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