1930? General Electric T-12

[legenbass77]

I got this radio from a relative to fix for them. Cabinet is in great shape, as well as the speaker cloth. The only repair I can see is the 10uf electrolytic capacitor. I've never opened up anything this old. Tested the tubes, and the only weak one is the 80 rectifier. The wiring looks ok, hooked it up to the isolation transformer, and it came to life. It actually works pretty well, nothing smells like it is burning, but has a slight buzz in the background. Also it just has two wires dangling for an antenna. I have a schematic for it too, so I've been getting familiar with the components in the chassis. Looks like there are 2 metal boxes with capacitors inside that probably need to be replaced. Has anyone ever worked on one of these radios?

[Sandy G]

Its ABSOLUTE Junque-You need to box it up & send it to me ASAP. I'll say Words over it & give it a proper burial. Got a BIG Gooleyhole in my backyard I'm tryin' t'fill up... Seriously, GREAT score ! That Bad Boi is GORGEOUS ! You're VERY lucky !

[bandersen]

Nice set

According to radiomuseum.org it's very similar to the RCA R5. You can download the schematic for that set here.

[Reece]

Nice little set. This is a TRF set and needs a good antenna to work well. About 20' of wire stretched across the room would be enough for a test, if you don't have an outdoor antenna.

The internals of the capacitor boxes are laid out very clearly with regard to color coded wires. I'd take plenty of close-up digital pictures and make drawings of the wiring and remove the boxes with the long wiring attached. Be sure the color coding on the wires still shows up clearly and is not faded so that it could be confusing later on; label wires if needed. The caps may be bedded in tar in these boxes: heating the boxes with a hair dryer or heat gun should release the block. It's not necessary to refill the boxes with any kind of potting material when replacing the caps. There may be reusable terminal borards in there or you can make up pieces of perfboard with copper traces to mount your new caps on and connect the old wires to. Mount the boards back inside the cans maybe with a bit of hot glue. Be sure the terminals can't short to the cans by wrapping the board in some plastic material or fish paper.

[legenbass77]

Thanks for all the tips! The heat gun worked great to get the capacitor pack out. I used sharpie markers to color code wires according to the schematic wiring diagram before I clipped away. It worked well for me to make sure everything got put back where it belongs! All the capacitors are replaced, still have a buzz, but not as bad as before the re-cap. Is this normal? I turn it on, it buzzes almost immediately, the tubes warm up, audio begins to play, and the buzz isn't as noticeable. Also sprayed the volume control, and it's working better. I found one resistor way out of value. It's supposed to be 500K, but was 1.2M that on my meter, so I'll order a replacement. All the other resistors are close to what the schematic calls for.

[Reece]

Good neat job on that cap block. For the buzz: what mfd. electrolytics did you use to replace the old ones? I notice that C18 was originally 2 mfd. Back then mfd's were expensive. You could try bridging that cap with a 10 or 15 mfd. electrolytic and if that kills the buzz, substitute it. I wouldn't go over 10 mfd. for C17, the other 'lytic, as too large a one here can strain the rectifier.

[legenbass77]

Thanks! I do have a 2uf in right now. I'll give a 10uf a try, and see what happens.

[legenbass77]

Oops, I hit enter before I finished my thought. I have 10uf for C17, and 2uf for C18. So I should make C18 10 as well, or would that stress the rectifier too much?

[Reece]

The general rule is keep the cap next to the rectifier the same value as original, but you can increase the second cap farthest from the rectifier a bit. Yes, C18 can be 10 or 15 mfd. Check the high voltage before and after the change from the 2 mfd. as too big a C18 can raise the voltage. 10 mfd. should do it IF the source of hum is the B+ supply. You could also have a buzzy power transformer with loose laminations? Or is the hum definitely from the speaker.

[legenbass77]

The buzz is definitely coming from the speaker, not the transformer. Adjusting the volume control doesn't change the buzz at all. It's just not as noticeable when I'm tuned into a strong station. I clipped in a 4.7uf across the 2uf, nothing changed. Where do I test for high voltage? I clipped the positive lead of my meter onto a filament of the 80, negative on the chassis, and got 185 volts with or without the extra capacitor in the circuit. I hope I'm not asking too many dumb questions. This is my first old radio repair, so I want to get it right rather than destroy it guessing! I sure appreciate all the tips they've been very helpful.
Thanks again!

[Reece]

You're testing B+ at the right points. You said the buzz comes up "almost" right away: after the 80 tube lights up, right? Pull the 80: no buzz? Try another 80 if you have it. Put the 80 back and pull the 47 tube. Still buzz or not? Put the 47 back in and ground G1 of that tube to chassis. Any effect?

The volume control wouldn't affect an audio section buzz because it's way up front in the antenna circuit. The audio amp runs wide open in these sets.

Try a good ground to the ground wire provided on the set. Any difference?

Check R9, R10, R11: I think you replaced those? So they must be close to spec.

Is there any evidence that anyone was in there before you and changed the speaker wiring or does it look ancient and undisturbed?

Some of these sets were just buzzy. About the time this set was built they started putting hum bucking coils in series with voice coils to introduce a little voltage out of phase with the field, to cancel out hum.

[bob91343]

The buzz is coming from 120 Hz currents through the field coil of the speaker. That is used as a filter choke. The rectifier starts working within a second or two but the other tubes don't function for about 15 more seconds, so it has to be the power line through the speaker. Pull the output tube and it will still hum.

Increasing output filter capacitance probably won't fix it, since if anything it will increase the hum current in the field coil. Increasing input capacitance will help, as long as you don't go too far and exceed the peak current rating of the rectifier tube.

[legenbass77]

Ok finally got back on the radio this weekend. It doesn't look like anyone messed with the wiring, as it crumbled when I began working on it. The only repair I saw was the 10uf electrolytic had been replaced at some time. The 80 I had replaced because it tested weak, no change. I found R 10, and R 11 out of value. I replaced R 10, and have R11 ordered. Just for kicks I pulled the 47 tube out of the circuit, powered up, and probably 95% of the buzz went away. Put it back in the circuit, grounded G1 to the chassis, powered up, and had very little buzz, but also no audio. Unclipped the G1 lead from the chassis, audio with the background buzz came back. Would I have a malfunctioning 47 tube then?

[legenbass77]

Here is a link showing the effect of grounding G1 of the 47 tube to the chassis.
http://www.youtube.com/watch?v=yPUeIcYP4iA

[Hemingray]

No Heater-cathode shorts in any tube?