Ad Widget

Collapse

Announcement

Collapse
No announcement yet.

Gibson GA79RVT Reverb motorboating

Collapse
X
Collapse
 
  • Page of 1
  • Filter
  • Time
  • Show
Clear All
new posts
Previous template Next
  • #1

    Gibson GA79RVT Reverb motorboating

    I have replaced all of the 20uf caps in the unit which got rid of the hum and made it sound much better. However, I have two more problems that I'm scratching my head over...
    1. The reverb motor boats when the reverb knob is > 2'oclock or just past 50% of the total knob travel. The reverb works but I can't go to far with it.

    2. The tremelo doesn't turn off with the footswitch. The switch is good and I get a good sounding tremelo...just have to use the knobs to turn it off.

    Hopefully someone can shed some light on this one for me.

    Thanks,
    CJ Landry
    Tags: None
  • #2
    Gibson GA79RVT Reverb motorboating

    Hey CJ. Have you removed the reverb potentiometer and measured it? It's possible there's a dead spot causing a short. Also, have you checked and or replaced any of the non-polarized caps associated with the reverb circuit?

    Anthony

    Comment

    • #3
      I have not replaced the coupling caps...that is what I am doing next. The pot measured OK.

      I hope to have this fixed for the Fort Worth Guitar Show this weekend.

      Thanks Anthony!!

      Comment

      • #4
        Did you replace all the filter caps? I thought motorboating was a filter cap related problem? Could be that the 'verb is just picking it up when you dial in it because the 'verb is the most sensitive circuit in the amp.
        Building a better world (one tube amp at a time)

        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

        Comment

        • #5
          OK, I confess, there is one filter cap that I have not replaced. I haven't replaced it because I can't find a multi cap that will fit in that spot. I'm going to have to rig something (that is if the coupling caps don't do the trick)

          Maybe if I dance around the amp and say a few words over it, I won't have to replace that one filter cap (my fingers are crossed)

          CJ

          Comment

          • #6
            Well you can "dance around it" 'til yer nuts 'r numb but you'e going to wind up replacing the cap eventually if it's true motorboating. If you wanna make sure before you do permanent work you can simply bridge each section of the bad filter with a good replacement cap and see if the problem ceases - if the filters were shorted you'd know it by now so bridging for diagnosis won't hurt. Is the cap you're having trouble sourcing one with leads at both ends held to the chassis underneath with a strap? Gibson was famous for those and I've never found a replacement.

            Rob

            Comment

            • #7
              I replaced the three coupling caps in the reverb circuit and the multi cap filter cap. That did the trick... no more motor boating. Finding a spot in the chassis for the multi cap was a challenge. It took some jerry rigging and big side cutters to form the cap clamp, plus a standoff or two to secure the new cap. Its not pretty but its functional.

              Thanks for everyones help,
              CJL

              Comment

              Previous template Next
              Working...
              X