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

[The_Marshall_Plan]

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!

 

[Crikey]

Tighten snd clean any jacks on rear if amp
My 2210 has loose foot-switch jack making all kinds of racket til i cleaned and tightened it

 

[RLW59]

When it does it, leave it running instead of powering down and see if it fixes itself the same way.

If the jack switching contact is weak, bent, got a little dirt/goo build up, and/or still a little corroded, it can make partial contact but drag a little and not make good contact immediately.

Especially since you know the jack was at least part of the initial problem, there's a real good chance the jack is still the problem.

Another really good cleaning might fix it. But it may be prone to recur so it might be best to just replace the jack.

 

[XTRXTR]

Sounds like you're on it, The Low jack is the likely culprit since the hi goes through the first stage and then through the low input stage as the second stage. Possible coupling caps there between high and low input stage too 43 years old might be leaking DC.

Does the preamp volume make scratchy noises as you turn the knob up or down when plugged into the High but not scratchy when plugged into the low?

 

[The_Marshall_Plan]

Sounds like you're on it, The Low jack is the likely culprit since the hi goes through the first stage and then through the low input stage as the second stage. Possible coupling caps there between high and low input stage too 43 years old might be leaking DC.

Does the preamp volume make scratchy noises as you turn the knob up or down when plugged into the High but not scratchy when plugged into the low?

no, no scratchy sounds when adjusting pots on either input

 

[Deftone]

no, no scratchy sounds when adjusting pots on either input

Better posts some pics so we can drool over your Marshall help you out.

 

[The_Marshall_Plan]

Better posts some pics so we can drool over your Marshall help you out.

haha, here you go...

 

[Chris-in-LA]

It almost looks like that black wire from the board to the input jacks is soldered to the positive terminal instead of a ground terminal. Can you confirm where it’s soldered?

 

[neikeel]

I think that the plating has gone on the low input jack. You might be able to save it with an old fashioned contact breaker cleaning file and some lube Deoxit Gold with brush works best but it has to be clean and corrosion free first. Failing that a new jack socket. Don’t buy cheap!

 

[The_Marshall_Plan]

It almost looks like that black wire from the board to the input jacks is soldered to the positive terminal instead of a ground terminal. Can you confirm where it’s soldered?

thanks chris, looks like i wont have time to get back inside this amp before it’s due at the tech’s. im going to pass along ur observation tho

 

[XTRXTR]

Noice!

 

[neikeel]

thanks chris, looks like i wont have time to get back inside this amp before it’s due at the tech’s. im going to pass along ur observation tho

Wiring looks correct to me - which is why I included annotated diagram for reference

 

[Chris-in-LA]

thanks chris, looks like i wont have time to get back inside this amp before it’s due at the tech’s. im going to pass along ur observation tho

@neikeel is right. That black wire is on correctly.

 

[The_Marshall_Plan]

back from the tech now. needed a new low input jack, a bias, and a grid...something...something...

anyway, sounds great now. thanks everyone for the input.

 

[XTRXTR]

Oh, I hadn't noticed the lack of grid stoppers on the power sockets. Definitely need those.

Cool you got it back into full working condition!

 

[myersbw]

Cool that you got it back! Did the tech replace both jacks? I usually do as one typically follows failure in a short time, too.

 

[The_Marshall_Plan]

Cool that you got it back! Did the tech replace both jacks? I usually do as one typically follows failure in a short time, too.

no, i believe it was just the one. probably didnt want to charge me for stuff that wasn’t broken. i guess ill just take on any issues as they arise

 

[yafal]

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!

Hi. Definitely I would replace the input jacks, maybe just the low input one. If you want to keep them for the sake of authenticity, just remove them and clean the three contacts that open/close when you plug/unplug. I use commercial clean/oil spray for electronics, and sometimes a very light sand paper, folded the way it sands to both sides, introducing it between the pair of contacts to be cleaned, one pair at a time. But, if the metal of the contact had loosened elasticity, you'll have to replace it.
Good luck!

 

[XTRXTR]

I use commercial clean/oil spray for electronics, and sometimes a very light sand paper, folded the way it sands to both sides, introducing it between the pair of contacts to be cleaned, one pair at a time.

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.

 

[myersbw]

About cleaning those jacks - spraying (even a small bit) lends the potential to create a short or partial short even when plugging the 1/4 plug in. It more easily happens when using the DeOxit D100 spray (a little thicker liquid). It's good to plug in a couple jacks to open the closed switch contacts. (I typically spray a plug and insert a few times while twisting.) If you spray the jacks, make sure you get the excess off the contact switch side. If you don't, when you insert a plug...a heavy droplet and expand and keep a resistive connection between the tip and the switch. What you can experience if that happens is a very weak signal. The easiest fix can be inserting a plug and sliding a folded sliver of paper between the tip and switch contact to wick the excess off.