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gibsonripper 19th January 2008 12:10 AM

Fender Twin Reverb VIBRATO PROBLEM - weak vibrato
So I've been stumped by this before...but (PLEASE READ THOROUGHLY: as you may find that I've already tried most suggestions that are typical)....
This Fender Twin I'm working on has a weak vibrato.

The 1st stage oscillator seems fine. Producing a decent sine wave off the plate...

The 2nd stage is more of a slight negative pulse (mostly flat line)... it seems that this tube is almost biased "off" (plate very positive / signal clipping positive / etc.).

I've swapped the tube.
I've installed a new 10M and 100k (feeding the LDR) resistor feeding this tube's plate, as well as the LDR. I've replaced the bypass caps, the phase shift oscillator caps and the cathode resistors.

The plate voltages and cathode voltages are right on (vs. the schematic).

What else could cause this?

Is it my imagination that in the past, I've seen a fuller cleaner sine wave at the output of the 2nd stage driving the LDR? Or is this just the way it is designed?

danFrank 19th January 2008 05:16 AM

If I remember correctly, the oscillator powers a neon lamp which causes the LDR to do it's thing. The neon lamps have a tendency to get flaky with time; replace it.
One more thing... the closer the side of the neon lamp is to the LDR the better the effect. You can use black electrician's tape to place the two in close proximity to each other and keep outside light out.

gibsonripper 19th January 2008 05:23 AM

Oops... it's fixed.
I hadn't noticed a weak solder connection at the grid of the 2nd stage. I touched it up and it's all right. strange, but i guess it had been capacitively coupled (out of design, via broken solder joint) and very weak.

And yeah, I replaced the Opto-coupler. When I said LDR, I meant Opto-coupler--my mistake.

It's true that keeping the light close and sealed with heatshrink helps wonderfully on the Opto.

Thanks for the feedback. Much appreciated.

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