Need help with IM-102 Digital Meter.

[suzanne grandy]

Hi All,

Hope someone can help. I'm trying to restore an IM-102 digital meter and
have a strange problem:  without any probes attached, the meter will
count negative for the two lowest scale ranges on DC (200 mV and 2.0V).
Counting stops at about - 0.3 volts on the 2V scale in about 30 seconds,
and it will "over-range in about 10 seconds on the 200 mV range.

If I attach probes, the two ranges will properly measure an external
test voltage, and the display will return to zero if I either short the
leads, OR just connect a high ohm resistor, such as a 1.0M-Ohm resistor.

I used an external Fluke meter and was able to read an increasing
negative voltage mysteriously build up on the input pin of the
integrator OP-amp (pin 3 of IC-1), without any probes attached!

I suspect that the integrator op-amp is bad, but I do not know what to
try to replace it with. The Heathkit manual indicates the op-amp is a
"SL11862". All I can tell from some old data books is that this was
(probably) made by Plessey Semiconductor (England).

Does anybody have an opinion of my diagnosis (leaky IC-1 op-amp) and
where can I look for a replacement. I don't even know if this is a FET
input or not. It's interesting that this meter presents an input
impedance of 1000 Meg-Ohms on the 2-volt scale!! At first I thought that
only a FET input op-amp could do this, but I noticed that an old
National LM108 series op-amp made with bipolar transistor technology has
an input bias current of only a few nano-amps.

There is one other strange problem: the "1" digit will not indicate on
the 2-volt DC scale for negative voltages only- works fine on all other
scales!! Seems like the same circuitry is used for all- so why not only
on the 2-volt scale??

I would appreciate any help on this- I spent about 15 hours over the
weekend completely tearing this meter apart (reduced it to about half a
kit) and throughly cleaned everything, thinking that the negative
drifting was due to leakage via the fairly dirty circuit boards and
switches- Afterall that- there was no improvement at all!! I hate to
give up now!!

Many thanks for any suggestions.

Mark Grandy
WD8RJJ

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