- 2025
- Jun
- 11
The Heathkit AG-7 Audio Generator part 3: Diagnosis?
When the AG-7 quit, I wasn’t really sure what caused it except that the oscillator now won’t. Tubes check fine on my cheapie tester, so I decided to start doing some measurements around the 6J7.
These didn’t seem right, much higher than they should be, but the individual components were (mostly) within tolerance after disconnecting from the circuit. It still didn’t seem right, things were bouncing around like there was another path to the filters.
I disconnected the line from the filter and that all went away. Ok…do I have a bad filter capacitor now? No, it checks ok, but that’s with the 10kΩ resistor across the filter disconnected.
And there it is. The 10kΩ resistor that goes from triangle to dash on the filter is bad. It’s not open, or high resistance, it’s 295Ω. Confirmed this with two meters…what? I’ve never had one of these old carbons short like this, but I suppose it’s completely plausible. It is 70ish years old at this point.
I have plenty of 10k resistors laying around to test with, so I’ll take some time and see if that gets me back to a semi-working condition so I can complete my troubleshooting from the last problem. I suspect this is going to need a complete rebuild in order to be functional with any kind of reliability…
Next part of this series: Coming soon.
Previous part of this series: https://wereboar.com … ator-part-2-testing/