Voltcraft VC270 multimeter with faulty resistance reading

[zionlion]

Hi everyone,

for the last couple of days I have tried to find the issue and repair my good old Voltcraft VC270 DMM. The problem is that it cannot measure resistance properly anymore. I get roughly 1 kOhm without anything connected to the leads. V DC, V AC, mV DC, mA and 10 A modes I have tested and they all work fine. When I first opened the DMM, there was quite some dirt inside, I guess from the manufacturer not removing the flux, so I gave everything a good cleaning with IPA and Q-tips. I also cleaned the rotary switch, which showed some marks from turning. This however did not solve my issue. Of course, I also tried a new battery and checked the 10 A fuse. In an attempt to nail down the problem I tried the following measurements:

displayed resistance on VC270, nothing connected: 1110 Ohm
displayed resistance on VC270, leads shorted: 0 Ohm

Using a working DMM I measured for the VC270:
resistance between COM and V, no battery, switch to ohms: 1870 Ohm
resistance between COM and V, no battery, switch to V DC: 14 MOhm (increasing over time)
resistance between COM and V, battery inside, switch to ohms: 10.1 kOhm

resistance across R2: 900 kOhm (as it should have)
resistance across PTC1 (WMZ11 75HV on bigger bubble, 75S on smaller bubble): 760 Ohm
resistance across transistor (S8050 D331): 1102 Ohm
resistance across R36: 434 kOhm (as it should have)
resistance across PTC2 (3 times KT600 160B S0908): 2 Ohm

So it seems as if I get the resistance of the transistor displayed when nothing is connected to the leads. Strangely enough the transistor has two legs connected (base and probably emitter) and it looks like this is supposed to be like that. Does anyone have a clue what could be the problem with my multimeter?

Thanks already in advance!

[zionlion]

some more pictures

[InductorbackEMF]

Hi, a common thing with most voltmeters is the input clamping transistors get leaky after a transient check that large transistor first.

[zionlion]

Hi InductorbackEMF,

thanks for your suggestion. Do you mean Q4 (see picture 3 or 13) with clamping transistor?

cheers!

[zionlion]

Hey InductorbackEMF,

you were totally right, thanks so much! The transistor Q4 is the culprit. I removed it from the board and it had 14 MOhm resistance from base to collector, but only the aforementioned 1100 Ohm between base and emitter. I tried the DMM without the transistor and it now shows the correct resistance values. I will try to get a new transistor the next days. Eventually, my suspicion with the transistor was not so bad. Thanks for confirming.

cheers

[InductorbackEMF]

Hi sorry for not responding i forgot );
Glad to hear it went well!