On Thursday, July 30, 2009 4:21:31 AM UTC-7, Avionics Tech wrote:
On Jul 30, 4:56*am, JW wrote:
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. ...

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.

Unfortunately, Fluke puts a lot of thought into their products. The
only electrolytics on this unit are the large ones in the power supply
busses. There's 8 of them, 50,000uF.

Actually, there ARE electrolytics, on the regulator board, other than those
big 'uns. I'm not sure why R114 and R115 would fail, but they supply
base current to the final transistors (Q103...Q106). So, could you
monitor test points TP3, TP4 and TP5, TP6, to see if the emitter currents
are in balance? If a final goes bad (low beta) the good one paired
with it would overheat, and then the opposite pair would have
to drive the leakage current of the hottie as well as the load.