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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