Fender Twin Vibrato Problem

[ElPedro]

Hey all.  Having trouble with my Fender Twin.  The Vibrato takes a little bit to engage after I turn it on. Once it has come on, I can turn it on and off and it works fine.

When I first turn it on, I can hear the vibrato thumping, but the guitar signal isn't getting through to the vibrato.

I'll try swapping the tube, but are there any other suggestions?

[sluckey]

Which model twin are you talking about? There's a big difference in the vibrato circuits of different models.

[ElPedro]

Sorry...

It's an ab763. Not a reissue.

[sluckey]

Replace the tube first. Then replace the roach (optocoupler).

[PRR]

> Vibrato takes a little bit to engage after I turn it on

Slow-to-start is more likely tube or cathode cap.

It *could* be that once it comes on, it holds a charge for a while so then it goes off/on quickly, but not if it has been dead a while.

Without looking, if this model has a kick-start (negative bias when off), that could have failed, but unlikely.

I would rate opto-coupler low on a list of slow-to-start reasons, but optos do have some strange failures.

[ElPedro]

Thank you for your thoughts and ideas. I will try the tube and cap tomorrow. This is a kickstart model - I remember the pin on the back of the rca jack read somewhere around -55vdc when I checked it.

[DummyLoad]

> Vibrato takes a little bit to engage after I turn it on

Slow-to-start is more likely tube or cathode cap.

It *could* be that once it comes on, it holds a charge for a while so then it goes off/on quickly, but not if it has been dead a while.

Without looking, if this model has a kick-start (negative bias when off), that could have failed, but unlikely.

I would rate opto-coupler low on a list of slow-to-start reasons, but optos do have some strange failures.

+1


if new tube doesn't do the trick, replace cathode cap with a 47uF 25V.


--pete

[ElPedro]

Would a 50u 50v work ok?

[sluckey]

yes

[ElPedro]

Thanks, all.  It was the roach.

[PRR]

So Sluckey was right.