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Topics - niftyprose

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Hello guys esp. Joseph, my first post on this forum. I'm a total n00b on amp repairs but have some electronics knowledge and experience gathered elsewhere. I have a pretty basic problem with a Roland Supercube. If that makes you yawn, the payoff is an amusing photo.

OK, here goes. The amp is a Supercube 100. It's a solidly-built 1 x 12 combo, the bigger brother of the original orange Cube. It will have been built between '78 and '82, likely late '70s because it has the stock speaker used in the Japanese units (later EU ones were assembled here in the UK with Celestions). Build quality is excellent throughout and the sound is good, if not quite my thing.

There are two input jacks, one for a footswitchable drive channel and one for a clean channel.

When I plug into the drive jack, there's a series of cracks and thumps that goes on for several seconds, that is, some time after the jack has gone into place. It's loud enough to make me worry about speaker damage. There's often a brief high-pitched squeal which sounds like microphonic feedback in a cheap pickup. The clean channnel jack is a little noisy but bearable.

I took the amp sub-chassis out of the main box yesterday. Here's the label on the reverb tank, which has nothing to do with my query but might give you a laugh:

http://unblotted.com/extmedia/FoldedLine.jpg

I'm inclined to think that the problem is with the input jacks, which are the only part of the circuit subject to mechanical stress. What I fiound was a neat PCB sub-board with both jacks soldered to it. There's a five-cable connector to the main board, presumably a shared earth plus connections to the two jacks. There are also two cables to the drive footswitch.

http://unblotted.com/extmedia/_0014934.JPG

The PCB looks pretty simple, and I can't see any problems:

http://unblotted.com/extmedia/_0014933.JPG

The obvious suspects are the earth wire from the outside of the jacks to the PCB, and the very tiny soldered lumps that you can see at the top of the PCB photo, which are involved in the footswitch. To my shame, I'm not sure how the footswitch to the drive channel works. I mean, it's obviously closing a connection on the sub-board to route the signal, but how is the connection made without a power supply? Is it the case that the signal is routed off the sub-board, goes through the footswitch and back to the sub-board? Sheesh!

It may be the case that someone reading this says, "Aha, the famous Roland jackboard problem!" and tells me a two-minute fix to identify and replace the notoriously weak link in the chain. If not, I'd like a 'best practice' type solution. It wouldn't break my heart to replace the jacks and the sub-board completely, although I don't think I'd relish having to connect to that five-way ribbon. Also I don't want to disable that footswitch...

TIA, NP.


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