# Marshall JMP-1 Problem - Tech?



## loopy_al (Mar 12, 2010)

Hey, 


I have a Marshall JMP-1 pre that is causing my power amp to blow fuses! (I know, difficult to comprehend) 
What is happening is, intermittently the signal degrades on the pre (very low signal, lots of hiss), then the signal will sometimes "pop" back in if i change a channel, this causes the power amp to blow a fuse. 
I have no idea what is going on with the JMP and i need a good tech with experience with these units to take a look. 
Any suggestions would be appreciated. I am located in downtown Toronto. 
Thanks! 

Al.


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## GuitarsCanada (Dec 30, 2005)

Sounds like it needs a going over for sure. Hopefully one of the members here can give you some guidance on how to proceed


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## loopy_al (Mar 12, 2010)

Yup. I know not many techs are familiar with the JMP-1. 
I took the unit to Buzzy Burak here in Toronto but he wasn't able to decipher the issue. 

Al.


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## Wild Bill (May 3, 2006)

loopy_al said:


> Yup. I know not many techs are familiar with the JMP-1.
> I took the unit to Buzzy Burak here in Toronto but he wasn't able to decipher the issue.
> 
> Al.


I would suspect a capacitor that goes leaky, starts to short (which pulls down the voltages and thus the gain) and then "heals" itself to do it again later.

This "self-healing" property is common with some caps. Caps are made up of layers of conductor with an insulating material between them. In the early years the simple method was to roll up two layers of foil with a layer of waxed paper between them.

If the insulating material started to break down at any particular spot it would allow the two foil layers to touch, causing a short. The voltage would blow a tiny hole in the insulating material and burn a bit of the foil away from around the hole, removing the short.

However, that spot was damaged and was prone to have the same thing happen again. So the capacitor slowly leaks and then shorts to cause the problem but it heals itself, works for a while and then does it again!

Coupling caps can do this but usually today we use caps with a much higher than needed voltage rating so we rarely get this problem with couplers between stages. I see it more often with a filter cap in one of the nodes of the power suppply! Especially with those old-fashioned Sprague Atoms. They are nice for the vintage look but capacitor quality and technology ratings have come a long way from when the Atoms were invented, back in the 1950's! I prefer the modern Illinois filter caps. Better specs and also smaller, which makes them easier to fit in some cramped amps, opening up some room.

Just a thought. Something else to check! Try alligator clipping your voltmeter across a filter cap and wait to see if the voltage dips when the problem occurs.

WB


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