Gp12smx problem...help!

[coleye]

Hi, new to these boards but really hoping somebody can help. I have a GP12SMX 300 watt combo, loaned it to a friend who was running a small festival at the weekend. Apparently someone plugged the output from a bass head into the speaker out socket on the back of the amp (don't ask me how is could be confused) now the amp blows the mains fuse immediately it's switched on. I don't see signs of burning inside, but by what I've read on the net, there would seem to be a fair chance it could be the output transistors. Does anybody have any suggestions how to narrow down what the problem is? I'm gonna go now and disconnect all the boards from the power supply to rule out the transformer, then re-connect the boards one by one to try to isolate the problem, am I on the right track? Thanks for any help you can offer!
Update - disconnected all outputs from the transformer- fuse holds. Fuse still ok with cable reconnected to main pcb, so I'm assuming the fault is in the board with what I assume are the power transistors on the aluminium heat sinks. There's 3 on one heat sink and 2 on the other, is it worth just replacing them all and hope it was one of them? Any tests I can do to diagnose the problem?
Thanks again

[coleye]

Further update - it's fine with the 'PL1' power connector plugged in on the output pcb, but blows immediately with 'PL2' plug connected. I'm struggling to find the correct schematic for this amp, it's the 300 watt 1x15 combo, model number on the back is 'T0661/472/398', 230v version. Anybody got a schematic available?

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[One shoe]

You sound like a man with a decent knowledge of trace amps. I have an AH250 SMX that after issues with a bad connection between the transformer?? the big circular unit with a copper wrap around - the cacble running from this connected to circuit board or not as it happened. Thats all good but ive had it since 96 and it never popped but has the issue with mains cable 3 maybe 4 times. I've been told a 1mega -ohm transformer ser across the stanby either + to + or some other configuration. I just need a sketch or PDF to inform the guy who Ive passed the info from this forum. (some of it probably way out) If you can help I appreciate. I wouldnt know how to pass on items would the email I gave to forum runners have some set up??? Hope you can help. (first new amp)