AB763 Vibrato problem

[Marcfrom]

Hi Guys,

The vibrato of my Vibroverb take a long time to be active (30 -40 seconds) sometimes more ,when the amp is cold (first start of the day). After that it works properly.

Does anyone have an idea?

Thanks.
Marc

[martin manning]

Sounds like maybe the oscillator tube is getting weak. Have you tried a replacement?

[Marcfrom]

Yes I already replaced the V5, I will try a spare tube in V4.

[martin manning]

A tired V4 won't keep the oscillator from starting quickly. Probably the next most likely problem is the caps in the feedback loop are leaking. Do you have any DC voltage to ground (term off) at the lead going from the 0.01 and 0.02 caps to the "speed" pot?

[Marcfrom]

Ok, I will open the amp and check the voltage at this place.

Do you think is probably caused by one of the three ceramic disc ?

[martin manning]

Ok, I will open the amp and check the voltage at this place.

Do you think is probably caused by one of the three ceramic disc ?

It's possible. You should see at most a few mV DC there.

[Marcfrom]

Ok, I will check it.

What do you mean by (term off) ?

[martin manning]

Sorry, the spell checker got me... should have been "trem," short for tremolo. What Fender really meant when they called it "vibrato."

[Marcfrom]

Ahh, yes

Do you think it's possible that's caused by one of the 22uF Filter caps?

I had a problem before that with one of the 70uF, it was completely closed (as a wire) after to put a failing 12AT7 in V3

[martin manning]

No, as long as you have something close to the schematic value for the screen node voltage (~450V after the choke) that's all you need.