Fluke 343a DCV calibrator fix: The Good,The Bad and The Ugly - Page 1
Forum section: electronics / testgear

author: onlooker
author_rank: Frequent Contributor
timestamp: May 04, 2013, 07:30:10 pm
content: I recently won an ebay auction for a for-parts Fluke 343a DCV calibrator (up to 1000DCV) for under $50 (incl shipping). Apparently, the seller was from an instrument service shop and described the 343a as"...it's dead, as dead as Marley. The power transformer failed and it blows fuses. Sometimes they're really dead! Other than being dead, it looks in fair shape internally."I liked the straight talk of the seller (and good communication), though I do not really have the need for a DCV calibrator, but I figured, if I can win with low bid, those precision resistors will be worth it; If the 15V reference is still ok, I will have more to gain.In understanding the design of the 343a, I realized that it can be a dangerous thing since there are over 2kV difference inside the box. Also, a good part of the control circuitry was +SENSE referenced; while for external use, chassis GND was supposed to be connected to the -SENSE. There can be 1kV crossing the two SENSEs.Upon the 1st look of the PCB, I found that there were some re-solder/de-solder marks, a few cold joints, one missing 1.6kV gmv cap and a few caps with higher ESR. But, none of these were in the critical areas and they are easily fixable.The main problem was a winding short in the multi-winding transformer and the short winding was one of the two windings for the sink-supply and crowbar driver. Fortunately, the sink-supply and crowbar are not essential for the operation when the output is under 300V and the bad winding was the outer-most one on the transformer. My solution was simply cut the winding without taking apart the whole transformer.I do not yet know what had caused the short, but during the fixing, I was surprised to see the 120ACV bad winding directly on top of a 227ACV winding without extra insulation. There were potentially 700V crossing the two windings.The other point I learned was that even though the sink-supply and the crowbar are not essential for under 300V output, the remaining good winding for this part still needs to be connected in order to cut some leak path that interferes with the voltage regulation.The 343a is now fixed and works in spec for output less than 300V. For all these the calibration pots are NOT touched. The seller had said he and, he believed also, the previous owner did not touch them either. That is, the calibration is still in the somewhat "original" state.I have no plan or need to connect the windings for output higher than 300V, though it can be done by using a small separate isolation transformer to provide the function of the shorted winding, so that the crowbar driver can work again.For this fixed transformer, I would not dare to leave the 343a on unattended.Now, Let the pictures speak for themselves and enjoy:300V test:100V test:10V test:1V test:1mV test:the inside view:beta and sample string:bad winding:the ugly open view:

author: SeanB
author_rank: Super Contributor
timestamp: May 05, 2013, 05:19:23 am
content: Nice rebuild. The transformer is rewindable ( once you persuade it apart, you may lose a laminate or three along the way) but doing it the way you did is usable with the reduced functionality. They must have run out of winding space on those last windings, and were not going to redo and use thinner inner insulation material. If I was rewinding it I would use class H wire, class H tapes and sleeves and thinner paper separators, then it would still run as class B temperature rise but with a lot more reliability.

author: casinada
author_rank: Frequent Contributor
timestamp: May 05, 2013, 07:09:37 am
content: Congratulations on a good find. There is another thread on the Fluke 343A. My 343A wasn't very stable when I got it. I had to change The 2N3739 (power transistors) and a couple of electrolytic capacitors (c37 and c42). Later models have leds instead of lamps. R180, R181, and R239 are 240 ohm on the lamp version and 1.5K ohm on the led version.

author: kripton2035
author_rank: Super Contributor
timestamp: May 05, 2013, 08:09:04 am
content: I would also carefully unwind and rewind the transformer to have a full nice rebuild !try first on a small other transformer, and there are videos on youtube about that.

author: bingo600
author_rank: Super Contributor
timestamp: May 05, 2013, 08:32:28 am
content: @onlookerNice work with the flukeBut from my point of view ...Even nicer work with the backlight of the 3457AI just got one of those , and would love to see some pics and a description .... So i can do the same ...TIA/Bingo

author: onlooker
author_rank: Frequent Contributor
timestamp: May 05, 2013, 10:47:38 pm
content: I agree I had only done a partial fix. The transformer is likely to still have hidden damages that will need to redo all windings soon. In fact, I did consider redo the outer most one or two windings. Without knowing what materials are needed, I bought a roll of 50mm wide Kapton type and looked into buying the transformer wires. But, the amount of work involved eventually scared me away. Now I know what I need to buy, it surely will be helpful when I have the time and will to redo the transformer.I did read all the 343a and 341a related posts with interests. For the component defects in my case, c65 was missing with burnt thru holes. c42 and c18 had ESR about 50~80R comparing to a more normal value of 4R of similar caps on the PCB. Actually, one leg of c18 was not even soldered back from apparently a previous repair. Then, it looks like none of these caps are critical, but I replaced them anyway.This 343a comes with lamps and 3 of the lamps were burnt. I replaced them with 2 white and 1 red LEDs and R181 was changed to 2.7kR.As for the HP 3457A backlight mod., I think I'd better open another thread to answer it.

author: bingo600
author_rank: Super Contributor
timestamp: May 06, 2013, 05:01:06 am
content: Quote from: onlooker on May 05, 2013, 10:47:38 pmPlease do ....I'd like to get BL on my 3457A , but would like a few hints (pics) to mechanical mounting and where to get the led power from.TIA/Bingo