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Help with Marshall Lead 12 combo amp


 
3/25/2006 1:43 PM
steve
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Help with Marshall Lead 12 combo amp
I'm trying to fix my Marshall Lead 12 combo amp. This is a solid state 12w practice amp.  
 
Originally, the low input jack was flaky. When playing plugged into the low input jack, the sound would cut out.  
 
I opened up the amp and resoldered all the joints for the low input jack and now it squeels.  
 
Here is a link to the schematic:  
 
http://www.drtube.com/schematics/marshall/3005.gif  
 
Any advice on where to start?  
 
TIA,  
 
steve
 
3/26/2006 7:25 PM
John D
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I know you want some high powered help, and I'll let you know up front that's not me. But being no one else has answered I'll take a neophite shot at a possibility. I resurected a Traynor about 3weeks ago. One thing I noticed on that was that the input jacks were supposed to be isolated from ground. I took a look at the schematic for this amp and though there is no note to say they are not to be grounded, the schematic does not show a ground from the jacks. I would try to make sure you remove any possible grounding from the jack and see what happens. You can't hurt anything and it just might work.
 
3/27/2006 5:08 PM
Mark Black
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I've had the same experience with those amps. One thing you need to make sure of is that the "normalling" switch contacts in the input jacks are functioning perfectly. Even if they are chances are good it will squeal if you dime everything.  
 
I've got another one pending (squealing - go figure...) and am thinking about devoting some time to finding out the underlying cause if possible even if it turns out to be a design flaw. Probably be a while before I get to it though...  
 
If somebody here has already figured it out please do post.  
 
Meanwhile good luck!  
 
Mark
 
3/27/2006 7:44 PM
Enzo
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Make sure that the traces are not cracked. The pad where the lig from the jack solders on, may have torn free from the board anad is electrically separated from the trace that goes off to who knows where. Especially the ground traces, use an ohm meter to verify the actual leg of the jack is electrically connected to whatever on the other end of a trace.
 
3/27/2006 7:44 PM
Enzo
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"leg" not lig
 
3/28/2006 8:54 AM
steve
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Thanks for all the suggestions, guys. They are greatly appreciated.  
 
steve
 
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