[Test-Equipment] HP 403B pegging meters

[Grant, Ken]

 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


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