Crucially, the first link mentioned fake parts. With this is mind I tried two more UK suppliers specifically asking that they guarantee their parts were genuine – both said they did guarantee this and guess what? Both failed. By this time we were wise to the explosions, so had rigged up the old light bulb current draw test and variac to avoid losing any eyes or gaining life-changing scars. Again, both suppliers refunded. Eventually (and this is where I hang my head in shame for it not being my first port of call) I contacted Marshall who were incredibly helpful. They told me that they themselves had experienced problems with supplies of this exact part and that they had located a source of new old stock to get around it. What is not clear is whether in some later revision to these chips, the unusual way in which Marshall implements them causes a problem. I guess it’s that or the fact that these were all fakes – though I can’t believe the fakes wouldn’t work in some application. We didn’t have the energy to test that.
The difference in appearance between the failing parts and the new old stock is pretty clear (ignore the cut legs on the good part):
Should you experience this problem, I would advise you to go straight to Marshall and they will provide you with a working component. At time of writing, they can be contacted as follows:
Lee Bates firstname.lastname@example.org
Marshall Amplification plc.
Spares Department telephone:
Special gratitude goes to Pete for his efforts on this repair – it really did chew up some hours whilst we tried to work out what was causing the fireworks.