Hello everyone,
I recently acquired an optioned-up Fluke 8840A, but I'm having trouble with the AC measurement board ("option -09"). For some reason it keeps causing a filtering cap on AC board's -15v rail to short out. I've been looking through all the posts about this meter I could find, but haven't seen anything like this come up, although it looks like a lot of people have experience with this model so hopefully you guys can help me sort it out.
The schematics can be found here:
http://fablab-lannion.org/wiki/images/b/b1/8840A.pdf and the board in question is on the last page.
I found out I had a problem when I did the built-in self test and tests 1-4 (AC measurement tests) error'd out, but everything else checked OK. I pulled the cover off and found R833 (a 10 ohm resistor right where the -15v rail arrives on the board) was burned to a crisp. After a bit of trial and error and poking around, I discovered that C823 (10uF, 25v dipped tantalum) was failed short. After removing the cap, there was about 3M ohms between the -15v and ground. I checked the board over for any debris, bent pins, etc. for anything that could cause a short, but didn't find anything obvious, so I just replaced the burned resistor and the cap (observing polarity).
I then powered up the meter without the AC board installed and checked all the power supply voltages, which were all within limits. I also scoped the rails for ripple and that didn't look too bad either (about 4.5mV RMS).
I re-installed the AC board and powered up the meter. Almost immediately the same resistor started to let out the magic smoke. I powered it down right away, and went straight to checking that same cap, which had shorted the -15v to ground, just as the one before it did. I went through the whole routine again, but still couldn't figure out the cause. I checked my charred resistor, which still showed 10 ohms, and then pulled the shorted cap out, and powered up the board again. With the cap out, the resistor stayed cool, all the self test passed, and AC measurements appear to be accurate and stable. I also poked around the AC board with the scope, and all three of it's power supply voltages look good.
I'm hoping someone may have a suggestion as to why the cap keeps blowing. I promise, I checked polarity, realizing that it's marked on the + side instead of - like electrolytics, and the anode has the longer lead, and in this case the anode goes towards ground.
If nothing else, do you suppose there's any harm in going on without the cap, considering the voltage & ripple looks fine on the scope and the meter is functioning perfectly without it?