[Test-Equipment] HP 8405A For Sale

[Richard Knoppow]

----- Original Message ----- 
From: <k3hx at juno.com>
To: <test-equipment at mailman.qth.net>
Sent: Sunday, March 29, 2009 1:23 PM
Subject: [Test-Equipment] HP 8405A For Sale


> HP 8405A Vector Voltmeter for sale.
>
> Does not work, I was told there are special transistors in 
> the probes
>
> that are shot and I now lack the fine motor control to 
> repair them.
>
> Unit comes with a pair of the special BNC Tee adaptors and 
> a copy of
>
> the parts and service manual.
>
> $50 firm pick up only north of Pittsburgh, PA within 5 
> miles of
>
> turnpike exit 39.
>
> Be Well,
>
> Tim
>
     I used to work in these guys for -hp-. I am not sure 
about the transistors but a common failure was burning out 
the sampling diodes. I think these are Shotkey type and 
similar diodes are probably available. Replacing them 
requires the use of low temperature solder. We used silver 
solder and a tiny soldering iron. The alignment following 
the diode renewal requires re-balancing the diode circuit. 
This is done with a pot in the probe to minimise the leakage 
of the sampling signal at the probe tip, not at all 
difficult.
     When the 8405A came out the probes were supposed to go 
back to the factory for repair but, after looking at the 
circuit and talking to, I think, the designer, I found out 
it was really a quite simple prodedure and began to repair 
them successfult.
     You do need a steady hand and also, I am afraid, a 
magnifying glass. I have not done this job for some forty 
years so I don't remember the details. The chances of the 
transistors in the probe being damaged is far lower than the 
diodes being bad.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com