[time-nuts] 10638A Degausser Update

[paul swed]

Congrats John. Lots of fun. The interesting thing is you most likely have
enough info so that anyone could build a degausser if needed. It doesn't
sound to bad.
Regards
Paul WB8TSL

On Fri, Nov 23, 2012 at 2:06 PM, John Ackermann N8UR <jra at febo.com> wrote:

> 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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