Electronics > Repair

Power Transistors for old HP-6920b

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factory:
OK I checked the positive side of C6 with ref to the chassis, in AC mode, set to 0000, with the output off I get -856mV and with the output on I get 0V.



Also checked the voltage & ripple on C6 with the Tek THS720A scope (fully isolated inputs). DC voltage was 25V, ripple with output off was 30mV and output on was 900mV.



I've also checked the other capacitors for voltage & ripple, don't have enough time to add all the results tonight, but I can tell you the results agree with the Peak ESR meter, C17 is dead, it has 18V of ripple.
C16 & C17 have approx 52V across them, this is just above their rated voltage.



David

RichardM:
Thanks very much David. That confirms my suspicion about the 2-3V offset measured on the positive side of C6. I have ordered some 2N1304 at great expense from overseas but I don’t think that is the issue. I will have a look at the ripple and your other measurements. I may also try another power transistor in place of the MJ15004. It is lime I have something leaky connected to the positive trace connected to Q6.

Richard

RichardM:
A couple of measurements from my 6920b using my Siglent scope. I am not convinced I know how to really measure ripple accurately.
Only DC mode at the moment as AC is not working at all

DC Mode:

Picture 1 is negative side of C6, 0000 setting, DC off, AC Coupled. Shows 1.28V ripple
Picture 2 is positive side of C6, 0000 setting, DC off, AC Coupled. Shows 464mv ripple
Picture 3 is negative side of C6, 0000 setting, DC off, DC Coupled. Shows -21.8V min
I see about +2 - 2.5 VDC on positive side of C6, 0000 setting, DC off as presviously mentioned

Richard

DC Mode:

Picture 1 is negative side of C6, 0000 setting, DC off, AC Coupled. Shows 1.28V ripple
Picture 2 is positive side of C6, 0000 setting, DC off, AC Coupled. Shows 464mv ripple
Picture 3 is negative side of C6, 0000 setting, DC off, DC Coupled. Shows -21.8V min
I see about +2 - 2.5 VDC on positive side of C6, 0000 setting, DC off as presviously mentioned

Richard

RichardM:
Ok, my germanium 2N1304 arrived from the other side of the world. Unfortunately, and unsurprisingly, it made no difference to the DCV output issues described above. At a dead end :(

RichardM:
Hi All

Very grateful for the help so far. It looks like I am at a bit of a dead end. Am I able to post my issue on another outside forum (ie one specifically for this brand of test gear) ? Or that considered "cross-posting".

Richard

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