Kaz Kylheku wrote:


Reported as consistent intrusive mains hum with any settings of
controls, but of course with me it is perfect.


Thus, a likely hypothesis is that it is picking up some EMI in its
regular location, with the regular instrument and other signal chain
components that it is hooked into: stomp boxes, and their power
supplies, cabling.


** Long as the amp exhibits no AC frequency hum when used alone, it has no fault.



Step one is to reproduce the problem. Get the user to demonstrate it,
if possible. Re-create some of the original conditions, if necessary.
You can hardly be effective in investigating a problem that isn't
reproducing.


** Of course, but few customers are capable of comprehending even that simple idea.


Not to mention fixing it; how can you be sure something is gone, that
you never observed!


** Of course, but having outlined a perceived fault customers expect it to be fixed and never return.

Only thing is to write on the invoice:

" Fault not seen, not fixed and not warrantied "



..... Phil