Another power supply repair: when it makes economic sense

In my previous article, I wrote that repairing power supplies is usually not cost-effective, and even gave examples of cases where it really does make sense. But I just remembered a simple power supply I worked on fairly recently that’s a good example of when a repair is worthwhile.

This example is quite different from the previous ones: the power supply is significantly less powerful, there’s more damage, and the repair is even simpler.

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So, a friend’s brother asked me to repair his power supply. It had failed because of power outages—or, more precisely, because after the power comes back on, the mains voltage can sometimes jump to around 260–280 V, and there’s no voltage protection relay in the house...
By the way, I strongly recommend installing an overvoltage protection relay at home. I’ve reviewed several similar relays made in Ukraine, for example:
DS Electronics
ZUBR-D2-63 — link
Novatek-Electro
RN-263t — link
RN-119 — link
EM-126T — link
VC-115 — link
EM-129 — link

I can recommend any of them, but it’s best to read the reviews first.

Today, we’re looking at a power supply repair on the control board of a kitchen range hood. This is exactly the kind of situation where a repair makes sense. Of course, you could replace the burned-out section with a 12 V power supply, but in my opinion that would not only be a hack, it would also be inconvenient. Buying a new board for a specific range hood model—or even just a particular series—would clearly be more expensive and complicated than repairing the original. Besides, power supplies that fail after a high voltage reaches the input often look much worse than this one.

I was brought the board, along with the remains of some components in a little bag.


Judging by what’s left, these were an electrolytic capacitor and a varistor—everything that remained of them.


Someone without repair experience might take one look and think, “It’s all over,” and the smell of burned components would only reinforce that impression :)
But let’s see whether things are really that bad.


The first thing to do is remove any obviously burned components. In this case, that’s the X capacitor and the fuse.


Then clean the circuit board.
I should point out that the manufacturer marked the value/type next to every component, which is a real gift for anyone doing repairs.


It’s a good idea to clean the underside of the board too, especially if it’s dirty. I also recommend that beginners in electronics repair photograph the entire process; sometimes it can really help.


The capacitor burned out and left a charred area on the circuit board. As I wrote in the previous article, carbon conducts electricity, so the charred area needs to be completely removed mechanically. No cleaning solution will help here.


This power supply is built around a Viper12A PWM controller. In my experience, repairs like this are relatively straightforward, especially since this is a common controller and is really cheap.

First, check for a short circuit or very low resistance between points 1-2, 2-2, and 3-3. Then check that there is no short circuit on the power supply output.
If everything looks good, you can try replacing the input capacitor and powering it up through a light bulb.


In fact, the power supply will work even without the input capacitor, as the example in the previous article confirms.

I applied power, and the bulb didn't even glow, so things aren't too bad.


There's voltage at the power supply output. Great.


The components that needed replacing were:
1 - fuse
2 - X capacitor
3 - varistor
4 - input capacitor
5 - PWM controller supply capacitor. It was still working, in principle, but I replaced it with a name-brand part since the board was already on my bench.

I couldn't find a 470 V varistor at home right away, but I did find a pair rated for 240 V. Together they add up to 480 V, so this is a perfectly acceptable replacement.


I soldered in the components, then made sure to clean the board and coat it with protective varnish.


And of course, I coated the top with varnish too, especially since the board will be installed in a range hood. I couldn't find a silicone cap for the fuse, so I protected it with heat-shrink tubing. This is necessary to avoid having to pick up fragments from all over the appliance later.

And that's the whole repair.


As you can see, the repair was relatively simple, though it still required inspection, diagnostics, component replacement, and a final test. The board looked scary, but nothing serious had actually happened, and the appliance has been restored to its original condition.

Why it made sense to repair it:
The parts for the repair were relatively inexpensive.
Replacing the appliance would have been impossible, difficult, or expensive.
The appliance has been restored and, in fact, is now even a little better than it was before.

Of course, someone will ask how much this repair cost. I won't hide it: 250 UAH, parts included. Since everything was done at home and the owner dropped it off and picked it up himself, there was effectively no delivery charge either. In my view, that's quite reasonable by today's standards.

I hope this was useful. I hadn't originally planned to write this up, so I don't have detailed photos of the process. Sorry.