mg100hdfx humming

[engman]

When turned on amp it made loud humming noise. Checked cables, turned off and on, changed outlets, and still had the humming. I slapped the head and it quit the humming and started working fine. Turned off and on several times over the next few hours with no problems. The following day I turned it on and had the same humming again. I hit the head again and the humming stopped. Have been turning on and off over the past few hours and seems to be fine. What could be causing this? Anyone have any ideas? Thanks.

[Enzo]

Broken solder on a main filter cap comes to mind.

[engman]

Thank you Enzo for the reply. 
Does anyone know of any easy way to check the PCB for a loose/broken solder point.

[Enzo]

One can grasp each filter cap amd wiggle it.  If the hum comes and goes when you do that, you have a loose connection of some sort.  A loose connection is usually cracked solder, which we simply resolder.  But the wire lead from the cap might be broken, or the copper lines on the pc board might be cracked.

An even simpler test would be to ball up your fist and whack the top of the amp - no disassembly required.  If that stops or starts the hum or indeed if the amp reacts at all, that tells us some connection is loose inside.  Turn the reverb down when you whack.

One might also just bite the bullet and flip the board over and resolder them regardless of how they appear, fresh solder can;t hurt, and while under there, look close at or resolder the controls and jacks.

While the amp is running, use something insulated, like a wooden chopstick, to push on each part on the board, looking for any that react.

[engman]

Thank you Enzo,

As a manufacturing Plant Engineer, I usually don't have time to do much PCB troubleshooting. I usually just yank the problem one out and put in a new one so the machine is up and running again. Obvious problems like burned or broken components are easily repaired and put on the shelf for spares but intermittent problems are hard to find and often not worth the time/risk to troubleshoot/repair.

However, since this is my own personal amp, I want to repair it.

I will be following your advice and let you know within the next few days exactly what I find.

Again, thanks for your help. It is much appreciated.