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

Tom Miller tmiller11147 at verizon.net
Wed Jul 29 03:12:37 EDT 2015


How about any aluminum electrolytics that might be used for coupling 
capacitors? I had a problem with a DC reference that had some aluminum 
electrolytic caps that were leaky and caused feedback problems.

If memory holds, they were in the 10 to 30 uF range.

Just a thought.

Tom

----- Original Message ----- 
From: "Chuck Harris" <cfharris at erols.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Tuesday, July 28, 2015 11:35 PM
Subject: [volt-nuts] Fluke 5200A instability...


> 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
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