A technician in a medical company brought this power board for repair. While the technician in my workshop, I powered the board and I could hear a clicking noise coming from the relay. After few clicking noises, the clicking stopped and the power board was working fine.

The clicking noise was coming from that area where it is marked with the red square. It made sense because there is only one relay on that board and it was easily identified.

When I got closer to the relay I found a big surprise. Take a look

The relay is located next to a resistor that is 47 ohm 5 watt. These kinds of resistors produce heat and it seems like it was very close to the relay that caused a big hole there. The relay was still working fine but it had to be replaced. I took the old relay out and replaced one with the same specification. It was 12 volts with 30 amps rating. I did not want the resistor to be close to the relay, so I extended the legs of the resistor to be higher than the relay itself keeping the heat away from that part.

I powered on the device and to my surprise, the new relay that I just installed started clicking faster and never got to a point of stopping and the board failed to produce power. A relay that is clicking is due to the fact that the source of power to the pins of the coils is not steady and I believe it was a capacitor issue.

While I was checking the board, I came across few parts that needed replacement.

This capacitor was open and it was replaced. I also found another capacitor that had to be replaced as well.

Shockingly after replacing these faulty capacitors and powering up the board, the board worked fine for about few seconds and then, the entire board went into shut down. The relay was not clicking anymore but there was a sign of power on the power stand by light on the board.

That is the transistor that feeds 12 volts into the one of the pin of the relay. When I tested for voltages there, there was nothing. I took it off board and tested, it was shorted. The part number ZVN4206 and I replaced it with IRFZ20 which not an SMD transistor but it worked fine. Below are the specs of the transistor:

I powered the device, the LED light on the other side of the board came on and the relay clicked once and everything seems to be working fine.

Mission Accomplished.

This article was prepared for you by Waleed Rishmawi, one of our ‘Master Authors’ and currently working in the Bethlehem area of Palestine repairing electrical and electronic equipment.

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Note: You can check his previous repair articles in the link below:

https://www.electronicsrepairfaq.com/a-deserted-calculator-printer-brought-back-to-life-model-casio-dr-t120