MP-3314 multi-voltage plugpack - fails under load

[dkneyle]

I know these things are only worth a few dollars, so this is perhaps more an intellectual exercise in understanding what is (not) going on here.

The model is marked as MP-3314, sold in our consumer electronics stores, and is variable between 3V and 12V at 1.5A.

Under no load it shows as the appropriate voltage - set currently at 9V. But under load it fails, the LED flashes/blinks as it tries to restart. Meter shows volts come up but fall back with a fraction of a second, repeated continuously.

What would be a typical component that would fail, producing this effect?

Images attached of both sides of board.

By the way, have confirmed the device it is driving is OK. I have wired in my bench variable supply and all works 100%

[stj]

3v @ 1.5A
probably means 6v @ 750mA and 12v @ 375mA

whats your load and where is the primary's heatsink?

[R_J]

There is a small value electrolytic in the primary I would replace first, In your picture it is the one located next to the transformer below the mosfet.

[petehall347]

clean the solder side of the board and resolder the bad joints

[dkneyle]

Originally Posted by stj

3v @ 1.5A
probably means 6v @ 750mA and 12v @ 375mA

whats your load and where is the primary's heatsink?

It's driving one of these cheap and nasty Chinese all-in-one Hi-Fi (actually Lo-Fi) units with a turntable, CD player, radio. The specs on the unit say 9V @1500mA.

And what you see in the images is what you get. No heatsink to speak of.

I think you are probably correct. Simply can't supply the required load.

I found a 12V 2A plugpack in my junkbox and gave that a go. It powers up and runs. But there is a oscillation tone coming from the speakers that is reasonably prominent. Hard to image that 3 volts would make much difference? But I certainly didn't hear any of that when I was running it off 9V from my bench supply. Just cheap and nasty electronics all around I suspect.