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Fluke 343a DCV calibrator fix: The Good,The Bad and The Ugly

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onlooker:
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:

SeanB:
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.

casinada:
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. :)

kripton2035:
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.

bingo600:
@onlooker


Nice work with the fluke  ;)

But from my point of view ... Even nicer work with the backlight of the 3457A    :-+
I just got one of those , and would love to see some pics and a description .... So i can do the same ...

TIA

/Bingo

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