Mesa 2:90 left channel issues

[MightyBassa]

Morning Everyone

This is my first post and it's out of utter despair!

I picked up a knackered 2:90 over a year ago with the intention of using it for home audio with the amp in line mode from my hifi the preamp.

I cleaned up the stinky pcb, cleaned up some solder joints, recapped it and got a new set of valves/tubes installed.

The right hand channel has always had a hum that's audible at anything over 1/4 on the volume pot, getting louder as the volume goes up.
The left channel has always been OK up until 1/2 volume then the hum would get louder with more volume.

Wanting rid of the hum altogether and not really wanting to mod the amp with 100r resistors on the heater centre tap( assuming the hum was due to the heater line), i sent the amp off to a professional who appeared to fully understand the convoluted feedback these amps have...

The amp came back a week or two ago and upon connecting it all up and switching it on( in standby ofcourse) there was a loud bang and a spark from somewhere towards the right hand centre/rear of the amp, near where the two pair of caps of sit.

So i turned the amp off and thought id tend to it when i had a clear headspace to deal with what had become quite an expensive paperweight.

A few days later without remembering what had happened when I got the amp back, I powered it up and the right channel sounds as it did before, clean and clear with with that annoying hum that made itself known when pushed over 1/4 volume...but the left channel had low output and sounded really distorted.

I changed all the valves out and it sounded exactly the same, then i remembered the spark/bang and looked over the pcb for signs of charring/ exploded parts and it all looked fine.

I powered the amp back up and found that with none of the sound modes engaged(deep/half power/modern) the left channel made a horrible high pitched noise, enabling any of those modes stopped the noise, disabling all those modes and turning up the presence to anything other than off on that channel also stopped the noise, but the channel still sounded awful.

Would I be right in thinking something is wrong with the feedback circuit on the left channel?

Sorry for the lengthy essay, I thought it best to be as detailed as possible with regards to the functionality

[Jon Snell]

The wires connecting the standby switch and PCB are dangerously close to the chassis and the choke. This can spark if there is a bad solder joint under the board. Unfortunately the PCBoard must be removed off the chassis to see it, as one does to replace the four main smoothing capacitors.
Doesn't take long to do but a b1tch of a job. This is a photo of the issue.

Hum problems can be the selector setting to instrument instead of source or line. On the rear.

[Helmholtz]

Do you get the hum with nothing connected to inputs?

Replacing the heater CT with balancing resistors shouldn't make a difference at all as in both cases the center of the heater voltage is grounded.

[MightyBassa]

Thanks for the replies gentlemen, I'll get the pcb out later and check for any errant scorching etc.

Helmholtz; the amp hums on channel 2 whether there's any input or not