Classic Hifi Care

While the general physical condition of this little radio chassis is quite good, no rust or corrosion, and not all that dirty, the general condition of the electronics leaves a lot to be desired. The wafer tube sockets fall apart when I try to remove them. I had to drill out the rivets that hold them in, of course, but any twisting or prying at all on those sockets would result in them simply disintegrating right before my eyes. I'm not at all sure yet at this point whether any of this will effect a successful repair, but it sure can't hurt anything. The rubber insulation on the wiring is dried up and falling off, so that might be a part of it. I wanted to do this a socket at a time then test, but anytime you do anything to one of those sockets in the way of repair or soldering, you have to fix something else. I hope to be able to get the two sockets I have replaced wired correctly, then test.

A 4-tube radio was probably not expected to be in service 78 years later, and being one tube short of an AA5 means it was a budget item already, so it's not surprising that they didn't spec ceramic tube sockets or even Bakelite (or whatever that black plastic is). We're just lucky that they used discrete, modular components so we can replace them one by one as they give up. Wooden boats are the same way. You can replace the parts that fail. Some classic-yacht "restorations" are really new boats built to the measurements of the original, often with a few durable parts of the original incorporated. I've never quite figured out where a job moves from restoration to replica.

Chris Campbell

I still have 90 VDC on pin 6.

I did manage to lose the hum/buzz/noise in the audio, however. I do believe there was AC getting into the audio. The wafer tube sockets I replaced were "crispy critters". Not very technical, to be sure, but apt in description. The good news on the 90VDC on pin 6 is that now it is steady, doesn't go away when I wiggle the tube. No question that replacing the tube sockets in this set is a good idea.

I am beginning to wonder if my service information pertaining to voltage values has a mistake. I am going to remove a chassis from a known-good-working AA5 to see if there is a terminal on the oscillator coil that has high voltage on it, if the 90 VDC is correct. The schematic says 0 VDC is spec. Could this be a mistake? Oscillators are not my strong suite and I am struggling mightily with this one. I'm sure the original oscillator coil in the radio is bad. I don't want to replace it, then burn up the replacement. I need to verify the correct voltage for pin 6. If anybody has any suggestions on this, I am all ears and would be much obliged for any help with this.

I think this may be a hot little set if I use a good antenna with it. In fringe areas, maybe not so much..

I have put this chassis back into its repainted cabinet, pending a tad more research. All this to repair a 4-tube radio...

I still have 90 volts on the cathode of the 12SA7, even with the tube pin disconnected. How this is happening is the challenge. The cathode, pin 6, is connected to the primary of the oscillator coil which goes to common negative. I don't see any path for high voltage to get on this tube pin. There was some discussion about the 12SA7 and the 12SA7GT, which are wired slightly different. The diagram specifically states that the metal 12SA7 tube is the correct one. I need to find a correct oscillator coil, so there is that. None of the texts I own say anything about high voltage on the cathode. I need a break from this project....

I don't have the pin-out at hand, but if you have voltage on a tube pin with the pin disconnected, then the voltage is coming from the tube, right? Either an internal connection from another pin or a short from another element.

Chris Campbell

I subbed a 12SA7 from another radio with no change. The 12SA7 in this radio works fine in another radio. I have changed the tube socket with no change. I have replaced most of the wiring in the chassis, no change. I have changed every resistor, every capacitor except micas, no change. I changed the speaker, power cord, output transformer, no change. How can a tube element get voltage on it when it's not connected to anything? Voltages in the chassis are spot on except in the converter tube, of course, since there is a known dead oscillator coil. Resistances are good except on pin 6 of the 12SA7. This thing is going to be a shelf queen for awhile, until I figure out what is going on.

Another thing. Just in case I was reading the voltage on pin 6 wrong, when I connect the wire from pin 6 to the oscillator coil where it goes, I get a buzz in the audio, telling me there is indeed voltage on pin 6 and that the set is not happy with that wire connected with 90 volts on it.

Chris, the schematic is posted on page 2 of this topic....

You will get it sorted. It took me forever to get the Silvertone record player sorted. For me, after having all of the caps/resistors replaced that needed replacing, it would cut in and out. Finally, and by shear accident, I discovered it was the tube sockets. After replacing all of them it has not missed a beat since. Like some of the others have said, give it a rest and come back to it another day with a fresh mind.

Bill

The tube specs give a maximum heater-cathode voltage. Any possibility that's being exceeded in your radio, and not in the other ones that the 12SA7 works in?

Chris Campbell

I haven't checked that. How would I check heater-cathode voltage? I'm assuming that would be measured as AC voltage, right? If I find it high, how would I correct it? Would I measure from heater to cathode on an AC setting?

I'm not at home an don't have the RCA manual at hand, but this online source has the Tung-Sol specs and ghat's where I found it:

https://frank.pocnet.net/sheets/127/1/12SA7.pdf

I'm assuming it's a DC differential.

Chris Campbell