JMP 2204 issue -- have I met due diligence before tech?

Ron Carter

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Hello,

Looking for some advice on this...

Picked up a '79 JMP 2204. High input sounds great, plug into the Low input, also sounds great, plug BACK into the High Input and I get all kind of crackling/gating. If I let it sit for about half an hour and come back to it, the High Input sounds good again.

I pulled the amp chassis out, thoroughly deoxit cleaned both inputs and all the tube sockets.

Now all the gating is gone, and crackling is mostly gone. BUT -- when I switch from Low Input back to High Input, there's a noticeable (negative) tone change -- sounds brittle and splatty.

Same as before, when I let it sit for about half an hour and come back, the high input is sounding good again.

Swapped out all preamp tubes and tested, issue the same.

I'm bringing it in to a tech next week, and I understand switching issues are common with corroded low input jacks on this circuit, but I find the high input tone change kind of puzzling seeing as how it corrects itself after about half an hour of non-operation.

Want to make sure I'm not missing an "easy" maintenance fixes before I bring it to the doctor, because the seller is going to reimburse me for any repairs needed.

Anyone have some input?

thanks in advance!
Have you tried using deoxit on the input jacks to make sure they're clean and also using a barnishing tool to make sure the input jacks are grounding as well, sometimes this can be the cause of a lot of problems and weird losses of tone and signal at that.
 

yafal

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I am not a working tech but I have seen a few videos that say not to use a sand paper but just paper card soaked with the cleaner on both sides. Sand paper can cause striations, marks which can make the contact surfaces not fit as well, even though they are clean. In other words less actual surface area makes contact.
Of course you would have to use a very light sand paper (the ones normally used along with water), but paper card is OK too.
 


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