HP 4277A/4276A power supply issues

[ZGoode]

I got my hands on a cheap HP 4277A LCZ meter a while ago that seems to have power supply problems.  I have been trying to work my way through the service manuals for the 4277A and 4276A, but can't identify the bad components.  I have checked all fuses, and replaced all RIFA caps.  I have also replaced the Q10, Q11, Q12 transistors with known good parts.  So far all other parts I have checked appears to be within spec.  For the first check in the troubleshooting guide it says to check for 12V at a test point on turn on that goes quickly down to below 3V.  When turning on it goes to around  8.5V and then dips below 3V very quickly.  Anyone have any other ideas as to what could be going on here?

[ZGoode]

Reply to add picture of board.  There is that burnt section near the D2298 transistor, but it appears to be fine now, and that resistor connected there appears undamaged.

[jonpaul]

Our 4277A   never had any issues.

 random parts replacement is not reccomended. This switching power supply is complex and hard for troubleshooting if you have little SMPS expericence or equipment.

(variac, isolation trsf, probes, etc. )


1/ Find full PDF and printed service manual for your model and serial number range.
Artech still prints good reproductions. Beware that The PDFs vary widely in quality

2/ Read and understand theory and curcuit description

3/ Follow troubleshootong proceedure for affected section , in the  flow chart

4/  find the issue.  Fix. Test, adjust calibration if needed.

DONE!

Good luck,

Jon

[ZGoode]

I've been mainly working off the 4276A manual, since it appears this board is most likely the same between the 4276A and 4277A (and since that version of the manual appears to have better scans that I could find)
This board is behaving really weird, and makes me feel as if U1 might be the problem.  Since when I remove the 250mA fuse (F1), it appears to test like how it should in the first sections of the flow chart (12V on startup, going down quickly to less than 3V).  As soon as I put the F1 fuse back in and then turn it back on, it goes back to behaving weird with the test point measuring around 8V on startup.  I'm thinking due to this test with and without the fuse, I'm assuming the circled section is most likely where the problem is.

[ZGoode]

After making a replacement for the U1 (1813-0255) and replacing the transistors that tested bad with my DMM I have successfully gotten the 4277A to turn on.  The analog section isn't working, so I need to figure that out, but this is a step in the right direction.