Dan Sawyer wrote:
> All,
>
> I have an 8405a with a bad probe. The unit makes it through most of the
> calibration until the noise cancellation step. One board works and comes
> in at less then 5 mv. However one shows a 1V peak to peak distorted sine
> like wave. This probe actually works, however due to the fault it has a
> phase frequency error. This error is of course disaster for a phase meter.
>
> Does anyone have experience in diagnosing these probes?
>
> Does anyone have a lead as to what part to suspect?
Usually it's blown from overload - the four diodes in the sampling gate
and/or the p-channel fet go. There's a guy on ebay sells replacement
kits. I haven't got round to repairing the one I have, but I reckon four
shottky diodes from the same batch should work (HP2800 or 2835, for
example), and probably any general purpose p-channel fet. No promises on
that, but it won't bankrupt you to try. As an alternative to four
discrete diodes, I was thinking of trying one of the quad arrays you can
get in a surface mount package.
Steve
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