Can anyone help me understand/repair the tremolo in my sentura II?

[cooldude666]

Hello, I've been trying to repair my sunn sentura II's tremolo circuit. I've checked over the caps and resistors and they all appear to be ok but I'm not sure I understand the function of that SM-1 unit. It looks like an LED and a phototransistor in a little package, but it's not putting out voltages/resistances like I'd expect.

My version of the story is that the LED activates the phototransistor which oscillates resistance, creating a square wave that gets added to the guitar's signal to get the tremolo effect, but the resistance stays constant. It does change when I change the resistance of the depth pot, so the LED appears to be functioning internally. Am I grossly misunderstanding how this is supposed to function? I imagine this could be the fault of the transistor before the LED? Please help!!

[Jazz P Bass]

From the Ampage archives, current member Loudthud gives an excellent description of the tremelo circuit & the photoresistor.(it is not a 'phototransistor')
Link: http://sunnforum.ampage.org/sdp/inde...ic,4772.0.html

[loudthud]

Also check this thread: http://sunnforum.ampage.org/sdp/inde...ic,4836.0.html

[cooldude666]

Loudthud gives an excellent description of the tremelo circuit & the photoresistor.(it is not a 'phototransistor')
Link: http://sunnforum.ampage.org/sdp/inde...ic,4772.0.html

That's what I thought, but the guy at the electronics supply store here told me otherwise. Shows what he knows. Thanks for this information, but I still don't understand what is supposed to be switching the light off and on. Id guess that 2N2102 transistor before the LED, but I'm seeing a constant voltage running into it. The unit was rebuilt at some point because this is what's in it now, replacing the SM-1 module.
Excelitas Technologies Sensors - VTL5C3 - Optoelectronics & Lighting - Optocouplers/Optoisolators - Allied Electronics

I've also noticed that I get intermittent volume drops when I'm playing. Thanks for pointing me to that other thread. I'm going to try jumpering this out of the signal path.

[nickb]

Yes, it's the 2N2102 that turns the current on and off. You say you see a constant voltage - are you using a scope to check that? The signal might be changing too quickly for for a meter to see.

On the volume drops, you can disable the tremolo by shorting the collector to the emitter of the 2N2102. Full volume should return and you should not get any drops. You can turn it off by shorting the base to ground - the volume should reduce dramatically. This will tell you if the opto-isolator is OK.

FWIW, I would have guessed the VTL5C4 would be better as it has a resistance range that is better suited giving a deeper tremolo and the response time is slower giving a more natural feel to the tremolo effect. That's something you can experiment with and let us know.

[cooldude666]

Yes, it's the 2N2102 that turns the current on and off. You say you see a constant voltage - are you using a scope to check that? The signal might be changing too quickly for for a meter to see.

No, I'll check this with a scope. .. now to find a scope.

On the volume drops, you can disable the tremolo by shorting the collector to the emitter of the 2N2102. Full volume should return and you should not get any drops. You can turn it off by shorting the base to ground - the volume should reduce dramatically. This will tell you if the opto-isolator is OK.

How will this tell me if the opto-isolator is OK? If it's not what will I hear? Will the volume not drop if the opto-isolator is bad?

FWIW, I would have guessed the VTL5C4 would be better as it has a resistance range that is better suited giving a deeper tremolo and the response time is slower giving a more natural feel to the tremolo effect. That's something you can experiment with and let us know.

Thanks. I'll order one and test it out once it's working at all.

[Jazz P Bass]

The two 2N2923 transistors and all the components to the left, are your low frequency oscillator.
Any faulty capacitor (or resistor, transistor, connection) in that part will kill (or diminish) the LFO.
When working correctly, the LFO will vary the base of the 2N2102, thus varying the lamp.
The lamp will vary the photoresistor.
You have to get the LFO running.

[nickb]

How will this tell me if the opto-isolator is OK? If it's not what will I hear? Will the volume not drop if the opto-isolator is bad?

There will be limited or no volume change if the OI is bad. You could also try shorting out the LDR side of the OI to remove it form the circuit all togther and see if your odd volume drops go away.

[cooldude666]

volume drops were from a dying power tube.