JCM602 heater question...

[mnexsen]

I have a JCM602 that came to me because "it fell over and didn't work anymore." I found some solder joints broken where the jack pushed them when the amp fell, repaired those, and all was well. I played the amp several times over several days with no issues. While waiting for the amp to be picked up, suddenly the first preamp tube doesn't light up. All the others light up just fine. Changing tubes doesn't help. Looking at the board where the tube sockets are mounted, it appears the heater circuit is 2 substantial traces on the board and I see no cracks, breaks, no bad joints at the socket. I have to believe the issue is a bad tube socket, but I'm flying blind without a schematic. Is there anything else I need to look for here? The main power switch feels a little iffy like it doesn't always catch, but still if the other tubes on that trace are good why wouldn't the first tube come up?

[dukeofurl]

Think your schematic is here: Marshall Amps Info & Schematics

[Jazz P Bass]

The heaters are in parallel.
With the power off, check continuity from V2 to V1, each heater leg, at the socket.
If it is a cracked trace this will prove it out.
If the crack is at the socket pin, you can try to scrape off the solder mask from the copper trace & lay in a bead of solder.