[volt-nuts] Fluke 845A mystery

Will willvolts at gmail.com
Sat Feb 4 22:50:13 UTC 2012

This is actually not mine. A friend got it when he was in the army.
Now we know why they gave it to him...

Opto choppers in this one. The 845AB manual (page 3) from Fluke
website tells more (but not much) about the new version.

Thanks for the suggestions. I measured all the transistors and they
worked fine. I even removed the output isolation stage (which is after
the meter in the 845AB) but there was no effect.

When I replaced the internal +/-15V supply with a lab psu, an
oscillation of 0.5 Hz in the meter signal appeared. Clean sine wave
approx. 10Vpp.

So far all the electrolytics changed, zero potentiometer changed, all
transistors measured, DC points checked, chopper drive and output
signals/timing checked.

It must be easier to build a new one than repair this...


2012/2/4, Frank Stellmach <frank.stellmach at freenet.de>:
> Will,
> Following your description, any output deflection seems to load the
> +/-15V supply abnormally.
> After having a look at the buffer caps C117/118, I would suspect the
> transistor pairs Q113/114, Q203/204, or Q111/112 for having a strong
> mismatch of current gain, or a simple malfunction.
> (For the refs.: I own the old schematics from the 60's only)
> Btw.: As your unit is of younger vintage, does it still have the optical
> choppers with neon bulbs, or has this been replaced by a MOSFET chopper,
> as in the 332/335 calibrators?
> If the latter is the case: Do you have a schematic and would it be
> possible to get a copy, please?
> Frank

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