[Test-Equipment] HP 3468A Question

[k2cby at optonline.net]

I have an HP 3468A DVM in which the Auto Zero function doesn't seem to work.
The unit executed "Self Check" with no discrepancies shown.

In the dc volts mode with no connection to the input terminals the screen
shows an indication of several volts either plus or minus.

If I short the input terminals, the meter shows zero within a microvolt or
two.

Still in the dc volts mode, if I shunt a resistance substitution box across
the input terminals the meter still shows roughly zero up to 200k; above
that value the reading  continues to increase and at 10 Meg it is above "1
volt" just like it was where the input is open-circuited.
Connected to a voltage source with a reasonably low impedance, the meter
reads accurately.
I've got the tech manual, but the service section doesn't seem to discuss
this particular symptom.

Any ideas where I should start looking? I'd love to find a fix that doesn't
require recalibration or, in the alternative, an el-cheapo cal lab, since I
don't need an NIST certificate.

 

 

Miles B. Anderson, K2CBY
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Email: k2cby at optonline.net