My Plexi's bizarre behavior

[page361]

My '69 Plexi 50w is behaving very bizarrely...so hoping someone has a suggestion...

When I remove the chassis from its head cab and mount it in my amp cradle (and subsequently the amp is upside down now) , it play and sounds great - can play it all day long. Wonderful!

But...when I install it back into the head cab (now right side up), the amp (both channels) will simply cut out...
Amp is 99% unmolested with only the volume pots being replaced. I've inspected all the solder joints all look good. Like I said - stick in in the cradle and it sounds fantastic...strange.

I am dumbfounded! ...help!

[OnTheFritz]

Has to be some grounding issue with the Cab shielding. Any pics?

[neikeel]

Been there with similar problem - maybe related to yours.....................

I doubt it is a grounding issue, sounds much more mechanical/elelectrical

- have you tried chopsticking when someone plays?

What happens if you have amp in case and upside down?

Mine was a dry joint on grid wire between second stage and cathode follower admittedly on amp with under board connections, your 69 may have wires through holes and around turrets which kills that option.

Mine would go intermittent with the amp normal way up and you tapped the preamp end of the chassis.

I stripped back the wire and hooked it through and over the turret reflowed the joint

[page361]

Well I re-installed the chassis, tightened up the bolts and flipped it upside down - cranked it up and played it for 90 minutes - both channels from 1 - 10 with no problem whatsoever...
...suppose I could get used to having the head upside down...

[herbvis]

I had a similar issue one time. One of the nuts on my ptp board must have been a little tight and causing a short. I discovered it by chopsticking around. I loosened the nut and the issue went away. Probably just a simple thing. Good luck to you

[lo_watt]

Look VERY closely at solder joints EVERYWHERE. Just had a 77 JMP 2203 with the same exact thing. Found a bad solder joint on V3 heater pins.

[page361]

Well I was getting kind of used to playing through my JMP with it upside down - thought if I take it to my next gig maybe I'd start some weird trend!

...anyway...with Memorial Day being a wash out here in North TX yesterday, I fired up the soldering pen and touched up a few joints on the two volume pots that had been replaced (these are the ONLY non original solder joints) Put her back in the head cab and played through it right side up this time. So far so good...going to really put it through it's paces today in between meetings...one of the benefits of having a home office. Just have to make sure my Polycom is on mute....