Lee Jackson Metaltronix Amplifiers

[BM]

My VL 502 has a problem ...
When I when play through the amp without using the FX loop, or with even with using it the sound now fizzels to a shitty low crackly/breaking up type tone , turning the stand by on then off again brings it back to life...only to fizzle again after a few minutes, I am using Ernie Ball speak cables, all new Electro H tubes
do you know what this could be?
also, the amp hums, kinda like the sound when you tap the end of a plugged in guitar jack ...I see the hum adjuster in the back but I can't make out what kind of screw driver it takes...or even how to properly adjust it...sadly none of these things are covered in my manual