577 repair

Craig Sawyers <c.sawyers@...>

Some of you might remember that a year or so ago I blew up my workhorse curve tracer. Shorted tant
somewhere, pulled power supply to test which rail had died, and then after repair, reconnected the
supply - with one of the connectors one pin out. Smoke came out. I was really miserable at my
stupidity, and parked the unit.

Anyway I eventually decided to find out what was dead. The only two transistors that were dead were
two dual FET's, where the gate was resistive on three of the four FETs. 2N3958, which fortunately
Linear Systems have remanufactured. Two 741's crispy and easy to replace.

As a precaution I'm replacing the op-amps and logic. The only difficult one is a long-obsolete 7454
- basically four dual input AND's into a NOR (and confusingly the LS part is not a direct
equivalent!); I thought about dusting off my Boolean and stitching together currently available
logic to synthesise the part (I did that in my HP counter), but in the end just bought the part from
eBay. Anyway all parts are now ordered, so short of collateral damage to diodes and passives I'm
keeping my fingers crossed.

I'll let y'all know how it goes.

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