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REVERB FROM HELL


 :
7/14/2005 4:49 PM
jim
 REVERB FROM HELL
Running a dual channel amp in a Fender 75 w/EXACT fender type reverb. I`m getting a NASTY RFI induced junk off first stage of the mixer tube where it feeds the reverb potentiometer. Due to space, I`m feeding the coupling cap( .0022) right to the pot entrance ( reverb pot at back panel)I have tried regrounding( star and fender/marshall) new reverb tranny, new tank,TOTAL SHIELDING on ALL reverb wiring, new cables, new tubes, NOTHIN works, I`ve never seen it this hard before to dial out the noise! Sure appreciate some help from you awesomely smart techs out there!!  
 
thks in advance,  
 
jim
  
7/14/2005 6:04 PM
R.G.
Put a 1K 1/4W resistor in series with the signal input right at the 0.0022 and a 47pf to ground.  
 
In general, you can't stop RF with shunt capacitors. You need some series R or L, or both to attenuate it and then a capacitive shunt to ground. The "ground" needs to be the shield of the return cable.
  
7/14/2005 9:26 PM
jim
much OBLIGED RG, will implement ASAP!!  
 
deepest appreciation, jim
  
7/15/2005 6:45 AM
Randy Re: RFI trouble shooting
RFI is a bear to fix in some cases. Did this amp always have it? or did it appear suddenly or after a mod ?  
 
I would ohm out all grounds for zero resitance. Also, the grounds for each jack. Another problem can come from the switch and its cable if the reverb lead and the channel switch are reversed or dirty.  
 
I used to work on tube and solid state televison transmission equipment. Sometimes the stage the interference seems to be coming from is in fact not the source. It works in combination with other parts.Old cruddy tube sockets are also worth checking.  
 
Severe RFI can be induced from certain kinds of crud in jacks, switches and potentiometers. Cigarette smoke, with its tars and ashes is a particulary bad culprit. In really bad cases, the tag board can become conductive between suposedly insulated tags. The smoke residue forms little crystal radio type detectors and RFI arrives.
  

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