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Keithley 175A not working

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MikeK:
So I got a Keithley 175A multimeter off eBay for cheap.  It powers up, but won't properly measure anything.  I found that the 6.4V reference Zener diode only measures 4.47V.  This could be the problem.  But before ordering this part ($8 plus shipping) I'd like to do more testing.  Any reason not to do either of these?

1. Remove Zener and apply 6.4V with a floating power supply.
2. Or, remove Zener and use 5.1V Zener (1N4733) plus two small signal diodes (1N4148) to achieve the 6.4V.

The problem with the Zener solution is that the datasheet test current for the 1N4733 is 49mA, which is much more that what the 6.4V reference Zener is spec'd at (2mA).  And indeed the reference Zener appears to run at 2mA in-circuit (based on the math).  I ran the three-diode chain at 2mA, however, and it is almost dead on 6.4V.

Is the Zener solution reasonable to allow me to do further testing?  What could have caused the reference Zener to fail in the first place?...I know the multimeter is probably 30-40 years old, but there is a 5.1V Zener on the board that tests dead on 5.1V.

Attached is the part of the schematic showing the reference Zener.

Kleinstein:
The zener combination would be OK for testing, even if not very stable (low TC).  It is very rare that a 6.4 Zener difts towards 4.5 V. Chances are there is a different fault, that causes a low voltage.
A first point to check would be the supplies, if there is excessive ripple (e.g. due to failing electrolytic capacitors).   Another possiblity would be something loading U103 too much.

MikeK:
Yeah, I thought the failed Zener was odd, especially given it's low operating current.  All the supplies tested good, though...Except for the ones involving the 6.4V reference Zener of course.  I'm going to remove it and test it out of circuit.  And I think I found confirmation for the 5.1V Zener in the datasheet: Izt2 appears to be the minimum current, which is 1mA for the 1N4733.

Kleinstein:
The 1N4577 ref. zener is supposed to run with 2 mA to give a low TC. It would work with a higher / lower current, but normally with a higher TC.
The 1N4733 is a more normal low cost zener and the test currents in the data-sheet are more like example values. The 1 mA are more like a common point for comparisons not a real minimum for the diode.

Testing the diode sepately is a good idea, though unsoldering is some stress to the diode. So this may effect the calibration.

The +5 V supply in the K175 is quite important. The ADC raw conversion result is relative to the +5 V supply and than a seprate measurement of the -2 V ref. value is used to measure the scale factor ( 5 V supply relative to the -2 V ref and some resistor ratios). So a noisy 5 V supply can cause quite some extra noise. It is only a 4.5 digit meter - so it may be good enough.
IF the -7 V ref. part is way off this can cause quite some trouble for the ADC, as this is used to shift the zero.

MikeK:

--- Quote from: Kleinstein on June 29, 2022, 04:28:13 pm ---IF the -7 V ref. part is way off this can cause quite some trouble for the ADC, as this is used to shift the zero.
--- End quote ---

It is indeed way off.  It measures -4.5V instead of -7V.

How about if I inject current at that op-amp output?  I could apply 5.1V across the output Zener, but allow for more current.  If the voltage across the reference Zener then picks up I'll know that something is loading that op-amp.  Good?

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