This past weekend I was able to replace the larger 50-50 caps in my amp. It meant I got to use my capacitor discharge tool which I’d been toying with for a while. Soldered between two alligator clips, I have a 5W 5k Ohm wire-wound resistor. I left them clipped on each positive cap terminal for a few minutes. That seemed to do the trick; my test was to do “the screwdriver trick” and short them to ground. There was no “loud pop and sparks” so my tool was successful.
So I de-soldered the old wiring and removed the old clamps. It was easy to put the new ones in and re-attach the wiring. I had taken pictures along the way,so it was trivial to get the wiring back in the right place. History has shown that my memory is close to useless, so I didn’t even attempt to hold it all in my head.
You can see the underside of the main PCB through the hole in the chassis (see left). I left this cap, the one sitting next to the choke, sitting quite high in its clamp (see the picture at the end of this post) as I was paranoid about shorting the wires against the PCB.
You can also see in the above pictures that Marshall wasn’t too picky about employing people that could solder particularly well. Their preparation seemed to be the key…i.e. when they stripped insulation off wires they did not bother to spend half a second to twist the strands together before tinning it. I had to replace a few wires as the original wire was not going to hold up too well.
Once completed, I fired her up and checked the bias. All was normal, and it appeared to have greatly reduced the amount of hum…especially when cranked. Success!
Next step will be replacing the electrolytic caps in the grid bias circuit.