Hi - I recently picked up one of the original Mk1 units used and wanted
to share a little bit of troubleshooting I did to get it up and running
properly, perhaps someone may benefit from it.

In my case, I had two problems. At first, most test resistors showed
appox. double the correct value. I opened it up to check the calibration
and found that someone had shorted across R16, thus limiting the range
of adjustment for calibration. Removing that fixed the upper-range
calibration problem, but then I found that at the <1 ohm range it would
test my 1 ohm resistor as a 0.25 ohm resistor. Above the 1 ohm range it
would properly read a 2 ohm resistance.

Having narrowed it down to that, I found a reference mentioning that the
range scaling utilized Q3 and R6 (10K) at the 99 ohms range, so
conversely I concluded that Q5 & R10 (100) must be the low range. The
R10 measured perfectly, and I noted no short circuits so I swapped Q5
with a 2N3906 - fixed it on the spot!

This repeats a bit of the troubleshooting you provide, but here it is.

So I guess I'd summarize it with:
If you have a good calibration working on the 10-99 ohm range but the
0-0.9 ohm and/or 1-9.9 ohm range is not reading accurately:
1) Check for shorts/solder bridges on Q4 & Q5
2) Check resistor values of R8 (1k) and R10 (100)
3) Check C6 for a partial fault by shorting its connections. If the
reading returns to normal, replace.
4) Replace the transistor for the range that is inaccurate (Q4 for the
1-9.9 ohm range, Q5 for the 0-0.9 ohm range)


One of the next modifications I plan on attempting is changing the
seven-segment modules to a slightly brighter/more efficient module from
the same series.

Anyways, hopefully this is of help to someone else!


Regards,

Kiel Lydestad