[Amps] help diagnosing a bad 8450a probe ?

[Steve Thompson]

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