Steve,
There has been a guy advertising on ebay for probe repair kits off and on.
Search on hp 8405a
73
Gary K4FMX
Steve Thompson wrote:
> On Monday 28 February 2005 08:34, Ian White G3SEK wrote:
>
>>Bill L. Fuqua wrote:
>>
>>>The HP vector voltmeters are nice but often the probes have been blown
>>>away due to application of either DC or excessive signal. Also,
>>>adapters are very hard to find for them. The probes themselves have a
>>>sharp tips that are easily broken off and delicate matched hotcarrier
>>>diodes inside. But if you can find one that works they are great! I
>>>have one myself.
>>
> I have a feeling that (electronic) repairs to the input probes might be less
> difficult than mythology suggests. Matched diodes aren't difficult to come by
> - four bought loose will almost certaily come from the same batch and
> packaged quads are readily available. The other warning I heard was about the
> special high frequency JFET that's used after the diode gate. I've been
> reading the service manual, and I reckon that FET runs at the 20kHz IF - it's
> the diode gate that's the mixer and the 22pF cap at its input that does the
> integrating. If I'm right then a 2N3820 or J117 will do ok. I have a 8405
> with blown probes so I'll report back if/when I find out. If anyone's already
> been there, please let me know!
>
> G3SEK wrote:
>
>>I used to have one too, but let mine go after building N2PK's
>>50kHz-60MHz Vector Network Analyser.
>
> The N2PK thing is very nice, but for those who aren't familiar with the kit,
> the HP 8405 VVM goes up to 1000MHz.
>
> Cheers,
>
> Steve
>
> PS another good one to look out for is the Rohde & Schwarz ZPV
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