Startimes decorder hit by power surge

I received a phone call from my neighbor to pass by his place to check on his decoder which he believed was hit by power surge.

I passed by and the decoder was brought to me but since I had no tools to open it up I decided to take it with me to my workshop.

Once on my workbench I opened it and I scanned the board (power and main board) and I could not see any suspect component.

They all looked very innocent, I pulled my meter and tested across the main fuse and found it open. Fuses rarely die of old age, there must a reason why and in most cases they die when there is a short on the supply and therefore I started testing components on the power supply which are directly connected to the main power line.

So I decided to start with the rectifier diodes and I found one which was shorted and this put a smile on my face, this must be an easy fix!

After replacing the diode and the fuse and applied the power using the series bulb current limiter (you can make one easily on this link), I noted the bulb was bright throughout.

With this I concluded there are still shorted component(s) I did not pick, I continued checking and I could not get any other component and this caused me some stress because I had marked this assignment as simple.

I removed the diodes one leg from the circuit one by one and applying the power via the series bulb and once I removed the last diode I noted the bulb was now dim (off) and I realized the diode was failing under load since I was not able to pick it using the meter.

That is why when you find one diode is shorted, it is advisable to replace the four diodes otherwise the chances of the customers calling back with the same problem is high.

  

I replaced all the four diodes and after applying the power via the series bulb, the bulb was bright and then dim and with this I concluded no more shorted components in the supply.

I connected the power board back to the main board and the decoder worked like a charm.

Thank you friends

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