I picked up a Valvetronix AD15VT used for $60. The only problem is that the channel button does not work. It sometimes clicks if I really push hard, but even that doesn’t do anything. I took the amp apart and didn’t spot any obvious way to fix the problem. Canned air did not fix it Does anyone have suggestions, or should I just pay a tech to pull out the boards and replace the button? (too many delicate solder points for me to not screw that up)