OP-amp help...

Discussion in 'General Electronics Chat' started by Marc0, Aug 28, 2012.

  1. Marc0

    Thread Starter Member

    Nov 28, 2011
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    Hi, I'm sudying the schematics of an old analog synth I have.
    I would like to know the gain of the OP-amp which is used to amplify the signal coming from a voice chip.
    The OP-amp is wired as follows:

    - the inverting input of the OP-amp is connected directly to the output of the voice chip.

    - the non inverting input is connected to ground.

    - there is a feedback loop between the OP-amp output and the inverting input, inside the loop there are a 47kOhm resistor and a 100pf capacitor in parallel

    Is there a way to know that gain? I have found many calcultators online, but they require R1 and R2 values...here I see just one resistor!
     
  2. Jony130

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  3. Marc0

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    Nov 28, 2011
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    yes, but there's also a 100pf capacitor on top of the resistor (in parallel), and I'm sure that the OpAmp here is meant to act as a voltage amplifier, because its inverting input is feeded with the waveforms (millivolts) coming from the voice chip.
    The chip used as an OpAmp is a Sharp IR94558 (it's a dual OpAmp, but only one half is used here)
     
  4. Marc0

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    Nov 28, 2011
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    [​IMG]
    Basically it's like pictured above, but there's no resistor before the inverting input (labelled V_in), because it's connected directly to the output of a voice chip.
     
  5. t06afre

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    Can you post a copy of that schematic?
     
  6. Marc0

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    Nov 28, 2011
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    https://www.dropbox.com/s/70ypfqqlbf0qki4/2012-08-28 13.05.49.png

    Here's a dropbox link (can't post directly the image, don't know why)...
    As you can see, pin 8 (which is the output pin of the voice chip IC59) is connected directly to the inverting output of the opamp (IC64).
    The +v and -v power supplies of the opamp are both marked as Vref, but don't know exactly how much they are.
     
  7. Jony130

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    It looks like that pin 8 of a VCA (voltage control amplifier) has a current output.
    And this op amp work as a current-to-voltage converter. The gain is equal to
    Vout/Iin = 47000V/A = 4.7V/μA. And 100pF capacitor don't have any effect on audio band range. This capacitor start to reduce the gain from about F > 0.16/RC = 0.16/(47K*100pF) = 34KHz
     
  8. t06afre

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  9. Marc0

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    Nov 28, 2011
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    Wow, do you mean that IC59 does not output a voltage signal, but a current signal instead? Never imagined that! Thank you for sharing your knowledge with me.
    I have been troubleshooting (looking for a fault) this circuit for almost a month and never realized that...
     
  10. t06afre

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    Also remember that you will measure 0 volt at pin 6 IC64 (or pin 8 IC59) even if the circuit is in working order ;)
     
  11. Marc0

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    Nov 28, 2011
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    Actually THIS is what was driving me mad!!!:D

    One (or more) of the 6 voice chips remains on... to understand which one I started checking each of the six IC59 pin 8 with the oscilloscope, to see which ones was still playing...BUT no voltage was showing (only a 1 or 2 millivolts noise).
    Now I understand why, but I also don't know what else to do...I can't use this forum to troubleshoot and repair a synth!
     
  12. t06afre

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    The output (pin 7 IC64) is marked with TP5. TP is an abbreviation for test point. You will probably find some reference info in the service manual, about this an the other test points. Saying something the expected voltages/waveforms.
    Do not give up that easy. Give it some time. Maybe some of the members not in your part of the world. Still sleeping and hence not logged in yet may help you.
     
  13. Marc0

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    Nov 28, 2011
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    The problem is that IC64 (and so TP5) collects the output of all the 6 voice chips...when the fault occurs I can of course see what's happening on TP5, but I can't tell which one of the 6 voices is failing (not shutting down).

    The circuit is arranged as follows:
    a 4051 demultiplexer sends 6 control voltages to each of the six IC59 chips, exactly at their fourth pin (which is for Linear VCA CV input).
    The connection between the 4051 demultiplexer outputs and the IC59 cv inputs is not straight: just after each 4051 output pin, and just before each IC59 4th pin there is a capacitor connected to ground... in between there is also a 1mOhm resistor.
    Basically the fault could be in the 4051, which sends a bad CV to the VCA of one (or more) IC59 voice chips, or in the voice chip itself...
     
  14. t06afre

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    Are IC59 or IC64 placed in sockets?
     
  15. Marc0

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    Nov 28, 2011
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    No, everything on the board is soldered directly, but I'm quite good in soldering/desoldering jobs....
    I was thinking about placing a resistor on top of my oscilloscope probe...this could let me read
    (indirectly) the current on pin 8 of IC59?
    If yes...what resistance value for the resistor?
     
    Last edited: Aug 28, 2012
  16. Marc0

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    Nov 28, 2011
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    After some research I have found that the 4051 demultiplexer - which should send the same cv to the VCA section of each voice chip - instead is sending a different and odd looking cv on the fourth voice chip. Different in shape and amplitude.
    Could it be that the demultiplexer is broken?
     
  17. t06afre

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    The 4051 chip is not a rare chip. So just go for it. In this case the I would have cut it loose with a plier. Then desolder the component pins. As the PCB is more valuable than a cheap 4051 chip.
     
  18. Marc0

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    Nov 28, 2011
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    Ok, I just ordered a new 4051. Probably I will not be able to cut the legs of the old one, as you recommend...there's not enough space to work with cutters, because there are other components too close around.
    Are there any other recommendations? I just want to do things right, that pcb is from 1982 and not easily (and cheaply) replaceable!
     
  19. t06afre

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    Since your board is from 1982. I assume it uses DIP packages. You can get side cutters for electronics that easy will cut a DIP package out from the board. Even from above. They are not that expensive. Just remember do not use such a tool for anything else than thin component legs or thin copper wires
     
    Marc0 likes this.
  20. Marc0

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    Nov 28, 2011
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    Ok, small side cutters... thanks.
     
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