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Thread: Theremin Repair

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    Theremin Repair

    I pickup up a non-functioning Big Briar Etherwave Theremin. The problem is extremely low, almost no output.

    I sometimes work on guitar amps, but this is a different world to me. For those unfamiliar, the volume output should be zero when hand is on the volume antenna, and increase as your hand moves away.

    You can hear a very faint output, and the pitch is effected by the pitch antenna as it should be. So the problem likely resides in the Volume part of the circuit.

    Voltages are ok.

    If I put my finger on the inductor L7 I get a strong output with pitch antenna functioning.

    There is a sine wave signal coming out of the Volume Oscillator into collector of Q8. Nothing on Q8 emitter.

    The LM13600 appears to be functioning. I initially swapped it with a LM13700 just to check, made no change.

    Schematic is attached here. Thank you in advance for any help!
    Attached Files Attached Files

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by garytoosweet View Post
    I pickup up a non-functioning Big Briar Etherwave Theremin. The problem is extremely low, almost no output.

    I sometimes work on guitar amps, but this is a different world to me. For those unfamiliar, the volume output should be zero when hand is on the volume antenna, and increase as your hand moves away.

    You can hear a very faint output, and the pitch is effected by the pitch antenna as it should be. So the problem likely resides in the Volume part of the circuit.

    Voltages are ok.

    If I put my finger on the inductor L7 I get a strong output with pitch antenna functioning.

    There is a sine wave signal coming out of the Volume Oscillator into collector of Q8. Nothing on Q8 emitter.

    The LM13600 appears to be functioning. I initially swapped it with a LM13700 just to check, made no change.

    Schematic is attached here. Thank you in advance for any help!
    What amplifier are you feeding it into

    on their webpage it says

    Audio Out - standard 1/4" phone jack, which delivers line level output to your amplifier.

    nosaj

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    Running it into a Fender Champ.

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    Quote Originally Posted by garytoosweet View Post
    I pickup up a non-functioning Big Briar Etherwave Theremin. The problem is extremely low, almost no output.

    I sometimes work on guitar amps, but this is a different world to me. For those unfamiliar, the volume output should be zero when hand is on the volume antenna, and increase as your hand moves away.

    You can hear a very faint output, and the pitch is effected by the pitch antenna as it should be. So the problem likely resides in the Volume part of the circuit.

    Voltages are ok.

    If I put my finger on the inductor L7 I get a strong output with pitch antenna functioning.

    There is a sine wave signal coming out of the Volume Oscillator into collector of Q8. Nothing on Q8 emitter.

    The LM13600 appears to be functioning. I initially swapped it with a LM13700 just to check, made no change.

    Schematic is attached here. Thank you in advance for any help!
    How about removing and reseating the connectors?

    nosaj

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by nosaj View Post
    How about removing and reseating the connectors?

    nosaj
    Does the Vol tuning affect anything?
    An anywhere there is a voltage on the schematic it checks ok?
    nosaj

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    Volume pot has no effect.

    All +-12V are correct.

    R16 is 0.135V, 0.2V on schematic.
    R6 is -0.65V, -0.6 on schematic.
    R2 is -0.65V, -0.6 on schematic.

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    Lifetime Member Enzo's Avatar
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    Keep it simple. Touching L7 makes it work loud? OK, then is there continuity from the volume antenna down to L7? In other words is there an open L8, L9, L10 or the connections between or up to the antenna?

    A loud signal when touching L7 tells me the circuits are working.

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    There is continuity from the volume antenna down through the inductors.

    I read the below passage from the service manual, and verified the junction of C12 and R14 fluctuates between -3V and 0V depending on hand placement.

    Click image for larger version. 

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    I thought the low voltage at Q6 and Q7 emitter was suspect, so I installed sockets and replaced Q6 with a new 2N3904. Voltage went from 0.135V to 0.212V, much closer to schematic.

    Problem still persists.

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    Line level signal is much higher level and a diffent impedance than instrument level signal. Can you plug it into an aux or tape input on a stereo or a line input on a PA , effects return, or main amp in on an amp? A can of freeze might be helpful. I would tickle that op amp on the output. Is that output coupling cap good? Do you have a scope.

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    I do have a scope. No freeze can unfortunately.

    Swapped the op amp for a new one and no change. With a finger near or touching pin 1 I get a loud (regular loudness) output that is NOT affected by the volume antenna, Pitch antenna functioning OK.

    Pulled C26 and tested, checks OK

    I don't have access to a proper Line Input right now, but guitar amp should be ok for troubleshooting, no?

    I have also verified front panel wiring, all pot/output connections look good, and all voltages check OK.

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    Last edited by garytoosweet; 03-25-2020 at 11:22 PM.

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    Post E,B, & C voltages for Q6,7, & 8

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    Q6
    E: .205VDC
    B: -0.057VDC
    C: 12.07VDC

    Q7
    E: .205VDC
    B: -0.014VDC
    C: 12.08VDC

    Q8:
    E: -2.842VDC
    B: -2.2VDC
    C: 12.07VDC

    Thinking there may also be a tuning issue, and the unit is tuned 'out of range', which would exhibit similar symptoms. I will need to tune it at some point, but first will need to pick up a frequency counter as my DMM only measure up to 50kHZ, L5 needs to be tuned to 286khz.

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    Old Timer olddawg's Avatar
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    Quote Originally Posted by garytoosweet View Post
    Q6
    E: .205VDC
    B: -0.057VDC
    C: 12.07VDC

    Q7
    E: .205VDC
    B: -0.014VDC
    C: 12.08VDC

    Q8:
    E: -2.842VDC
    B: -2.2VDC
    C: 12.07VDC

    Thinking there may also be a tuning issue, and the unit is tuned 'out of range', which would exhibit similar symptoms. I will need to tune it at some point, but first will need to pick up a frequency counter as my DMM only measure up to 50kHZ, L5 needs to be tuned to 286khz.
    Use your scope to count the frequency. Itís easy.

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