Good clarification, John - I assumed as much but I was in a bit of a rush and didn't explain myself very well last night! The back of the chip is internally connected to ground, so if the designer of the equipment didn't insulate it from the common heatsink, then the heatsink must be grounded as you say. I'd argue it's not ideal practice, and that insulating the chips individually is perhaps preferable. But best just reproduce what the manufacturer did, as it seems to work. Unless that was the cause of the failure of course....!

Well spotted regarding the contacts - they do look a bit iffy! Those are "turned pin" type sockets - high quality ones, which is a good start. But definitely worth following John's suggestion to check them carefully, or even replace them. That of course involves soldering :-)

Graeme