Capacitor in power supply problem?

[negativ3]

Hi all,

I am repairing an amplifier (2.1ch cheapo PC type) power supply and I think I have identified the mains rectification smoothing cap as having an issue. When I measure across it, I get 315V DC which seems correct, but only at the exact moment of measuring the voltage does the supply output start up. Does this indicate a bad cap in your experience? It is rated 450V, 47uF and reads 44uF out of circuit.

Thanks
Andy

[tautech]

SMPS I presume ?
Depending on just where you're placing the other DMM probe it could be the VCC supply storage cap or the HV DC dropper resistors as there's just enough current passing through you DMM to satisfy the IC's UVLO and kick it into action for the kickback winding on the primary side to take over and keep it running.

Identify the IC and get the datasheet for it and study how they are used and it should become clear. 

[negativ3]

Smells like an SMPS but I'm struggling to find the switcher...

MOSFET on the top of the board and a small SOIC on the bottom with marking 848L11 I cannot as yet decipher.

For sure the cap is ok, changed it out and still not functioning.

Any hints appreciated.

Thanks
Andy

[tautech]

Smells like an SMPS but I'm struggling to find the switcher...

MOSFET on the top of the board and a small SOIC on the bottom with marking 848L11 I cannot as yet decipher.

For sure the cap is ok, changed it out and still not functioning.

Any hints appreciated.

Thanks
Andy

Looks pretty small, can you confirm the package by measurement ? SOT-23/6 ?
You say 848L11, all on one line ?

Have a hunt for a resistor dropper for the HV DC. Maybe 2 or 3 in series for a total of normally 500k+.
Check any measure correctly.

[negativ3]

The hunt has begun  , see picture attached.

Than you.
Andy

[Seekonk]

Could you explain that.  Does the output power bump or does it work fine after it starts?

[negativ3]

It seemed to be working fine, now I have a dead short across the main cap location, after removing the cap, mosfet and transformer the short persists.

the mosfet and transformer measure ok out of circuit.

The main fuse may have provided protection but it did give me one hell of a suprise 

[negativ3]

Yea, so a stray leg of a clipped resistor was found under the dut. Totally my fault and a lesson learned many years ago but relearned again today.
The MOSFET, line inductor, and fuse were toasted. Replacements on the way and a scrupulously clean work surface has been arranged.