I have some photos but got the amp home today. There are five screws which go through the lid into the amplifier itself and once removed the back has to be removed as well in order to gain access to the rear of the speaker. You need to remove the speaker wires and the gunk which stops the wires rattling about. Once that's done the amplifier electrics slide out backwards.
With the amp on the bench I measured the speaker's impedance and found it to be 6 ohms so selected a 6 ohm speaker for testing. I hooked it up and the power light came on but no output. I then prodded about inside with a plastic chop stick to see if anything obvious was loose. The large white rectangular resistors are a favourite due to their weight. The next step was to photograph the inside and make a note of where all the wires go before disconnecting the cables and removing both boards.
Looking at the back of the board it became apparent that there were a LOT of poor solder connections. The places that are normally an issue are the sockets and pots. I re-soldered all these and was still having problems. Looking through the microscope I found that one of the tracks to the volume pot had broken and so mended this using some solid core copper wire. So with that fixed it worked and I re-assembled it to find it not working again!
Next I realised that all the faders were also loose so removed that board and re-soldered all the bad connections there too.
Re-assembling it again it stopped working again when I screwed the plastic socket retainer in place. Taking it apart again I realised that all the sliders and sockets were oxidised so cleaned them all and the faders with dioxit.
Now it all fully works.....
These Marshall amps are more often not working due to bad connections than component failure.
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