Electronics > Repair

Fluke 8840A Self test Problem

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Paceguy:
Hi Everyone. Maybe someone can help me out. Yesterday, I picked up a Fluke 8840A with the AC and IEEE options installed. It's in nice physical condition. Problem being it does not read DC voltages, and won't zero with the inputs shorted. I tried the self test and this is what occurred: ERROR codes on 1,2,3,4 and 5. and that's if it will test up to 5. It will not self test past 5. Most of the time it will only go to 2 and then returns to the momentary power up screen display. I suspect a power supply problem and will test them all first. If the supplies test ok both voltage and ripple wise, what would be the next area to suspect? Thanks in advance !

giosif:
A few comments on my side:
1. I think this thread is better suited in the Repair section and it's likely you would get more attention there. Try asking for the thread to be moved there.
2. I would disconnect the AC and GPIB boards from the main board, just so you don't need to worry about any of them having a problem and affecting the rest of the meter.
3. Yes, do measure all power rails and check they are in spec (the service manual is widely available).
4. What format is U202: plastic DIP with no window, or DIP with window, or DIP (usually ceramic) with piggy back EPROM IC?
5. What version is shown on the label of U222? Can you check the contents of U222 against the associated ROM dump file for the same version (you can find it on the Internet)?

HTH

xwarp:
Check solder joints for the transformer. Not sure if this is your issue, but that would be 1st place to start since these 8840's seem to suffer from cold joints when they age.

bdunham7:
Definitely reflow the transformer solder joints.  It will take a bit of disassembly, but you may as well remove the AC, GPIB and lower shield boards now so that you have access to work on it.  Do that, resolder the xformer and then retest.

If you can't get a DC reading, first try the rear terminals and make sure the switch moves properly.  If that isn't it, check the input circuitry right behind the front terminals to make sure that hasn't been blown up.  And check all the power supplies, of course. If your self test issues continue after you've verified that all the power is good, then you may be in uncharted territory.  I've not had one abort the self-test or have any such reset issues other than power supply problems.

Paceguy:
UPDATE: Problem resolved, repaired and working as it should.

First, I fluxed and reflowed all of the pwr. transformer connections on the board. As suggested, I have found units of this model with bad /cracked solders on the transformer in the past. In those cases, the display was blank or intermittent.

Next I checked the low voltage power supply rails and found that C605, 470uF, 50V electrolytic capacitor was open. Changed that cap and now all works perfect. All 21 self test show no errors, DC volts zeros when inputs are shorted, etc.

Many thanks to giosif, xwarp and bdunham7 for taking the time to give me good advice.

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