OK, so I bought a broken Fluke 87 off eBay. I could see in the pictures the low-battery symbol was on, so I took a chance that it only needed a battery. It did, but it had other problems as well, mainly pertaining to the milli-volt, ohms and diode test functions.
On inspection, I could see it had been worked on before - the LCD bracket was missing one edge holder and another was wobbly, so obviously the digits weren't there unless pressure was applied. OK, I can fix that later.
With no battery, the input resistance at the volt/ohms jacks was only about 400K on milli-volts, and 3K or so in Ohms mode. With no leads inserted, about 140 mV showed on the display, and in Ohms mode, about 138 ohms, IIRC. The unit could not supply the standard 1mA current output in Ohms mode. The circuit board appeared uncontaminated but I had not ruled out a problem here
The service manual, as good as it is, lacks certain things, like voltages at the pins of U4, a clear explanation of reference grounds for testing and repair (that I found anyway) so I spent time here and there over several days becoming more conversant with what the schematics were telling me, and testing various internal protection components. The MOV's, diodes, transistors etc were all ok. I was on the verge of condemning U4, but I really wanted to prove it was bad before doing that.
Additional poking around, opening the circuit by lifting the 1.5K RT1 thermistor, suggested to me the problem involved the switch itself.
Long story short the switch had been re-assembled at some point in the past - with the small black plastic shaft and it's associated black plastic switch / feeler assembly on the rear side of the board, precisely 180 degrees out of phase, such that the deposited carbon tracks were the source of my leakage. Positioned the black plastic shaft in the opposite orientation, snapped the rear switch back in place, all functions now work as intended.
A good learning experience and I got a "fair-to-good" condition Fluke 87 for 75.00 freight included.
Aside from me getting to brag a little bit, I hope this information may be helpful to someone. There was really no indication the switch was wrong or that anyone had messed with it.
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