- 2025
- Jun
- 30
The Heathkit AG-7 Audio Generator part 5: It’s gonna need everything.
I received some new tubes for the Heathkit AG-7, and that did nothing - so the unit is probably going to need most (if not all) of the parts replaced.
In the previous part, the oscillator just quit. I discovered a part in the power supply was radically out of tolerance, but replacing that did nothing. As I was troubleshooting, the lamp in the oscillator circuit that’s used as the PTC resistor started glowing red and flickering, indicating high current draw. The output would jitter in time to this flickering, like the oscillator was trying to start, but couldn’t. This is a symptom of wein-bridge oscillators, they require a balanced diet of feedback and amplification to start and oscillate properly.
Then the smoke started. Ok, there’s something seriously wrong here, so off it goes and discharge the filters. I decided to start checking the parts in the circuit that were isolated enough to give me a reading without something in parallel interfering. I found some good ones. Case in point, this 100kΩ resistor:
But as you can see, the meter tells me that this is actually closer to 700kΩ. Not surprising for a high-value carbon, and a spot check of other components revealed a similar story.
Now, you’re going to be saying that checking directly across a resistor in a circuit shouldn’t give a higher value, and you’re correct - at best, it should be it’s marked value but may be lower due to the effects of parallel resistance. That this part reads nearly 7 times it’s listed value says that it’s indeed very much bad.
I’d say this thing had a lot of repairs over the years, as there are plenty of mixed component types. So…it gets entirely rebuilt, because it’s going to need it. Stay tuned, I’m going to do this one in stages.
Next part of this series: https://wereboar.com … -removing-everthing/
Previous part of this series: https://wereboar.com … ator-part-4-repairs/