pot" in my first post (#10) here. A likely culprit, no matter what caused it.
But you should be able to verify this by rotating it or bypassing it altogether by a cap to the max volume, and subbed the rheostat itself by a regular resistor to load the detector, or whatever there is. If the fluctuation persists, it is not the pot.

[NostalgiaRadioTime]

Well after trying it for over 90 minutes this afternoon, it performed flawlessly. No hint of volume fluctuations, or crackling or popping. Nothing. I tried it at several different volume levels and, again, it didn't act up one bit.

I think the flushing I did last night with the WD-40 fast drying contact cleaner must've gotten the residual D5 out of the pot. I had tilted the chassis as I sprayed it inside so it could run out as I sprayed it in. Then used the compressed air to dry it and left it overnight. 

I will try it again several times but I think the pot is OK. 

So I learned something new that I hadn't previously encountered. And hopefully my experience will help others to be mindful of which DeOxit you use on your volume pots, and that if you inadvertently spray the wrong type inside, to consider that when troubleshooting intermittent audio problems as a possible culprit. 

Sometimes a mistake can be a helpful learning experience.... albeit a frustrating one in this case!