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Re: Marshall Amps/Gospel Radio


 :
3/26/1998 7:51 AM
Ted Breaux
Re: Marshall Amps/Gospel Radio
Well, the transistor would be a logical place for detection to occur. The station is definitely an AM station! It looks as though I should begin taking the schematic to work! I am printing out all of your suggestions, and I intend to fix this problem during the weekend.
 
3/27/1998 4:49 AM
GFR

Found two schematics of Marshalls with tremolo, the 1959T and the 1987T. Both use same circuit for tremolo, one is 50W the other is 100W. There's also a tube combo with tremolo, but this uses two triodes and a FET for tremolo.  
 
No wonder why tremolo is not as popular in Marshall amps as it is in Fenders. The circuit uses one triode as a LFO and a transistor as a modulator, and has some drawbacks:  
 
1) The transistor works as a variable resistance only for very low level signals. If the signal increases, it distorts more than a photocell does.  
 
2) To compensate for the above effect, the tremolo circuit is placed right after the FIRST gain stage, while in Fenders they appear after the LAST stage. Any noise introduced early in the signal chain will be more amplified, of course.  
 
3) The footswitch is not used to disable the LFO as in Fenders. Instead, it disconnects the transistor collector from the audio path. So the footswitch cable is wired to the audio path, and at a relative high impedance point. Very prone to pick up noise.  
 
4) The footswitch jack has to be stereo, an the footswitch cable has to have a shield and two conductors (stereo), since it does not perform a "short to ground" switching. Try wiring the switch so that in one position it shorts the two inner conductors, and in the other it runs the conductor wired to the audio path (the one that doesn't go to the transistor) to ground (shield) through the smaller valued resistor you can use without losing too much gain. This will give you less noise and interference than just short one condutor to another or leave them open.  
 
Having had a look at the schematics, the "antenna cable" theory looks very, very probable. I was a little reluctant about aceppting this theory because of my experience with the Fender design, that is much more robust in this aspect.  
 
 
 
GFR
 
3/30/1998 8:17 AM
Ted Breaux

At last, I have silenced the fanatical preachers.  
 
Thanks to all who offered their troubleshooting advice. I enhanced my shielding efforts of several runs of internal leads, as well as built a new, heavily shielded footswitch. I decoupled the footswitch leads via a 50pF cap to ground, and voila!......silent operation.  
 
I hope this wasn't a sin!  
 
Ted
 

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