[volt-nuts] Fluke 887A Modifications

[Randy Evans]

I recently picked up a Fluke 887A AC/DC Differential Voltmeter.  It kind of worked but it wouldn't zero completely. Suspecting a faulty chopper relay that is unobtainium, I decided to repace the chopper circuit with a Linear technology LTC1050 (simply because I had one available).  I ripped out most of the existing chopper amplifier and replaced it with a small perf board chopper circuit with just a few components.  I had to make a couple of changes in the 887A output circuit to stop some oscillations plus put a resistance in series with the -18 V to the _5 volt regulator circuit for the LTC1050, but now it works great.  Extremely stable, no noticeable noise on the meter, and seems very accurate compared to my 34401A DVM.  I plan on doing a complete calibration in the near future but it is fine for now.

If anyone is interested, I can forward the modification circuit to them.  Just send me an e-mail request.

Randy Evans