Fluke 343A DC voltage calibrator restoration - Page 1
Forum section: electronics / repair

author: Grapsus
author_rank: Regular Contributor
timestamp: December 02, 2017, 08:33:35 pm
content: Hi there !I haven't been posting here for a while but this week-end I finally got motivated to repair a cool DC calibrator I bought off ebay a while ago.It's a 7 decade Fluke 343A. Looks like it's an early unit, serial number 335, parts date from 1972 ! The output voltage was way too low and the "current limit" bulb never completely went off. The service manual is available online and I followed all the troubleshooting steps but everything was within specified ranges and the unit still wasn't producing the correct voltages. So I started measuring voltages with the unit ON (whereas the service manual asks to measure in STANDBY) and I noticed the high voltage doubler output drop and produce a huge ripple. With that observation I easily traced the flaw to a huge electrolytic cap C55. Once desoldered, I could see that this cap was leaking electrolyte underneath. A 80 uF 450 V Sprague 68D. Can't really complain for it failing after 45 years of service. I replaced it with two random 150 uF 250V caps from my junk bin in series to see if it was the only fault.Surprisingly the unit started to work really well and the calibration seemed pretty good. Actually I don't have anything remotely that accurate to verify this calibrator.So my question is: what should I do next ? Obviously I will order a good quality replacement cap. But 5 of those Srapgue caps are still in the unit (one seems to have been replaced in a previous repair) and I'm not sure I can trust them. Moreover there are like 20 other smaller electrolytic caps from the same era. Should I replace them as well ?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 02, 2017, 09:33:29 pm
content: I would replace the other one too. Chances are they are not much better - if 2 out of 5 or 6 already dead, chances are the other will break down too.The smaller electrolytic caps are usually not that expensive. Especially the blue Phillips ones don't have the best reputation.

author: Seekonk
author_rank: Super Contributor
timestamp: December 02, 2017, 11:24:54 pm
content: It has to be online somewhere because I got it.

author: TimFox
author_rank: Super Contributor
timestamp: December 03, 2017, 05:49:02 pm
content: If you need to replace a 500 V electrolytic with two 250 V units in series, due to difficulties in obtaining 500 V rated parts, remember to put "equalizing" resistors across each of the two capacitors to ensure equal division of the DC voltage despite unequal leakage currents.

author: Grapsus
author_rank: Regular Contributor
timestamp: December 04, 2017, 12:30:11 am
content: Actually I found suitable single caps on Farnell (Rubycon CXW), the series caps was a temporary fix just to check it was the only problemI've read the same advice about adding bleeder resistors to series caps while looking for replacements.What's your opinion on replacing all the other electrolytics ? It wouldn't be that expensive, like 30 euros total for decent quality components. But should I do it ? They're probably not part of the precision equation, or are they ? But it would imply soldering all over the place, near the reference etc.Also what are my options to check the calibrator or even adjust it ? It's a hobby so paying thousands of dollars for a cal lab is not an option and I don't have anything nearly that accurate...

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 04, 2017, 07:19:45 am
content: You should definitely replace all these old electrolytic capacitors.For calibration, you should quest for a volt-nuts in the Netherlands or in the neighboring countries, who own a 3458A, and who might do this calibration just for cost of postage.Frank

author: TheDefpom
author_rank: Frequent Contributor
timestamp: December 12, 2017, 07:23:04 am
content: In case you are interested, I did video series of my repairs to a 343A I also picked up from eBay.Below is part 1 of the series, there are 4 parts in total.