[volt-nuts] Fluke 5200A instability...

[Chuck Harris]

Before I spend a lot more time on it, does anyone have any experience
with the Fluke 5200A's feedback loop oscillating when in the 100Hz band
position?

Mine started to oscillate at about a 1/2Hz rate a while back. It is two
to three orders of magnitude better when in the 1KHz and higher band
positions.

I can tell it is oscillating because if I watch the 3 least significant
digits on my 3456A voltmeter, in the AC position, they keep repeating
over and over and over... sort of fits a 1-2-3-1-2-3... count.

I have replaced the bad capacitors in the power supply, and on
a few of the boards.  I have checked the supplies for voltage and
ripple, and AFAIK they are ok.

I tested the 2uf mylar integrator cap that is employed while in the
100Hz band, and replaced more than a few carbon composition resistors
that were between 20 and 50% out of tolerance with no apparent affect.

The carbon composition resistors I replaced were used to digitally
adjust the gain of the integrator, so seemed likely to have an effect
on the stability criterion of the feedback loop..

The DC Reference voltage seems to be very stable, and can be selected
up and down with the voltage selection switches.

I have tested all of the tantalums for shorts, and found none.

I am running out of ideas.

TIA,

-Chuck Harris