[Test-Equipment] HP 403B pegging meters

[Richard Knoppow]

At 06:46 AM 3/13/2008, you wrote:
>  When I first got mine, and not knowing better, I thought that the pegging
>meter phenomenon was a sign that the instrument was broken.
>
>  So, I disconnected the original meter ( 100 uA FSD ) and, in its place,
>hooked up an external 0-500 uA meter, allowing for different meter internal
>resistances.
>
>  The external meter would go over to about 400 uA on power-up, settle
>towards zero after a few seconds, then do it again before finally zeroing.
>This seems to be normal.
>
>  Here's my best guess as to what's happening.  There's a big 1200 microfarad
>de-coupling cap in the DC negative feedback loop. This is probably necessary
>in order to get the frequency response down to 5 Hz. Until it charges up (
>thorugh an 8.2K resistor ) and the loop can correct itself, theres an
>initial rush of current through the meter movement. I still can't account
>for the second 'peg', though.
>
>  It would be interesting to know if anybody has devised a way to prevent the
>pegging. Perhaps using some component or trick that wasn't available back
>when the instrument was designed?
>
>Ken
     I don't remember if there was ever a service note about this 
but, since some of the service notes appear to be on line, its worth a look.