Hi!
What model no. is your amplifier?
Typical krauts, not an inch of silkscreen or other legends on the pcb to assist you with!!
Never mind, all is not lost!
Instantaneous power MOSFET failure in this type of switching supply very soon after switching on can be caused by the various low–value small blue WIMA rectangular film capacitors dotted around the large switch–mode power transformer going low in value or open–circuit, I recommend you remove and test all of these, and those pale green electrolytics don't look the best quality on the sticky–up L4981AD FC control module either – I'd turf 'em all out and put good–quality Panasonic FC or similar dark blue/gold ones of reputable manufacture in!
Be aware that another cause of instantaneous power MOSFET failure is floating or high resistances in the gate (left–hand lead looking at the device with the type markings towards you) drive circuits – this will leave unwanted charges in the gate which will prevent it turning off correctly during the flyback–part of the cycle, resulting in immediate failure!
Catastrophic MOSFET failure can also damage the L4981AD PFC controller, any signs of burnt or discoloured resistors adjacent to it should prompt you to throw it out – you can't rely on the safe operation of these devices if there's been a power–stage blowout!
Remove the MOSFET from the pcb and look at all the resistors, any diodes or smaller driver transistors near it for obvious damage or faults (please post again with more pics if you get stuck) – they may be the surface mount type so do please look at your main board carefully!
Finally, please be aware that power MOSFETs, like many of the older obsolete bipolar audio devices, DO get "faked" by unscrupulous F.E. vendors, a typical warning sign is also sudden and catastrophic failure immediately power is applied or very soon afterwards – the best way of gaurding against this is to only order from RS, Farnell, Mouser or Digi–Key!
I have had a look to see if there's any sort of service manual, schematic, etc., and there doesn't seem to be a sausage!
Please go through your board carefully (a good DMM and one of those cheap Chinese LCR T4 testers will do for this!), paying attention to the components & points I made above, and please do come back if you're still stuck!
Chris Williams