SMPS making click noises and randomly working

[ratatax]

Hi,
I have the power supply from an old EMU synthesizer that doesn't work properly. 100-240V AC IN, outputs +5V, +12V and -12V.

When I turn the unit on, it repeatedly does clicking noises. The output voltages are coming very shortly in between each clicking sound (it clicks about 5-10 times per second, very quick).

If I unscrew the power supply from its chassis, or put pressure on it while it's screwed, it starts working correctly and once it has started it keeps working normally with perfect voltages.

For me it screams some sort of cold solder joint, so I redo all the joints but nothing changed. Connectors aren't loose. Everything looks fine on the board, no burn marks, no loose component, no bulging capacitor.

I'm not sure where the clicking noise comes from, there is no relay.

Also it behaves the same no matter if I load it or not.

I don't know where to start for this troubleshooting... Any ideas ?

[George Edmonds]

When SMPS's start clicking it usually means that it is overloaded, so check all of the components around the output

George

[ambrosia heart]

" put pressure on it while it's screwed, it starts working correctly"

Obviously, cracks on PCB, you should find out cracks with the help of magnifying glass. 

[Runco990]

I would agree on a bad connection too.  Though I'd check C108. 

[ratatax]

I replaced C108 without success. No apparent crack on the PCB, but I know they can be barely visible sometimes... The power supply failing only when it's screwed into the chassis make it diffcult to test anything with the multimeter.

[Audiorepair]

Check all the TO220 transistor legs, I've come across some in the past where they have fractured, but are still in intermittent physical contact.

There is one without a heatsink you might check first.

[stojke]

Re-tin the ground points.

[shakalnokturn]

Adding a few comments to things that have been mentioned:

When SMPS's start clicking it usually means that it is overloaded, so check all of the components around the output

George

True, that could be a sign of a leaky secondary diode or a bad capacitor.
A dead short will usually not allow the voltages to come up to normal even briefly.

Check all the TO220 transistor legs, I've come across some in the past where they have fractured, but are still in intermittent physical contact.

There is one without a heatsink you might check first.

Although I've seen this happen on old and thermally stressed transistors on this issue I'd look more towards the components on the PWM auxiliary supply including:
Broken transformer wire right next to transformer pin, bad contact in diode (the one between transformer and transistor heatsink), CR104.

If output voltages are generously overshooting while it tries to power up, check on opto and regulation loop in general.

[asis]

Hi,
I see that next to the varactor VR201 there is some kind of SCR thyristor.
If so, then it opens when the voltage on the + 5V channel is exceeded and bypasses the + 12V channel,
to prevent this critical excess of + 5V. (or even blow out the mains fuse for warranty).
It seems to me that the thyristor is clicking.
Check or replace ECAP C108. SU40P31L.pdf (198.35 kB - downloaded 54 times.)