JCM900 noise issue – this is a weird one...

[nevetslab]

[ATTACH=CONFIG]53923[/ATTACH]
nickb for the win!
This was an easy one to spot once I pulled the back panel board.

Ahhh........solder joint fracture! My old 'friend'. I wouldn't have guessed it being the Ground terminal of the IEC-320 Power Socket at first, though on all of the Marshall amps that come thru my shop, I'm pulling that rear panel PCB off for inspection as a general rule, since those boards are supported entirely by the solder terminals of the rear panel parts. Glad to see the culprit was found, and thanks for sharing the process to the end!

[SoulFetish]

Ahhh........solder joint fracture! My old 'friend'. I wouldn't have guessed it being the Ground terminal of the IEC-320 Power Socket at first, though on all of the Marshall amps that come thru my shop, I'm pulling that rear panel PCB off for inspection as a general rule, since those boards are supported entirely by the solder terminals of the rear panel parts. Glad to see the culprit was found, and thanks for sharing the process to the end!

A little more follow up:
It's interesting, on the schematic, it is drawn more like you would see a traditional "star" ground point. Looking at it (the schematic) over the past couple of days with the rest of you, I was thinking back and didn't really remember seeing any kind of obvious place for a star connection on the PCBs, but couldn't be sure.
After looking at the back panel board today, it is laid out much more like a bus ground style trace. However, the connection to the chassis and rest of the ground connections is made through the earth IEC terminal mounted on the board. Then they run the earth/safety wire, soldered to the IEC jack, to the chassis through a crimped ring terminal.
I'm surprised I haven't run into this problem before, this inlet is definitely a place of mechanical stress and a prime candidate for a cracked solder joint.

[Chuck H]

Actually... I think I called out the flying lead ground and the probability of a fault specifically But to be fair all the pertinent info to track it was in Nicks post first. All's well that ends well.

[SoulFetish]

Actually... I think I called out the flying lead ground and the probability of a fault specifically But to be fair all the pertinent info to track it was in Nicks post first. All's well that ends well.

Chuck, you're absolutely right. Not only that, but it was also Pedro Vecino who first drew our attention to the problem lying in the area of the star ground. So I'd be a bit of a shit if I didn't give you two honorable mention. Big team victory for us here

[Chuck H]

Well don't actually do repairs. I've had to do diagnose symptoms often enough to be sure though. The well of experience here is profound. Just another day in paradise.