"touchy" Reverb on AB763 SR

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j allen shaw
Junior tube assistant
Posts: 32
(4/2/04 8:26 am)
 "touchy" Reverb on AB763 SR
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 Can anyone tell me how to fix a reverb that is very touchy, as in - when the reverb pot is on 2, there is no reverb, but at 3, reverb is full on.
Is it the slope resistor, or what exactly controls the reverb?
Which one is the slope resistor?
What resistor(s) or cap(s) would I look at on the Fender AB763 circuit to fix this problem, to make the pot bring the reverb up more smoothly/slowly?
Thanks on advance, Jeff.

 
Tiny Daddy
I will work on all amps
Posts: 590
(4/2/04 9:19 am)
 Re: "touchy" Reverb on AB763 SR
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 It shouldn't work like that, the 763 is a good circuit. Maybe a bad part such as shorted cathode bypass cap, have the caps been replaced yet?
 
j allen shaw
Junior tube assistant
Posts: 33
(4/2/04 10:49 am)
 Re: "touchy" Reverb on AB763 SR
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 Yes, all the caps have been replaced w/ Sprague orange drops & Atom electrolytics.
Which bypass cap would I look at? I'll take it out of circuit to check it w/ the meter.
Also, is possible the pot is bad?

 
Tiny Daddy
I will work on all amps
Posts: 591
(4/2/04 11:03 am)
 Re: "touchy" Reverb on AB763 SR
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 It's possible the pot element is cracked causing it's grounded end to loose it's connection. This would cause a sudden jump to full volume. Does the reverb volume change suddenly at 3? Or does it seem like there is just too much gain someplace (maybe a previous mod needs to be removed)?
 
j allen shaw
Junior tube assistant
Posts: 34
(4/2/04 11:36 am)
 Re: "touchy" Reverb on AB763 SR
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 I found a couple of carbon comp resistors in the reverb circuit that have drifted alot. One is supposed to be 470k, but is 156k. Another should be 220k, but is reading 153k.
I'll replace these with the correct values & see if it makes a difference.
The jump is not sudden, just rapid from 2 to 3 on the dial. If I bump it slightly one way or the other I can get an acceptable level of 'verb, it's just very annoying. So I suppose it's not the pot.
I don't think there are any other mods, as all other specs match up w/ the Fender schematic & layout.
Once again, I'll order all new resistors (there's also a very loud hiss in the amp) to see if it helps.
Thanks T.D. for the help! Jeff
 
Tiny Daddy
I will work on all amps
Posts: 593
(4/2/04 11:42 am)
 Re: "touchy" Reverb on AB763 SR
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 "very loud hiss in the amp" is usually a plate resistor that has incraesed in value. I would suspect the 100K on the plate of the 7025 that recovers the reverb, and the 100K on the next stage.
 
 
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