[SOLVED] Fried ohmmeter circuit of Vellman DVM890F

[ro0ter]

Hi everyone,

I was the proud owner of an Vellman DVM890F but unfortunately I can`t seem to accurately measure resistors anymore. Actually it can`t measure resistors at all, I get LOTS OF NOISE when doing so.

After a couple of days I decided to open it and I digged in for half an hour. I then found that the dial corresponding to 200MegaOhm is toasted (burnt). I hope I will be able to fix it with conductive silver paint.

The thing is that I could not see anything on the circuit board, I carefully inspected it but I was unable to detect any other anomalies. No broken resistors, no swollen capacitors, no exploded transistors.

Please help me fix my multimeter, I really need it.

Thank you


Here, some pictures with my multimeter`s board and closeup of the problem. This is the first time I opened it (if changing battery doesn`t count), the white areas are from the manufacturing process.

 

[mister_rf]

That’s pretty hard to rebuild the PCB area, need to check(replace) also Q1 and D12, see the attached diagrams.

 

[ro0ter]

Thank you, mister_rf, I`ll try to verify Q1 and D12.

Q1 is a s9013h but unfortunately I don`t know the specifications of D12. If you have further details, I`ll be grateful.

The main problem is that it won`t measure any resistance (not just the 200ko). If I use a crocodile cable and directly link the appropriate terminals, the ohmmeter displays fictive noisy data. The continuity module works as expected.

I`ll get back with details. Thanks again,
ro0ter

 

[mister_rf]

D12 = 1n4148

 

[ro0ter]

Thanks for the quick reply. Couldn`t see what D12 is in the attached scheme.

 

[ro0ter]

Praised be mister_rf, for he just saved my ohmmeter.

Although the transistor was not fried, the diode was actually a simple jumper wire. I replaced both and I get EXACT readings.

It would have been a pity to throw it away. Regarding the fried part of the PCB, I was able to fix it with silver paint (Electon 40 AC from amepox). I hope it would last. Maybe I should have used epoxy conductive glue.

Thank you,
ro0ter

PS: I couldn`t read the components in the scheme you attached because I had a low resolution at home. I could clearly see everything from work. Need to upgrade my screen