Currently my JVM410HJS squeals periodically. Initially this would only happen occasionally and I was still able to gig with it, but now it has become to frequent to use. It will do this even if unplugged from a guitar and will do it no matter which channel it is on. Channel switching does not stop it. I am not sure where to start. Am going to change out the tubes and reset the MIDI switch, but otherwise will have to travel a good distance to find a qualified tech. Anyone else ever had this issue?
With regards to your JVM410HJS, we would recommend checking the pre amp valves and if they are not a fault it could be the input socket not closing down correctly.