I do not normally consider Evapo-rust to be an #electronics repair chemical and yet here we are. Only late-era #EICO would use a light bulb to add impedance to a circuit. Or at least I think that is what they’re doing in this 379 signal generator, anyway.
This is how far I’d got up the main board when I had to pull the evaporust. Everything down from here out to the power plug is working again. (It… had not been. Yes, I am stacking capacitors. The originals have weird values and this matches to 1%.)
The original power smoothing caps were... inadequate. (You can see from impacts on other components that they got _rather warm_.) I have improved the situation in my repair.
I'm pretty confident that this was originally a vacuum tube design and that this has something to do with the internal light bulb as impedance but there are no circumstances under which I would testify to that under oath
@moira @Joel
It's a Wien Bridge Oscillator! A classic circuit that stabilizes the output frequency.
https://en.m.wikipedia.org/wiki/Wien_bridge_oscillator
It's very likely you can still get replacement bulbs. They don't have to be exact, as they're not exactly precision references. They kind of just settle into a steady state after warmup, assuming the proper current and time constant.
@stephlahs @Joel I learned something today!
I will understand this better later, as I'm fixing this for an online electronics class. (Not something accredited, just hobbyist.)
@Joel Oh my god. That's terrible.
Not saying I don't believe you, just being appalled.
Doesn't it vary meaningfully over time? (And across makers?)
This one fortunately still has continuity but it also looks like it's a GE 1819, and that's still made. But is it to the same specs? Who knows? xD