Hi,
I've been working on fixing a Datron 4200 AC Calibrator, and sadly think I've introduced more problems than I've fixed (the original post is here:
https://www.eevblog.com/forum/repair/datron-4200-ac-calibrator-fail-2-during-selftest/)
I think I've narrowed the problem at hand down to the oscillator that starts, increases amplitude and frequency, but then damps quickly to zero (over a few cycles.)
I think I've tested pretty much every input to the oscillator and can't see anything changing at around the time of the damping, so I'm really stuck now and would love to get some pointers ... I am way way way out of my depth in terms of understanding how all the control circuits work.
I have attached the schematic, you can see it's a oscillator built with two integrators and an inverter, then there are some control circuits that handle frequency and amplitude control.
I've also attached a couple of scope traces ... the first (ending 223) shows the tail end of the oscillations, and is setup as follows:
CH1 - TP4
CH2 - TP5
CH3 - M23 pin 6 ... output of the amplitude control
Math - CH2 - CH3 (I think, I was trying to check that M15 was doing as expected comparing against CH1)
The second (ending 236) shows the overall activity from power-on through to the oscillations stopping, and then the various signals moving to fixed positions. This one was setup the same.
Other things I have tried:
1. I've disconnected LK.C to disconnect the output, to ensure it wasn't causing the problem -- no change.
2. I've moved LK.A to LK.F to use a test signal for frequency control -- no change, which presumably rules out the FREQ ERROR signal.
3. I've removed the logic that drives the relays (RL1-RL8) to eliminate the switching in of different caps during the power on sequence.
I haven't eliminated the switching in of resistance by the FET's ... could that be an issue? Dodgy FET?
I haven't eliminated the possibility that the +/- Vsin and Vcos could be pulling things down?
With the FET's I did consider that they might be all turning off, however my assumption is that would break the loop and hence you'd see an abrupt stop rather than a damping?
Any clues or anywhere I can look next would be really appreciated!