Boss OC-3 - CPU/DSP Power Supply issue

[duff2]

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.

[Jazz P Bass]

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.

[duff2]

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!

[duff2]

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.

[Jazz P Bass]

I would suggest that you also check the mosfet that EXT is driving.
Check to see that it is not shorted.

[duff2]

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?

[duff2]

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.

[g1]

Double check for any kind of reset procedures in the service manual.