analog multimeter damaged ohm meter

Thread Starter

Research Survey

Joined Feb 27, 2016
16
i have this china made basic multimeter. i tested acv using ohm meter and i heard small boom. i opened up and found the fuse blown. i tested thr probe, acv, dcv and everything is working. except for ohmmeter. ohmmeter x 100k works but i suspect no longer accurate. but still i can adjust to 0 ohm. for x1, x10, x1k. all not working. i checked x1 resistor has very high resistance ,i replaced it with 20 ohms but still not working. for the left diode, it is showing always shorted both polarity. if i replace x1 resistor and diode, will it fixed the problem or are there any chance the transistor, capacitor and diode damaged?

also appreciate if u anyone can help me understand how the transistor and diode used in the circuit.

i did some research and found this circuit which is very similar to the circuit of my multimeter.
 

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sagor

Joined Mar 10, 2019
490
I don't see any transistor in the schematic, nor in the pictures. It is possible that the meter protection diodes, D3/D4 are also blown. Check all diodes. I don't see any burnt resistors in the pictures, so the solid state devices are the ones most likely blown.
Schematic shows a B2, another battery, which is not visible in the pictures. It is on the high resistance range of the schematic. It is possible the schematic is not exactly right for your version of the meter.
 

Thread Starter

Research Survey

Joined Feb 27, 2016
16
I don't see any transistor in the schematic, nor in the pictures. It is possible that the meter protection diodes, D3/D4 are also blown. Check all diodes. I don't see any burnt resistors in the pictures, so the solid state devices are the ones most likely blown.
Schematic shows a B2, another battery, which is not visible in the pictures. It is on the high resistance range of the schematic. It is possible the schematic is not exactly right for your version of the meter.
Thank you sir. I tried replacing the diode and it worked. Then x1 resistor replaced to make sure meter will have full deflection. By the way can you help explain how the diode protect the meter. On the picture upper left there is transistor connected to 0ohm adjuster. Not sure what is its purpose
 
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