HM305 power supply repair

November 10, 2022 - Reading time: 2 minutes

I bought a cheap power supply off of Amazon only to find it was dead on arrival. It was long passed the window of return, so let's see if we can fix it.

Setting the scene

The power supply is a Hanmatek HM305, but you can find it under various Chinese brands. It claims an output of up to 30V at 5A. This particular model caught my attention because it has an output ON/OFF button separate from the power switch, so I can safely fiddle with the knobs without having to disconnect the load.

Unfortunately it had been sitting in a box for months, and when I finally got around to testing it was dead as a Norwegian Blue. Powering it on did nothing but make a quiet clicking sound, about once or twice per second, which stopped

Obviously something was happening inside there, so it was time to crack her open and see what's what.

(Lack of) Start-up

There are two bridge rectifiers, a large one going to the main storage capacitors, and a smaller secondary powering a switch mode controller IC, a DK112 (datasheet). Rectifiers check out, voltage looks normal. There is a small voltage present on Vcc of the DK112 (pin 4), but it's pretty low. According to the datasheet the chip requires about 5V to start up, after which it will run down to about 4V. So we're getting power, just not enough.

Time to check capacitors! The datasheet provides a typical application diagram.

There's a 22μF cap (C4) on Vcc which seems promising. Checking the board, there is indeed a cap on pin 4, rated for 47μF and 16V. Let's pop it out and take a closer look.

The manufacturer is Jackcon. The LCR meter shows a capacitance of around 26μF, well outside of the typical ±20% tolerance. Dissipation factor is very high. I had trouble finding a spec sheet, nothing seemed to fit the 4mm x 8mm form factor, but a lot of the 16V caps showed a max DF of 0.16, which seems reasonable. Obviously this is a dodgy capacitor.

I didn't have an exact match on hand, so I used what was available, a 100μF 35V cap from my bin. Quite a bit higher than the original, but that won't hurt. I tagged it in for testing purposes, it's the blue one standing proud of the board. I'll fit it properly if it works. Note the big black cap slightly to the right, soldered directly to the legs of the bridge rectifier.

Moment of truth, and we have success! Everything looks happy now, and I've got a spare power supply for who knows what. I'm just glad it's not going to end up in a dump.

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Datasheets: