[Jonny H]

No secrets here. This was traced to a solder joint that was contaminated during manufacture.

Yes, the output stage was damaged but on closer inspection we also noticed that one of the solder joints on the PCB had evidence of cracking. The joint in question is where the main charge capacitor is soldered through the board. But why?

Removing the solder revealed that there were traces of silicone compound in the ' through hole' where the leg of the capacitor was soldered. (We use silicone to bed down the main capacitor to stop it vibrating which would ultimately fracture the legs). Only a small amount is needed but it this case too much was used which was squeezed into the hole which prevented the solder from flowing into the hole and around the leg. From the top, it would look 'normal' hence passed visual inspection but only the top of the joint would be making contact. Over time, given the high voltage and energy passing through the joint it spark eroded the solder!

We have subsequently trained the assembly house to take particular care in that part of the assembly process.

Any faults due to manufacture are covered by the warranty.