Modification on a cheap digital multimeter

The cheap Chinese digital multimeter that I had was quite good when it was new. But it was not able to last very long.

After a while, I found that the DC voltage selections were not showing accurately, so much so that I thought of buying an analogue multimeter to replace it.

Suspecting that some contacts of the selector switch were not touching, I decided to dismantle and investigate. Well, the electrical sliding contacts looked alright, and there was no sign of any oxidation coating that would very often caused faults like this.

Putting the selector knob back required some extra amount of care and dexterity. There was a ball loaded with a tiny coil spring that I had to be careful to put back in the required position. This was the mechanism that clicks and allowed the selector switch to remained in place when the required function was selected.

The multimeter did work for other selections, and I was somewhat puzzled. Was it the circuitry for DC voltage that was at fault? Somehow, I could not believe that only the DC voltage was giving faulty readings.

After a while, I noticed that if I pressed the circuit board against the selector switch, it often gave the correct DC Voltage readings.

With new found hope of solving the problem (the multimeter seemed to be working and need not be replaced, goody!), I decided to solve the problem by adding a spacer washer to press down on the circuit board permanently.

But the circuit board could not be pressed in place by just adding an ordinary washer. It must be a special washer which allows the surrounding circuit board to be pressed for about 1 mm, while allowing a screw to be tightened at the center.

Luckily, I had my own homemade movable bed cnc machine which I could use to fabricate such a washer. I used some phenolic board and milled it to the required dimensions that I wanted, and installed it in place.

And it worked!