Nice job! Quite a few years ago I repaired, without the original diagram, a Mesa Road King. Due to a short circuit and an electric arc between the tracks with a large potential difference, two holes the size of an average coin were burned in the main PCB, along with discrete elements - resistors, capacitors. All JFETs and integrated optocouplers also went to hell. Nobody wanted to do it. I managed. And it's hard to believe - the furnace started from the first shot.
The 16 ohm speaker jack is supposed to lift the ground from the other two speaker jacks when it's in use. The typical 16 ohm jack will fail after years of use and the flex tang gets burned and will go to 32k ohms or more, not good for the other two speaker jacks. The proper solution is to replace the 16 ohm $5 speaker jack. I had the failure at 8 ohms, tossed the head off to the side for a year, then when I got to fixing it there were no problems as I was testing with the 16 ohm cabinet. Checked the resistance from ground to the 8 ohm jacks and found the issue.
Thanks for the video, useful pointers on this Amp, I have one that needs sorting! HT fuse, bias etc....been standing a while...One point I did see elsewhere was installing the Top Grill from the Head version of the Amp to vent the top of the combo....kind of maintains the Marshall style....although not a always a good idea with people putting stuff on top of your amp...like Beer! Any accidents go straight down the fan / vent hole onto the PCB... Another pointer is drawing air in or out of any electrical device attracts charged static particles and small fragments of dust, as well as metal fragments, believe it or not....as I work in IT...my experience shows you need to filter your intakes with a grit or dust filter sheet at the very least....fragments being drawn across High Voltage parts are a recipe for tracking and arcing IMHO. My suggestion would be to put the Vent and Fan each side of the Valves area and drawn the air across the valve areas not across the PCB area. Any air movement across the valves will by default reduce heat in the PCB shield, you have less to worry about in terms of particles being drawn into the PCB area and you can filter the side more easily and you dont have to worry about people placing stuff on top of the amp. I may give that some further thought, perhaps even place the Fans on the Back Panel Grill work and draw cold air in from the Bottom of the Amp by making larger holes in the lower grill....Anyway enjoyed the video and all very useful tips on the nice repair work.
Hello! Why did you used a thin strip of copper to repair the 16 ohm ground socket? If i use a wire to bridge the gap the results will be the same right?
About to do the Dr Tube mod in a couple weeks to my JCM 2000 TSL 100 watt head and my JCM 2000 DSL 50 watt head the parts are in the mail. I ordered some 1 watt metal film resistors to replace some that are known to be the wrong value in the amps from the factory. I'm also going to install a Classic Tone vintage style 3 Hy choke while I'm in it. I was under the impression that a fan would not be needed after these fixes?
Fan is optional. Mainly did it because the old JCM2-60-00 circuit board becomes slightly conductive above 85°c. Internal temperatures around the valve bases were way over that.
Good job...thou you have to change those shitty piher trims. They are already cooked and easily loose contact...you can loose bias and whole job is whortles. Put some new Bourns or similar and you are safe...
Yes I agree with your point, they are a bit flakey. As it's not being used out on the road, hence no bumps, knocks and drops, it's been absolutely fine in the studio for around 2 years now. Probably change them out when I service the amp though.
