On Wed, 29 Jul 2009 18:12:25 -0700 (PDT) Avionics Tech
wrote in Message id:
:

We have a Fluke 5220A Transconductance Amplifier with a problem
that's been getting progressively worse over the years. It's supposed
to put out at least 20 amps, ac or dc, with light inductive loads.
The overcompliance circuit is tripping the unit out at around 19A DC,
and much less for AC, around 13A. These are both with loads connected
with minimal voltage drop across them. The loads, a shunt and coil of
wire, have been used for years with no problems.All supplies are clean
and stable. After troubleshooting for hours and getting nowhere, we
disabled the overcompliance circuit to see if it could indeed put out
full current. R114 and R115 on A7 burnt up (page 129,
http://assets.fluke.com/manuals/5220a___imeng0300.pdf ). We replaced
all the transistors on A7 although none were blown, re-enabled the
protection, and we're back where we were.. I'm curious why those
resistors burned. I'm not familiar with this type of amplifier
configuration. We checked, and the amplifier output is stable and not
oscillating. At this point, we'll take any input. Next up I'm
planning on replacing all the active parts on the driver board, I'm
tired of troubleshooting and wasting time, it's shotgun time.

I'm not at all familiar with that piece, but I've found that when a
problem gets progressively worse as you say, that usually it's one or more
electrolytic caps going high ESR. I'd start by replacing them.