A couple of weeks ago I posted about problems with my HP 10638A
degausser (that's used with the 5061 Cs units to remove residual
magnetism from the high-performance option tube).
My troubleshooting indicated that the problem was likely in the op-amp
that is used to generate an exponentially decreasing current to drive
the degaussing coil in the tube. The circuit is basically a 10uF/10Mohm
network on the input to the op-amp, with the output driving a Darlington
pair to feed 28V through the coil. A couple of large resistors are in
series with the current path.
The op-amp was driving full power, full time, into the coil. Thus,
rather than degaussing, it "gaussed"! It's clear that this had been a
problem for a while, as *both* sides of the PCB under the current
limiting resistors (which are mounted a half inch or so above the board)
were cooked.
The op-amp is an Intech A-233, a potted device with FET input. You
don't find those at Digikey, unfortunately. But a couple of the
wonderfully helpful people on the list gave me leads.
First, CharlesS pointed me to a message board where one of the former
Intech engineers had posted about their products. I contacted "wsj"
(name deleted for his privacy, since he's not on this list) and had a
fascinating chat with him about the history of the company and its
products. He also kindly reviewed the 10638A schematic for me, and
suggested that a more modern high-impedance op-amp like the OPA130 would
likely work with a few bits added (trim circuit and supply bypassing).
Second, RichMcC pointed me to plccenter.com, a parts liquidator which
showed a couple of A-233 units in stock. Now, in the past I've seen
places like this that have inventory and price lists last updated five
years ago, so I frankly didn't expect much. But amazingly, they
actually had two new-in-box A-233 units available, and they are now in
my hands (they have a $50 minimum order, so there was no point in
getting just one).
I soldered the new part in, and the degausser is now working just like
it should. After a couple of degauss cycles the 5016B/004 appears to be
within about 1e12 of GPS with the C-field set at the nominal value shown
on the meter reading log, so all seems to be well.
Thanks to everyone who jumped into the thread, and especially to Charles
and Rich for giving me incredibly useful leads. And, if anyone else
finds themselves in the same situation, I have a spare A-233 a couple of
OPA130s!
John
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