Hello,
I am repairing a dumpster find Agilent 33220A. The symptoms are that the output has a negative DC offset. I have probed around and noticed that the isolated -15V is noisy, measuring 0.3V on my DMM. it is also closer to -16V DC. Is this a failed linear regulator? The noise of the input is a lot lower, both by measuring it with my DMM and observing the signal on my oscilloscope.
I just probed around some more and discovered that the noise is only present when the output is turned on. Thing is, the configured setup is to only output 0.00 VDC, no AC signal. When the output is turned on the -15V rail drops from -15.9V to -16.4Vdc which I imagine is way out of spec. Could the linear regulator be the culprit? How else can I narrow down the issue.
Have you looked at the input and output to the linear regulator with a scope? I'd suspect dried up electrolytic caps that have lost most of their capacitance so the regulator is dropping out of regulation under any load due to excessive input ripple. Second guess would be an open rectifier diode causing a 50/60 Hz rectified signal instead of 100/120 Hz.
Often it is enough to make DC measurements of input and output voltages of a regulator. You don't really nead an oscilloscope but havin it makes picture possibly more clear.
I have a IR covid thermometer and I measured the temp of the regulator, and it is HOT I got peaks of 140 degrees Fahrenheit. On a good unit it doesn't get nearly that high. I'm still not sure what it is wrong, is something sucking up a huge amount of current on the -15V line? The input of the regulator is only about -17V or so, on a good unit it's always a clean -20V. If something else is drawing all that current then something else should also be getting mad hot, no?
update i removed a 0 ohm resistor that disconnected the output of the regulator. It was still getting hot so it guess the part is toasted? Imma replace it and see if that fixes it.