Tillquist DT-447 TRUE RMS / Model 187 Fine Instruments Corp

[wiss]

My first multimeter, bought in -93, suffered some damage some time ago.
As I remember I plugged it into  230VAC when it was on the ohms-range.
Now it only measure low values of resistance, Infinity is currently mapped to 2.3 kOhm.
Does anyone have any clue to what might be damaged?

[retiredcaps]

Does anyone have any clue to what might be damaged?

I would check the PTC (just right of VR1).  Measure the resistance of it and report what you find.  I'm guessing a good one will be in the 1,000 to 1,500 ohm range.  If yours reads something like 100,000 ohms, it is likely bad.

If I'm correct, you can replace the PTC with a normal 1,000 ohm resistor for test purposes.

[wiss]

The PTC measures 1.1k, but that got me looking at the traces, the inside end of the PTC is connected to a diode-bjt (Q2 PN2222), after replacing that one with a bc546 it now works again! 

[retiredcaps]

Good job finding the problem.  Was Q2 shorted?

[wiss]

I didn't measure the transistor.
Before it only went up to 2.5k with open leads, now I could measure a 500k resistor and it goes overload when open.
This one is using low voltage in resistance, a parallel diode should not influence the resistance measurement (according to the manual).

[Jon86]

Well there's your problem, rubbish multimeter. It doesn't pass Daves '230V on the Ohms range' test

[wiss]

  It actually looks quite a bit more professional on the outside than on the inside
Yes, it is re-branded crap! But not the worst I (we) have seen.

[wiss]

I went to my parents this weekend, and what was lying on the kitchen bench when I got there? The manual for the DMM!
It claims to be tested according to IEC Publication 348 (whatever that is) and be able to take 600 VDC or Peak AC on all ohms-ranges as well as Beeper and Diode ranges.