Stuff could have been overloaded and changed value. Those resistance values are low and thus the relay contacts may or may not need to be looked at. Which I said before.

I agree with your assessment here. I checked values of most of the components in this section of the device. All the component values appear to be reasonably close to expected. There is only one relay associated with the 50 ohm output and it's functioning normally for now. I do have a few spares if it gives trouble in the future.

AD811: https://www.findchips.com/search/ad811

Nice website! I will keep this one bookmarked. I was able to find a replacement that will work in this application from Newark. The only real difference is the new IC has a wider range and a higher frequency limit than the old. Neither will impact the operation in this application.

An OP-amp not used as a comparitor, can USUALLY checked with a voltmeter between the (+) and (-) input. the value measured should be close to zero.

Not sure I understand this one. Checking voltage to the input should read nothing on DC, but should show the input voltage in AC at approximately 5V. Checking the voltage (power supply) to the chip from the (+) and (-) inputs yields a reading of approximately 29VDC. Is that what you were describing?

FWIW:
There is always the inspection. You could find a sandwich in a VCR or a roach nest for instance. Cat hair on top of an audio amplifier. Rattles, like an electron tube out of it's socket. regulator for 1 15 kV supply at 1.5A.

First thing I did. Fortunately the unit in question was being used in a clean environment and is pretty much solid state components. I did check IC seating in the few socket locations, but inspection showed a very clean, minimally abused device.

Verify the customer's complaint.

Easy to do. I found the issue, therefore the complaint was mine and verified with my scope.

Always check the power supplies. Ripple and value. Especially look at the ripple on the negative supplies.

I verified voltages, but did not inspect them for ripple. When I get it back on the bench before installing the new chip and resistors, I will check. I didn't think about it as prior to be being damaged, it was a great unit. I don't have an original baseline for how much ripple to expect, so not real sure how much would be too much.

So, far, I think you did well

Thanks! I do have a few years under my belt in the field. Unfortunately, our disposable electronics have robbed me of opportunities to actually dig in and repair things like I used to back in the "day"

 

Not sure I understand this one. Checking voltage to the input should read nothing on DC, but should show the input voltage in AC at approximately 5V. Checking the voltage (power supply) to the chip from the (+) and (-) inputs yields a reading of approximately 29VDC. Is that what you were describing?

U24, the AD811,: pin #2 to Pin #3. Not used as a comparator. DC Voltage close to zero - amp good.

 

U24, the AD811,: pin #2 to Pin #3. Not used as a comparator. DC Voltage close to zero - amp good.

Ah, ok. I will have to check it when I get it back on the bench.

I assumed signal leaving U15, no open components between there and U24, proper power to U24, no output from U24, bad U24. I will double check my work. New 811 coming from UK so I have a few days before it gets here.

 

Well, here's an update. I ordered the 811 from Newark and it came in as a surface mount. I ordered the wrong part. I need to find a part number for the socket mount version and try again.

Found the replacement for an obsolete part on Mouser... good price, in-stock, and PDIP-8. Next time, more inspection before purchase.