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Thread: Boss OC-3 - CPU/DSP Power Supply issue

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    Boss OC-3 - CPU/DSP Power Supply issue

    Hi,

    I have a Boss OC-3 Super Octave pedal that has stopped working.

    I suspect that this issue may be related to a doubling of the CPU and DSP supply voltage from 3.3V to 6.8V.

    Here is the schematic and the voltages I measured (in red) for the SMPS section that supplies power to the CPU and the DSP chips.

    https://drive.google.com/file/d/1H65...ew?usp=sharing

    Is it safe to assume that I need to replace the switching controller (S-8520E33MC-BJS-T2) IC6 or could there be another downstream reason for a doubling of the supply voltage?

    I am powering the pedal with a 9V Boss PS-1 adaptor.

    Thanks.

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    Supporting Member Jazz P Bass's Avatar
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    Something is not right.
    Either the IC or the mosfet. (or both)
    The EXT pin should be modulating the mosfet.
    Do you have a scope?
    The IC itself is a voltage regulator that will bring the battery voltage down to 3.3Vdc.

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    Quote Originally Posted by Jazz P Bass View Post
    Something is not right.
    Either the IC or the mosfet. (or both)
    The EXT pin should be modulating the mosfet.
    Do you have a scope?
    The IC itself is a voltage regulator that will bring the battery voltage down to 3.3Vdc.
    I do have a scope. I'll check the signal on pin 4 of the voltage regulator later today and post the results here.

    The area near the IC and the MOSFET gets quite warm after a few minutes and the pedal is drawing 70mA of current from the battery.

    I guess that would make sense if the circuit is now behaving like a Series Transistor Regulator instead of a Buck Switching Regulator.

    Thanks for the reply!

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    Quote Originally Posted by duff2 View Post
    I do have a scope. I'll check the signal on pin 4 of the voltage regulator later today and post the results here.

    The area near the IC and the MOSFET gets quite warm after a few minutes and the pedal is drawing 70mA of current from the battery.

    I guess that would make sense if the circuit is now behaving like a Series Transistor Regulator instead of a Buck Switching Regulator.

    Thanks for the reply!
    According to my scope there's no square wave signal on pin 4, so I guess the IC is the source of the problem. I'll update this thread as soon as I have sourced and installed a replacement.

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    Supporting Member Jazz P Bass's Avatar
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    I would suggest that you also check the mosfet that EXT is driving.
    Check to see that it is not shorted.

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    The MOSFET appears to be fine. No shorts between pins 1 and 3, 1 and 2 or 2 and 3.

    Someone also also suggested removing IC6 and the MOSFET from the board and replacing them with a conventional 3.3V voltage regulator wired up this way:

    Input on pad 5 of IC6
    Ground on pad 2 of IC6
    Output on pad 3 of IC6

    Is that worth trying while I'm waiting for the replacement IC to show up?

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    Replacement of the switching controller (IC6) restored the DSP/CPU DC supply voltage to +3.3V.

    Unfortunately, my OC-3 still does not function.

    A self test run according to the procedure described in the service manual indicated that there is still a problem with the DSP and CPU, even after correcting the power supply issue.

    So I guess the pedal is officially toast.

    Bummer.

    I think I'll stick to using analog effect pedals in future.

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    Don't forget the joker g1's Avatar
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    Double check for any kind of reset procedures in the service manual.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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