Repairing Heathkit nixie multimeter

[rea5245]

Hi,

I scored a Heathkit IM-1202 nixie tube DMM at a flea market. I've cleaned it, replaced the power cord, re-soldered a broken connection, and I've been going through the calibration procedure. It read DC volts correctly, but resistances always show the meter's version of "infinite" (an overrange light is on and the display shows 86.3).

I'm not too good with anything non-digital, but based on the manual and the schematic (attached), it looks like +15V is supposed to go into the emitter of an MPSA20 transistor that acts as a zener diode, so that 7V comes out the base. That goes into the base of 2N3638A transistor causing a constant current to flow through it. That current goes through the resistor under test and the multimeter measures the voltage drop.

When I measure the voltage at the MPSA20's emitter, it's 14.3V instead of 15V. The voltages at the 2N3638A's base, collector, and emitter are also a lower than the schematic says they should be, by 0.7V or so. I can understand that would cause the resistance to be measured inaccurately, but I wouldn't expect it to be reported as infinite. Am I wrong? What could account for this?

It also brings up the question of why the 15V source is low. Is this the sort of thing old capacitors could cause?

And advice on troubleshooting and repairing this would be appreciated.

Thank you,
   Bob

[floobydust]

Given the age, I would replace all electrolytic capacitors, sight unseen. It's just less time and effort than troubleshooting them down to the component level, and there are only 7 totaling a few dollars.

The +15V rail is low at +14.3V, possibly due to open C117 or excessive load on the +15V rail, or bad 15V zener D14 1N4166A (1N4744A). I'd start with replacing the caps.
But the voltage low by 0.7V should not make the ohms function not work at all, unless there is too much AC ripple which again points to the cap.

With input leads shorted or a low value resistor in place, the A/D should not see "over" voltage on ohms function.
The function switch might have a bad connection. With the meter powered off check continuity from the input jack back to the A/D input (C101). Polarity switch must be (+) for ohms function.