Fender Bandmaster AB763 Vibrato problem

[StratTone]

Anybody know how I can look at my OPTOISOLATOR to see if it is stock? The tube is fine that drives it but not vibrato. I'm going to crack it open tomorrow but wanted to get some tips on what to look for before I go in. Also if my osolator is bad who sells a proper replacement? Any good testing methods? Thanks again guys.

[tubeswell]

The Light-Dependent Resistor, which might need replacing, is shown in the attached .pdf document. (Was that your qn?) I think any LDR that can take 500V will work

[StratTone]

I've never acutally opened up the roach. What is the resistor value? So any LDR that will handle 500V will work? Value doesn't matter?

[52 Bill]

Originally Posted by StratTone

I've never acutally opened up the roach. What is the resistor value? So any LDR that will handle 500V will work? Value doesn't matter?

There is little or no voltage on the LDR, only audio from the second pre-amp output. The LDR grounds out the signal path to create the effect.

The "roach" is an LDR (light dependent resistor) and an NE-2 neon lamp bulb, coupled together in a heat shrink tube. If you can see the light flashing through the end of the heat shrink tube, then your LDR is suspect. Normally they will become very unresponsive and will not give you a very deep effect. Almost any modern photocell will work as a replacement, but due to reaction time, some will handle the fast speed settings better than others.

If you don't see the lamp flashing, then there is a problem with the circuit. Do you have a foot switch or shorting plug in the pedal jack? Is the oscillator itself running?

In all the years that I've seen these things I've only seen one neon bulb failure, and that was due to a broken lead.