Help repair a power inverter

[xeclipse]

Hi guys,
I got this 300W inverter that 'just quit'.I opened it up to find two 20amp fuses blown.I replaced the fuses and now i measure 250V dc after rectification.I measure 170 V ac on the output but drops to zero on connecting a load.
The irf740 output mosfets look ok,ie no burn marks,no short on the pins,they do not heat on connecting a load.
Another thing i almost forgot:there is a sound coming from the inside (near the output),a vibration that is clearly nowhere near 50hz(<50hz) on connecting the load.
what could be the problem?
Regards.

[Andy5112405]

Repairing these is VERY HIT AND MISS. Quite often they are designed to a strict budget and the circuit is only (sometimes) designed to prevent damage to its outputs.

Quite often the original failing component is the output rectifier diode, this can and often does take the transformer out with it. As the transformer fails so do the switching MOS-FETs, etc etc. FRIED.

Unless you are experienced with SMPS repairs, leave alone and replace.

[olmec]

I have a 12v -> 240v AC inverter that also works as 12v battery charger.

Recently it stopped working as an inverter, still works as a charger.
When in inverter mode it makes a pulsing beep. Power LED and fans go fine and when switched off the power LED stays on for a long time.

Measured output at AC plug with volt meter shows fluctuating voltage less than 1 volt.

Any ideas or hep appreciated