Fender 1000 Roc Pro circuit issue

[The Magic Rabbit]

The guitarist in our band gave me this amp to clean up and see if I could get it going. There's a strange issue where it jumps channels and makes horrible crackling noises, which I've discovered happens when the amp vibrates, and if you tap it. This was initially discovered when the amp seemed to work at low volume but played up when driven, and also when moved.

I'm experienced in basic electronic circuits, can solder reasonably well, know my way around basic test equipment, and am sensible enough to give mains electrics the respect it deserves. I don't know much about amplification circuits etc. as most of my experience is with microcontrollers, digital circuits, and LEDs.

I systematically worked my way around the board and have identified that R173 (120ohm 10W resistor) seems to be the issue, as just touching it gently and rubbing the top (ceramic part) with the end of a plastic stick causes the amp to go absolutely crazy, jumping channels to and fro etc. quicker than you can blink. This didn't happen when any other components are gently prodded, tapped, or wiggled. I suspect maybe a dry joint, so I removed the board to resolder this joint.

Upon removing the board, I noticed some discolouring on the board. The discolouring lines up with what I believe are two 16V 5W zener diodes.

Should I be replacing the zener diodes?

Is it likely the fault on R173 caused them to overheat, or is this an indication of a separate issue?

Should I be replacing R173, and its opposite partner R172, or if resoldering cures it, "job done"?

Schematic and photos attached for reference if required.

Many thanks
http://cloud.tapatalk.com/s/595d57d54e9ae/Roc_Pro_1000_schematic.pdf


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[BurningTantalum]

I think this amp is the same as the 1000 Performer. If you need any info I have one on the bench at present, out of its case still after an electronic repair because the case is awaiting new Tolex vinyl covering.
BT

[BurningTantalum]

I had a look at the 'Performer 1000' PCB in the workshop today- it is more burnt than yours so we must assume that this is normal. This one has had a tough life with an Aussie pub rock band for many years.
Maybe replacing the two resistors with the next wattage rating up, and mounting on ceramic beads to lift away from the PCB would be wise. The same with the two zeners.
BT

[Armadillo]

Looks like very bad solder joints thus creating heat.

I would advise;

use a sand paper, sand the black and green solder mask away until you see clean copper areas. Then clean the areas with alcohol. Then you solder the whole pad with solder to add conductive areas and solder mass on the whole areas and made proper joints.