- 2025
- May
- 30
The Heathkit AG-7 Audio Generator part 1: Observations
If you’ve been reading this site, you’ve probably seen this guy. It’s a Heathkit AG-7 Audio Generator that was purchased at the Breezeshooters Hamfest in Butler, PA during their 2023 event. It’s not something I needed, but was interesting because it had a decal from General Telephone’s Erie, PA office, and had drill points that looked like it had been set-and-forget. Some sort of tone generator for the telephone switch, perhaps? I’m not sure, save that this device has some interesting history behind it.
Here’s the decal on the side:
The chassis shows some signs of work over the years. J-hooked parts show someone probably did some investigative repairs, and that resistor pair that’s tied together certainly wasn’t factory…
What’s the initial diagnosis here?
Power supply seems to be working, and filter capacitors aren’t dried out. They’re not getting hot and nothing else is smoking.
The device has an output:
You can see this device has the same issue that a lot of Wien bridge oscillators have - the lower half of the signal is partially cut off. Later devices took care of this by putting adjustments in the oscillator circuit itself, but this one didn’t have that:
So there’s probably some resistors that have drifted in the oscillator circuit itself. No surprise there, and the repairs are going to be the first suspect.
It does have another, less apparent issue - the oscillator will stall and restart when you rapidly change the dial. There are trimmers in the unit to help alleviate this, and an adjustment procedure for them. This isn’t really that big of an issue, so I’m not concerned with it right now. The main thing is going to be figuring out which parts needs to be adjusted to bring this device back to a pure sine wave, at which point a partial or full rebuild may be in order.
Next part of this series will be determining where the sine goes bad. Stay tuned!
Next part of this series: https://wereboar.com … ator-part-2-testing/
Original post: https://wereboar.com … -7-signal-generator/