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

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

    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!!


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  • #2
    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

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    • #3
      Also check this thread: http://sunnforum.ampage.org/sdp/inde...ic,4836.0.html
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !

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      • #4
        Originally posted by Jazz P Bass View Post
        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.

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        • #5
          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.
          Experience is something you get, just after you really needed it.

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          • #6
            Originally posted by nickb View Post
            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.

            Originally posted by nickb View Post
            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?

            Originally posted by nickb View Post
            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.

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            • #7
              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.

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              • #8
                Originally posted by cooldude666 View Post
                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.
                Last edited by nickb; 09-28-2012, 02:16 PM. Reason: Typo!
                Experience is something you get, just after you really needed it.

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                • #9
                  volume drops were from a dying power tube.

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                  • #10
                    BACK FROM THE DEAD!
                    I pulled this guy out after sitting to try and fix it again. Going to sell it but wanted to try and get it in full working order before I did so (in the interest of $$). Problem seems to be at the lamp. The LFO is operating as I can now hear it clicking away. Seems the tech who had it before me did a god awful job soldering and almost every joint was cold, some so badly that the solder hadn't even bonded to the lead and the lead would slip right in and out through the solder. I don't read that 4.5 between lamp leads, however here's the thing- the sm-1 has also been replaced by the opto-isolator previously mentioned in this thread. Is that 4.5 still relevant? This thing has an LED inside, not a little incandescent lamp. I'm figuring that this thing is going to need modified with that in mind. Any input?

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                    • #11
                      yee-haw! solved. Awful solder job strikes again. I noticed that a fonz style pound on my desk would kick the trem into action. I re-soldered the board and voila! This is the first time since I bought this baby that it's fully functioning. Trem, and verb. Sounds nasty. I almost don't want to sell it.

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                      • #12
                        Are we to assume from your response that the VTL5C3 worked well without circuit modifications?

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