On Tuesday, June 25, 2019 at 8:50:33 AM UTC-7, John Robertson wrote:
On 2019/06/25 8:09 a.m., Don Brown 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, and will not stay in calibration. It looks like noise comes and goes and distorts it. I have replaced most of the semiconductors. The 15v power supply is a bit low at about 13.9v, though the regulator transistor and zeners have been replaced.

I have the manual, schematic, parts and usual test equipment.

Thank you,
Don

So the short answer is Yes.

OK, caps and resistors all good.

Then I'd scope the existing power supply to make sure it is clean & stable.

If you have a good bench power supply see if using a different +15VDC
source clears up the issue.

John :-#)#

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I can do both. I did not think of an external power supply, will report back and post results.

Thank you