Troubleshooting tremolo on a Twin Reverb

[BloozeGit]

Hi all,

I'm helping a friend fix up a non-functional tremolo on a 60's BF Twin Reverb (AB763) and I'm trying to narrow down the likely problem areas.

The oscillator circuit and lamp work fine, but max-ing out the intensity only gives a faint motor-boating effect. I suspected the photocell and removed it from the circuit to test it. At normal room illumination it shows about 60ohms and shining an LED directly on it gives a decrease to about 30ohms so it does function as expected, but I'm not sure if these resistance values are normal spec.

Can anyone shine any light on whether the photocell needs replacement (and if so, where to get one from) or if there is something else I need to look at in the circuit? Your assistance is much appreciated.

[MWJB]

If you suspect the photocell, just buy another, Mojotone should have them.

Failures are usually either 1) the photocell, 2) one or more of the 3 oscillator caps (0.01/0.01/0.02), or 3) cathode bypass cap (25uf/25v) - it's usually quicker just to go straight in and change these parts than to do bit, by bit, testing along the way.

[BloozeGit]

Mojotone only seems to sell it as a combined optoisolator rather than just the photocell. Not ruling that out but I was hoping there'd be an off-the-shelf replacement photocell that I can get from my local electronics shops, if I could just get the specs.

[Enzo]

You haven;t tested the one you have. What does it do in the dark?

Have you verified proper operation of the intensity control?

[billyz]

Do you have a shorting plug in the vibrato jack? Also, Check the ground at the footswitch jack on the chassis. Often the jack loses good ground there, it is a mechanical ground, so loosen the jack nut spray some cramolin and retighten. Works most of the time.

[BloozeGit]

The shorting jack is in place and the intensity pot measures fine. The voltages are close to spec as well.

I'll test the photocell again, but can anyone tell me what the typical resistance should be?

[MWJB]

"I'll test the photocell again, but can anyone tell me what the typical resistance should be?" I don't know why anyone would measure it, if it works you leave it in, if in doubt you sub it out...it takes a couple of minutes. You can't charge clients for time spent logging the values of suspect parts...well, obviously you can but it's not really ethical. If you're not charging for your labour, then you're just wasting your time.

Is it the bulb, or the photosensitive resistor? Who cares? Buy the complete unit, take it out of the packet & fit it. Again, why waste your time assembling & shrink tubing the whole assembly, when it's done for you?

[BloozeGit]

I'm doing this as a favour and out of "academic" interest, so time charged is not an issue...

To put it in perspective, I live in SE Asia and buying stuff from US/Eur incurs much more cost and shipping, whereas I could just pop down to the multitude of local electronics stores and get the LDR if I just knew what to get.

If anyone can provide a part number for either the optocoupler (assuming the LED would work in place of the old bulb) or LDR, I'll be on my way.

[pdf64]

In the pre internet days, when bugs were hard to source in the UK, I tried using some off-the shelf photocells to replace a dead one on a TR.
I didn't find anything as good as a proper one. None that I tried had a sufficiently large light to dark resistance range or speed of response.
You'll need something which has a dark resistance of >>100k, light resistance <<1k, and response time (both light to dark and dark to light) <10mseconds. Peter.

[Enzo]

Both Mouser and Jameco have a selection of photocells, as do other suppliers I am sure.

[Gtr_tech]

And....rumor has it that photocells will be banned in EU soon. So...stock up if you think you'll be needing these as repair parts. That goes for vactrols too BTW. Its ROHS round 2....cadmium and arsenide this time.

[Sweetfinger]

If the neon lamp flashes, the oscillator/lamp driver part of the circuit is fine. one of the photocell legs will go to ground. check that connection. if that's fine, then short the two legs of the photocell together. If you hear the sound go off, suspect the photocell. If the sound does not go off when shorting the photocell, you've got a bad connection to the intensity pot or somewhere before the pot. In a pinch, I've used off the shelf CdS cells, I just looked for ones that had the highest dark resistance, and the lowest lighted resistance. The ones sold as replacements for opto-electronic Morley pedals work pretty well. And for the obvious check- ......plugged into the vibrato/reverb channel?

[BloozeGit]

Hi folks,

Sorry I forgot about this thread for a while. Just to round things up, I changed out the optocoupler (turns out the photodiode wasn't easily available) and the ticking was still there, though only when reverb was turned up.

A few swear words later I traced it down to the tube being shared by the reverb and vibrato portions of the circuit by swopping it out. It must have been microphonic or some stray conductance.

Thanks to all!

[spy]

Found this thread while searching about tremolo issues. I have something similar with a Twin Reverb. The weird is that it happens only on low frequencies. For example when hitting the low E string it flaps! Not exactly ticking. Unless it's the same. When playing the high strings or not bassy chords it sounds awesome.
I have tried everything, cap at 10M, change grounding on cathode cap from vibrato RCA, new el. caps, wiring under channel B pots, except changing the optocoupler. Is this optocoupler's fault? Something else? Also, it gets louder if you turn the volume louder. Don't know if this helps.

[52 Bill]

Doesn't sound like a tremolo problem to me. More like a power supply issue. Pull out the oscillator tube does it still do this?