Heathkit AA-121- What happened to this thing?

[H713]

Just bought this thing with plans to restore it, and I was hoping for some advice as to how I should proceed. I have not tested this thing yet, as when I got it the selenium rectifier was still in place and the surgistor was burned out. Moreover someone had bypassed the fuse holder (uh oh...). So I've been cleaning up that mess, but I also noticed that one of the two channels is also a bit of a mess. See attached photos, but some of the OPT leads have been sleeved, and there is a blackish soot in the chassis. The sleeving is old and crumbly, and the lead wires that haven't been sleeved look a bit mutilated (by fire). The 6 ohm cathode resistors have also been replaced by enormous carbon comp resistors. Also, the tube sockets on this channel don't match. My guess is that a tube socket failed and there was a small fire inside this thing.

I measured the resistance of the output transformer windings from the center (B+) to each of the plate leads, and it's within 10 ohms of the other transformer. I have an extra OPT from an AA-40, and it reads the same as well.

How should I proceed with this? The sleeving definitely needs to be replaced as it's very brittle and crumbly. I don't have any EL34s to test with, although I do have a matched quad of 6L6GC tubes (might work for testing?). I also have the 6AN8 tubes. I don't want to buy a bunch of EL34s until I know the transformers are good.

Other Questions:

-I have a bunch of CL-90 thermistors. Since this needs more than 2A of current, can I just put two in parallel (see pics)?

-The coupling caps are branded "mica mold". They don't look like any mica capacitors I've ever seen. Will they be okay for testing?

-I replaced the Selenium rectifier in the bias supply with a silicon diode. Does anybody know if I'll need a resistor in series to compensate for the lower voltage drop across the diode?
Thanks for the help.

 

[Mike Sweeney]

A recap is in order at the very least. I've restored at least 50 pre 1955 radios and each and every one had bad caps. Not always shorted but they all leak like a sieve. Quite a few of the carbon resistors drift a fair bit too. The "mica" may test good. Those seem to be more durable than the rest, even the plastic bullet caps. You selenium rectifier drops about 5-20 volts ( more or less, this is based on my own experience, I was taught that you lose about 1volt per stack.. so 6 stacks is ~6 volts) and a modern diode like the 1N4005 drops a bit less than 1 volt so you need something around 50-100 ohm 5ish watt resistor to make up the difference. The resistor helps to compensate for high line voltage and protect the diode. For example, my house voltage is about 115-117 but spikes to 120 often. The old sets were normally designed for 120 volts as the voltage tended to vary quite a bit around the country back in the day. But a cheap set might forgo the resistor ( or poorly designed). I know more than a few techs that keep the old selenium in place for looks and put the modern part behind it out of sight. There is quite bit of info on youtube for replacing these

 

[H713]

Thanks... the original electrolytics seem to work okay, and don't self discharge too quickly, so I'm planning to leave them in until I've tested it. Any thoughts on the transformer? I figure I'll have to remove it for re-sleeving, but I was hoping to make sure it's okay before I do all that work on this thing.