HP 3455A DMM Intermittent Power-Up

[Bimmerdude]

Greetings to all....

I have an old school 3455A DMM that has an intermittent power-up issue.  In most cases, it powers up just fine.  Unfortunately, there are a few instances when you power-up the DMM, the display goes blank as well as no response from the front panel push buttons.  Once you cycle power, it comes on like nothing's wrong.  I've checked the power supplies (+5VDC and +12VDC) on the A1 board.  Both are good (even when the DMM is acting up).  Used the flowcharts and ended up on the outguard troubleshooting procedure diagram (figure 8-H-26, pages 8-131 and 8-132).  When I'm at step 8 (is the signature 401F when each key is pressed), some of the keys do not display 401F.  I then follow the no direction and end up at "is the signature at A1U51 pin 2 42AP?", the answer is yes. In lieu of switches S2, S7-S9, S13, S15-S22 and cable A2W2, it has me check out A1U50 and A1U57.  These two IC's check out good.

Any suggestions?  I'm stumped at this one.  Throwing money in the wind, I did swap out the A3 board with a known good one with no success.

I'm lost on this one.  Any suggestions?

Thanks,
Daniel

[Kleinstein]

Quite a few of the older meters can have problems with the data transfer part.  The 3455 service manual is still very detailed. It may be worth checking the waveforms for the data channels between the 2 sides of the isolation. It could be a boarderline wafeform.

[wn1fju]

I've had a few 3455A units with similar start-up problems.  In most instances, the problem is in the inguard circuit which is not completing its auto-cal and/or measurement process.  The outguard microprocessor just sits there waiting for that completion and the machine hangs.

In one instance, there was a problem on the A/D daughter-card.  In another case, the problem was with one of the switching FETs that caused two voltages to be simultaneously applied to the measurement.  But in your case, which is intermittent, I would suspect what I found with the last one of these I fixed.  Namely, the power supplies on the inguard board.  Look at all the voltages carefully with an oscilloscope - not just a voltmeter.  In my case, the voltage feeding one regulator had terrible 120 Hz ripple on it and the voltage dipped below the required regulator input voltage.  Thus, the regulator would drop out, the inguard measurement wouldn't complete, and the 3455A just sat there blank.  A new electrolytic cap cured all.