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HELP: DR has reverb on normal channel

8/12/2000 10:16 PM
Bob P
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HELP: DR has reverb on normal channel
Sorry about the long post, but this one has me stumped.  
 
I was recently brought a 1970ish SF Deluxe Reverb model AB868 for repair and a conversion back to blackface. The amp had been recently purchased. It sounded terrible and had loss of gain when the intensity pot was turned up. I did the following:  
 
* filter and bypass cap job  
* converted to blackface  
* put in a .0047 cap across the vibrato photo resistor to kill the ticking.  
 
The amp sounded great and worked fine and I released the amp back to the owner.  
 
Next day he brings it back saying it now has reverb and vibrato in both channels. Opened it up and found a broken tube in V1. Changed the tube and checked all other tubes, no luck. Removed reverb cables, resoldered everything in sight, lifted the board off and inspected, shortened the grid wires to V1/V2, even put back the 1200pf caps on the power tubes, no luck.  
 
If I pull V4 the problem stops but further inspection of components shows nothing...all components seem fine. If I remove the wires from the plates of V1 I still get reverb/vibrato. Something seems coupled. A Scope confirms a signal on V2 pin 6 even though V1 is disconnected. If I remove the grid wire from V2 pin 7 it stops. Again, all components, tubes, etc. check out.  
 
This is very strange and I am having no luck tracking it down.  
 
* Could the blackface conversion do it?  
* Maybe the broken tube in V1 shorted something out?  
* Conductive board?  
* Parasitic oscillation?  
* Sunspots?  
 
The wiring is stock with no mods for the double channel effects. I never checked for this problem originally and could have been there all the time. Also, very intermittently (only when the amp is back in the cabinet) the amp loses gain when the intensity pot is turned up (whether or not the vibrato is engaged or the foot switch is in or not). Checked to make sure the wire mesh wasn't touching but it still does it very, very intermittently. I don't think that the two are related problems because the reverb is always there, even if I pull out the vibrato tube.  
 
I have trouble shot this as much as I can. Any help here is more than greatly appreciated.  
 
Bob P.

 
Replies:
Richie{~}==::: Lets see.. a few things you might l... -- 8/13/2000 12:11 AM
Richie{~}==::: I also think this model has this ga... -- 8/13/2000 12:20 AM
Steve A. Bob:  &nb... -- 8/13/2000 7:33 AM