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 <[email protected]> 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 > > ______________________________**_________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
