My lab is down, failed switch and how to fix it


Nearly all my test equipment on my workbench is connected to a switch, this was dead today when I wanted to do some tests.
I did not want to run around in the city to find a new switch and most dealers with good models are mail order only anyway, this means I would have to repair it (if possible).

I noticed that the power lamp flashed on shortly every other second, this basically told my where the problem was. The switch mode supply could not start, each flash on the power lamp was a try to start.


A few screws later I had the switch mode supply out and there is some bad looking capacitors on it. The three at the output each has a bulge on the top. This means the are very bad or on the way to it.
I took them out and checked them for ESR: 9.2ohm, 1.53ohm, 6.5ohm (Tester would not read capacity). That is very high values, the replacement I used had a ESR of 0.04ohm.
But when the switcher has problem starting another capacitor is often the culprit, it is the small electrolytic capacitor next to the switcher transistor. It is used for local supply to the switcher. I replaced it, but it was not that bad: ESR:0.77ohm 9.1uF, the new one I used measured about the same.
I did also replace the main smoothing capacitor, I did not have any 400V types in stock, but found on in circuit boards from my teardowns. The original also measured fine with ESR:1.65ohm 20.24uF.



Supply with the new capacitors.


I mounted it in the switch and tried to power it up, it worked.


A switcher that stops working due to age can be easy to fix, in this case I did not do any measurement (I first checked the capacitors after I was done).


A schematic and explanation on how these switchers work