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Bypass issue after recap of Fender Reverb Unit

Discussion in 'Amp Tech Center' started by montana, Mar 19, 2017.

  1. montana

    montana TDPRI Member

    39
    May 5, 2003
    Seattle
    Hello. I have a 1964 Fender Reverb Unit that was completely original when I got it. It sounded fantastic but made the occasional noise here and there so I figured it was best to recap it. I only replaced the filter caps and the three electrolytics on the board. It sounds great--just as good as it did and without the extra noises. But now, when I turn the reverb unit off it doesn’t bypass the signal to the amp like it did before. The oddest part though—it actually does pass a signal, almost full strength, for about 45 seconds after I turn the switch off. Then the volume quickly fades to zero and no signal gets to the amp.

    I’m reluctant to use it for any length of time worried that I might be damaging something. Any ideas for what I should be looking for when troubleshooting this issue? Thanks for any advice.
     
  2. robrob

    robrob Friend of Leo's

    Dec 29, 2012
    United States
    If it's a 6G15 reverb unit the only signal path when the amp is not powered is through the 2.2M NFB/Voltage Divider resistor, 100k plate load, tone pot and mixer pot. I really don't know what could cause your symptoms.

    [​IMG]
     
  3. Wally

    Wally Telefied

    Mar 17, 2003
    Lubbock, TX
    I don't see how the signal could have been passed through the circuit with the unit powered off...before the recap or after. The signal has to pass through that triode in the lower right of the schematic. Without power, that tube cannot conduct signal. The 45 seconds or so of signal passing through the circuit exists as the caps are being drained of charge. When they are drained, that tube can no longer pass signal. This is normal, IMHO.
     
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  5. robrob

    robrob Friend of Leo's

    Dec 29, 2012
    United States
    Maybe the bypass signal takes the dry signal path then turns north through the 2.2M NFB & voltage divider resistor, 100k plate load, tone pot and mixing pot?
     
  6. robrob

    robrob Friend of Leo's

    Dec 29, 2012
    United States
    Hey, where is the grid leak for the 7025 reverb recovery triode in the schematic above? The layout also shows the lack of a grid leak resistor (7025 tube, pin 2):

    [​IMG]
    The foot switch when closed grounds the grid to kill the wet signal, but how can there be no grid leak? The blackface built-in reverb has a 220k grid leak resistor:
    [​IMG]
     
    Last edited: Mar 20, 2017
  7. montana

    montana TDPRI Member

    39
    May 5, 2003
    Seattle
    Wally, your diagnoses may be right. At least it behaves and sounds like a draining filter cap. But boy does it take a looooong time.

    Rob, those component labeled schematics are amazing. Super helpful in finding my way around the circuit. Thank you.
     
  8. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    The reverb tank itself should be the grid leak. Right? Grid to Tip, Tip through output transducer to Ring, Ring to ground.
     
  9. robrob

    robrob Friend of Leo's

    Dec 29, 2012
    United States
    I think you're right Clint, I forgot the output transducer coil would have to to have one end grounded.
     
  10. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I sense confusion here. Are you getting no sound when hitting the footswitch to bypass the reverb, or are we talking power switch on/off?
     
    D'tar and Wally like this.
  11. Wally

    Wally Telefied

    Mar 17, 2003
    Lubbock, TX
    Maybe I misread the OP, Clint, but I took it that the unit's power switch was being turned off...
     
  12. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I don't see that signal should pass with it powered off. Anything audio passing the 2.2M resistor should short to ground through the 40u/450V filter cap. Unless the old cap were really bad, or had a particularly weak ground, that is.
     
  13. robrob

    robrob Friend of Leo's

    Dec 29, 2012
    United States
    Good point Clint on the filter cap shorting the audio so it does look like the 6G15 should not pass audio when turned off.
     
  14. montana

    montana TDPRI Member

    39
    May 5, 2003
    Seattle
    Yes, I'm referring to the power switch. The filter caps might have been really bad. They were all clearly showing blisters.
     
  15. robrob

    robrob Friend of Leo's

    Dec 29, 2012
    United States
    Here's the matching 6G15 layout with signal path colors that match the schematic (if the colors don't match tdpri is caching an old image):

    [​IMG]
     
  16. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted

    Dec 11, 2009
    Bloomington, MN
    I've no answer for this, but I do know that the clone I made will not pass signal through when powered off. Never did, and I didn't second guess it. If you all can figure it out... I'll make the change on mine as well!
     
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