Re: Defective Power supply in HP8711a Network Analyser.

On Mar 11, 9:44 am, Max65 <mpor...@xxxxxxxx> wrote:
In the mean time, just got schematics from Ceasar by mail!!
I will try to find the ghost in the SMPS box....I will let you
know!

Great!
Ceasar is a great guy.

Let me know,
when you fix it, I can give you the mods to convert it to a full
loaded 8712 for free.
(If you want I can give you now, but it's better you fix it before).

Massimo

With the schematics by hand, it was not very difficult to pinpoint the
problem.
The SMPS of the 8711 is equipped with Under and Over-voltage lockout
circuitry; this is situated on a sub-PCB in the supply itself. This
OVL and UVL circuits are based on a common LM139 comparitor; this one
shows extreme bias on the inputs so treshold levels are wrong.
The simplest thing to do (IC exchange was difficult to realize) was a
complete disabling of the UVL and OVL circuit.
The supply works well and the analyzer is operational!
Thanks for the help !!!
Ivo.
.

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