Fluke 8840A AC measurement board keeps blowing cap (resolved) - Page 1
Forum section: electronics / repair

author: Rico
author_rank: Contributor
timestamp: July 04, 2014, 04:33:15 am
content: 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.pdfand 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?

author: motocoder
author_rank: Unknown
timestamp: July 04, 2014, 11:07:48 am
content: Is it possible you, even briefly, had a voltage across the capacitor polarized the wrong way? Here's a note about tantalum caps off of Wikipedia:Quote

author: Rico
author_rank: Contributor
timestamp: July 04, 2014, 08:47:32 pm
content: I don't think that's it, I double/triple checked polarity when I put the cap in. I also used the scope to see if there was anything funky going on during power up and switching off; everything looked good there too, going from 0v straight to -15v with very little settling time, and never going positive or anything like that.The only thing I can really think of is that the spare caps I have are unused, but they're probably 20 years old, I'm not sure if that would have and significant consequence....

author: xwarp
author_rank: Frequent Contributor
timestamp: July 05, 2014, 01:22:25 am
content: Can you post a picture of the a/c board you have?I am currently doing a performance test on two 8840's and one 8842.I also have 2 extra a/c boards.

author: Rico
author_rank: Contributor
timestamp: July 05, 2014, 03:38:51 am
content: Here are the pics.

author: xwarp
author_rank: Frequent Contributor
timestamp: July 05, 2014, 03:41:24 am
content: Could be that you replaced a bad cap with another bad cap.How about replacing it with an electrolytic?

author: mij59
author_rank: Frequent Contributor
timestamp: July 05, 2014, 04:03:08 am
content: 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?Noise on the power supply could affect the accuracy of the measurement.Probably you replaced a bad cap with an other bad cap.Try replacing the capacitor with a low esr elco, replace the charred resistor and place it a little bit higher on the board.

author: Rico
author_rank: Contributor
timestamp: July 05, 2014, 04:19:41 am
content: Sounds good guys, thanks.I'll do a little bit of testing on my stock of these tantalums, like I said they're unused, but probably 20 years old. I'll look to see if I have a quality electrolytic to try, and I'll replace that charred resistor.

author: Rico
author_rank: Contributor
timestamp: July 05, 2014, 07:03:21 am
content: Well, guess those caps weren't up to the task afterall. I tested a few on the breadboard with my power supply and a 10 ohm resistor in series. At first I was putting them straight on 15v and seeing what happens...Usually, this, and then dead short.5v/div, 1 ms/divI don't know if I'm somehow tempering them or what, but I've had luck by slowly ramping up the voltage to 15v with current limiting set to ~50mA. A couple started to "glitch" at the 10v mark, but then appeared to recover and work fine. However, most of them seemed to be OK. After the 15v ramp-up, they were able to take full current 15v pulses without issue.I picked a cap that survived testing multiple times and I installed it along with a fresh resistor on the AC board and it's working great so far.Thanks for the help, I would have never guessed my film canister filled with "new old stock" caps would have gone bad. Occam's razor FTW...I'll call this one resolved, but are tant's known to do this? I thought they were used for their reliability & stability?

author: ConKbot
author_rank: Super Contributor
timestamp: July 07, 2014, 01:17:09 pm
content: You are putting the band/marking on the caps to the + marking on the PCB, correct? Ive seen this cause problems for more than one person who is used to the - leg of electrolytics being marked.

author: Rico
author_rank: Contributor
timestamp: July 07, 2014, 02:18:13 pm
content: Yes, plus there's the longer anode lead.

author: motocoder
author_rank: Unknown
timestamp: July 07, 2014, 02:21:27 pm
content: Glad you got it sorted out. I like these old 8840A/AF meters. Taking mine to get it calibrated today.

author: Rico
author_rank: Contributor
timestamp: July 07, 2014, 06:35:04 pm
content: Sweet, please post back with the results, I'm curious how well these guys hold up to age. From what I understand they are quite reliable, even this old.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2014, 12:15:42 am
content: When I buy random NOS solid tantalum capacitors, I burn them in at their rated voltage and measure their leakage before installation. It is also often worthwhile to increase the voltage derating of the replacements.

author: Rico
author_rank: Contributor
timestamp: July 10, 2014, 12:28:05 am
content: Ah cool, I didn't know that burning them in was a legitimate technique, but it definitely appears to work.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2014, 12:52:38 am
content: Quote from: Rico on July 10, 2014, 12:28:05 amIt is just a way to find bad capacitors before using them in circuit. I have a local electronics store that has solid tantalum capacitors in bins for a good price but their pedigree is unknown so I test and burn them in. With new ones from a reputable distributor, I would not bother.

author: Rico
author_rank: Contributor
timestamp: July 10, 2014, 01:09:03 am
content: The thing that I noticed is that if I put them straight on their operating voltage (operating @15v, rated @25v) with a 10 ohm current limiting resistor (just like it's setup on the measurement board) they would fail every time. However, if I brought them up to voltage slowly, maybe over 10-20 seconds, they would work great thereafter.This article,http://nz.element14.com/pdfs/techarticles/kemet/Tantalum-in-Power-Supply-Applications.pdfSeems to say that tantalums will self heal through localized heating, which I imagine must be going on when I do the ramp-up burn-in.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2014, 01:23:08 am
content: Remember for life this little episode you have experienced.Tantalum Bead caps can be a real PITA.I avoid using them at all if possible and go to some length to replace them in gear I work on.Best advice is to use them well below their rated voltage.

author: Rico
author_rank: Contributor
timestamp: July 10, 2014, 01:37:00 am
content: Oh believe me, I've learned a lot from this!Almost everything I bought for my lab has been purchased as "for parts/not working", and I've learned so much from troubleshooting and repairing this stuff, with the added benefit of getting fairly nice stuff for really cheap.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2014, 01:44:45 am
content: Quote from: Rico on July 10, 2014, 01:37:00 amGood on you.Great you have the skill/knowledge to do this, not all do.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2014, 01:56:03 am
content: Quote from: Rico on July 10, 2014, 01:09:03 amThat is my understanding. Further if the current is not limited, then thermal runaway may happen causing catastrophic failure.