# '79 Marshall 2204 rewired with shielded wire, now no signal from high input?



## JDW3 (Sep 23, 2009)

I was getting some noise from my high input of my '79 2204, and it was suggested I install some shielded wire to the grids. The high input worked, but almost sounded like a gate, or bad connection. But when connected, was very loud. Something was wrong.

I did this, taped up the old wires (still attached to PC board) and tried it. The low input works and sounds very healthy.

The high input, which sounded bad to begin with, is now only audible with the amp almost dimed. Not good.

I basically put back in what I took out. I put the 68K resistor at the tube socket, as opposed to going to the board. 

Anyone have any ideas what is going wrong?


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## jcayer (Mar 25, 2007)

First thing that comes to mind.
How experience are you with a soldering iron ? Maybe a cold joint ?


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## Wild Bill (May 3, 2006)

JDW3 said:


> I was getting some noise from my high input of my '79 2204, and it was suggested I install some shielded wire to the grids. The high input worked, but almost sounded like a gate, or bad connection. But when connected, was very loud. Something was wrong.
> 
> I did this, taped up the old wires (still attached to PC board) and tried it. The low input works and sounds very healthy.
> 
> ...


The input jacks can be very tricky. You have to follow the schematic perfectly. You see, there are NOT two separate inputs!

What Marshall did is to have an extra tube stage of gain running THROUGH the extra switch contacts on the LOW jack! When you plug into the LOW input you open the switch contacts and the extra stage is disconnected. When you plug into the HIGH input all the stages run straight through the chain.

Once you understand this it's not hard to figure out the contacts but if you wire them up like two ordinary different channels you get EXACTLY the problem you have now!

Here's a link to the schematic:

http://www.schematicheaven.com/marshallamps/jcm800_lead_mstvol_50w_2204.pdf

Have fun!


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## JMC Amps (Dec 19, 2009)

JDW3 said:


> ... The low input works and sounds very healthy.
> 
> The high input, which sounded bad to begin with, is now only audible with the amp almost dimed. Not good.


Red Flag # 1: The contact tip on the Low input jack.

Perhaps when you tried the Low input, you gave the contact tip "one final stretch", enough for a bad connection.

I would try a jumper wire between the solder lugs on the tip end, or just move the wire to the other side and bypass the jack. Or, you could simply try placing something conductive (piece of wire) between the contacts of the jack and try to get a better connection. Mabey the jack just needs cleaning! Eliminate the mechanical problem possibilities first.
These cliff style jacks on Marshalls are kind of difficult to re-tension.


-JMc


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## Wild Bill (May 3, 2006)

Wild Bill said:


> Once you understand this it's not hard to figure out the contacts but if you wire them up like two ordinary different channels you get EXACTLY the problem you have now!
> 
> Have fun!


Forgot to mention, since the high input runs through the low jack switch contacts, if you used a piece of shielded wire the shield would short out the switch contact to ground!


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## JDW3 (Sep 23, 2009)

I wired the high input with shielded wire, just like this schematic.

http://www.ceriatone.com/images/layoutPic/marshallLayout/JCM800_2204Ceriatone.jpg

I can't see the copper traces at all with this dark blue PC board. Is there a chance something is disconnected? I figured it would be the same signal path on a PC as a turret board type. 

I have one new input jack. Should I replace the high input or low input? Not sure if I understand how they work.


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## JDW3 (Sep 23, 2009)

Wild Bill said:


> The input jacks can be very tricky. You have to follow the schematic perfectly. You see, there are NOT two separate inputs!
> 
> What Marshall did is to have an extra tube stage of gain running THROUGH the extra switch contacts on the LOW jack! When you plug into the LOW input you open the switch contacts and the extra stage is disconnected. When you plug into the HIGH input all the stages run straight through the chain.
> 
> ...




It was indeed the tip of the low input jack! I installed some shielded wire anyway, not sure if it was ever needed. I have the new jacks; just need to install them, along with some new resistors.

Thanks again, guys!


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