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Re: [Amps] help diagnosing a bad 8450a probe ?

To: Steve Thompson <>
Subject: Re: [Amps] help diagnosing a bad 8450a probe ?
From: Dan Sawyer <>
Date: Fri, 09 Dec 2005 07:11:47 -0800
List-post: <>

I have a cross match for HP2800 and HP2835 to NTE parts 112 and 484, 
they are available locally. What characteristics should I look for? In 
this circuit as long as they  match will they work in this circuit?

73 Dan kb0qil

Steve Thompson wrote:

> 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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