Roland JC-60 input jack mystery - HELP!?

[drasp]

I have a late '70s JC-60 that I bought as a parts amp to get the speaker for my late '70s JC-120. Turns out the JC-60 isn't in bad shape so I put a Celestion Seventy-80 in (had it laying around, should be a nice neutral speaker for the amp) and planned to give the amp to my Dad.

Only problem I couldn't sort out was that the input jacks only worked if upward pressure was applied to the cable end. This was the same for both the high & low input. I assumed that the tension on the jack wasn't what it'd been 30 years ago and installed a new jack. I used a Switchcraft mono jack with the same plastic chassis to match the style of the originals. Unfortunately, it's not an exact match and has lead to a really strange set of problems - I've even gone so far as re-installing the old jack which changed nothing, and installing only the one (high) input jack with nothing else wired in series with no fix in sight. The issue is that there is almost no volume. If I push up on the cable end and pull back slightly, I can sometimes get the volume to jump WAY up - maybe higher than it should be & with a higher noise floor than I'd expect based on my JC-120 (which has the same exact board layout, etc & the early chorus/vibe/verb board w/ the MN3002 chip & short 'assembled by beautiful girls' Z-spring tank).

Anyone have any experience with these who can explain the input wiring & why to jacks which LOOK just fine would BOTH opperate the same? Sorry if I'm doing a piss-poor job of explaining the problem. . .

Hoping to have this solved in time to send the amp back to Indiana w/ my Dad!!!

[Enzo]

You are assuming the jacks themselves are the problem. But when you press up on the plug, you are also flexing or at least putting pressure on the circuit board as well. There could be cracked solder or even cracked pc traces around those jacks, or it might not be around the jacks at all. If the solder was cracked on a nearby volume control for example, flexing the board could make that show up. ANd don't rule out a cracked volume control. SOmetimes where the legs come out of a control the brown phenolic wafer cracks across.

The real clue is that before you started throwing parts at it, BOTH jacks did this. It is really unlikely that both jacks developed the exact same odd symptom at the same time. However if both are soldered to the same circuit board, then both would flex or apply pressure to it.

[drasp]

No pcb anywhere near the jacks on the old JCs. These are panel mount closed chassis jacks with leads running down to the PCB. I've had the boards out of the amp & touched up the very few joints that had any flaws at all. I've already fixed the vibrato & chorus being inop, re soldered a broken connection inside the reverb tank & replaced a broken fuse holder that tryed to kill me. Maybe should've listed those problems in the original post.

My point should've been that I don't belive the problem lies w/ the jacks themselves. Wasn't trying to throw parts at it, but you're 100% correct that now that I've ruled out the jacks I AM totally confused/lost. How can a good jack wired straight into the signal & ground traces on the board still jump in volume when the plug is flexed? Only thing I can come up w/ is that the ground may be compromised as the barrel of the plug moves away from the inner bore of the jack. . . any other ideas?

[Enzo]

The concept still stands. Dismount the jack. PLug into it and hold it in your hand. Does it still react to flexing? Leave the jack loose and put a rag under it or something so it doesn't touch anything, leaving the signal applied. Now stick a large screwdriveer blade or something into the panel hole where the jack was. Apply some flex to the panel. Does the system react? If the jacks are not on a circuit board, are not the controls and such on a neraby board? and since they are tightened to the panel, any flexing of the panel might apply some stress to their connections to the board under them.

DOn;t think of this as specific to your amp, since I don;t know how it is laid out. Think about where mechanical stress can be transmitted to in the chassis. Sometimes stress on the panel can flex the whole sheet metal chassis. I have seen situations where such a thing was pushing by flexation ultimately on a board mount standoff in the center of the chassis, and the problem was really not close to the jacks. Downward pressure on the jack holes on the front was flexing the center of the chassis bottom upwards.

And we all do silly things, myself far too often. Have we tried a differnt cord?

[drasp]

Ahhhhhh! I'll give your suggestion a whirl tomorrow & hope for good luck! That makes way more sense (chassis/panel flexing & causing something to give in a pot (vol?) Man, that'd be a nice solution.

I did try the jack out of the chassis, sounds dumb, but I can't remember the outcome.

Will also take some other patch cables to work.