On Wednesday, June 26, 2019 at 2:46:50 AM UTC-7, whit3rd wrote:
On Tuesday, June 25, 2019 at 6:54:49 AM UTC-7, Don Brown wrote:
I have a Hickok 440 curve tracer that I am having difficulty troubleshooting, even though it is a relatively simple circuit.

The base generator staircase is not stable


Typically, there's a monostable making a current pulse into a capacitor for each step.
If the capacitor is an electrolytic, it may be leaky now (but run the staircase
overnight before you replace parts, it may be that some ON-time will reform
the oxide layer). If the monostable has a capacitor, it is also a suspect (unstable times).

Resetting the staircase there's a charge-dump (transistor?) that could be leaky, or its
drive might be picking up noise. Hanging an o-scope probe onto the step generator
to show the rise/plateau qualities will tell where the problem lies

Yes, this is the way it works. I should attach the schematic and circuit description. It charges a ceramic non electrolytic capacitor, with a diode in line that keeps it from discharging each pulse. But since ceramics rarely fail I did not change it, but maybe I should. And from previous scope probing I saw various qualities of pulses but did not really understand what was normal and what was not.

Thank you.