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Marshall JCM800 - no volume w/all tone controls at '0"


 
4/2/2006 3:59 PM
Bob S II
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Marshall JCM800 - no volume w/all tone controls at '0"
Hey there,  
 
Haven't posted here in quite a while, and am having a bit of a problem I could use some help with.  
 
1982 Marshall 4104 (2204 head in 2x12 combo form). I just pulled this thing out of storage to sell and just wanted to verify that everything still worked properly. It sounds good but seems to have developed a problem somehow.  
 
turning the bass, mid, and treble controls down to zero has the same effect as turning the amp's volume down to zero. Bring any one of them up past zero returns the amp to normal volume.  
 
I'll admit to being out of the wind for a couple of years, and I've yet to crack open the schematic, but I've gigged with and serviced a number of non-channel-switching 50 watt JCM800s over the years and don't recall any of them ever doing this. Have I lost my mind, or is there a problem here?  
 
Thanks,  
-Bob
 
4/2/2006 4:04 PM
Don Symes
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Re: Marshall JCM800 - no volume w/all tone controls at '
I'm reasonably certain that would be the expected behavior - the tone controls' job is to selectively shunt bands of signal to GND.  
 
All tone controls at '0' would necessarily have zero volume.  
 
If you posted with an email address, I'll toss you a schematic. If not, throw me an email and I'll get one on the way shortly.
 
4/2/2006 6:02 PM
Bob S II
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Thanks for the reply, Don. After some searching and looking back through my notes, I found references to the midrange pot on the 4104 combo being wired slightly differently than on the 2204 head. I just pulled the chassis and confirmed that to be the case, so I'll just consider this normal for this particular amp and button it back up.  
 
I do have another question though for you or anyone else who may know - the 2204 schematic shows the outer legs of each pot connected to the next pot in line, but in reality, one of the outer legs of the bass pot in every example I've seen is left disconnected from anything. This was explained to me once before here (I really wish old ampage posts were archived), but I no longer remember why this was. Examples:  
 
http://nobodycaneatfiftyeggs.com/stack.html  
 
Thanks,  
-Bob
 
4/2/2006 7:50 PM
Don Symes
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quote:
"I no longer remember why this was."
 
 
My take is that it's basic cheapness - the Bass pot is wired as a variable resistor (2 terminals shorted) rather than a potentiometer (3 independent terminals). Since there's no electrical difference between shorting those two terminals and just wiring to wiper and one end, and a wire costs labor (more than wire costs).  
 
HTH!
 
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