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.

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.

[Rob Strand]

My guess (as I did not analyse the expected voltage) is 6.8V is a bad voltage due to a fault.  That voltage could blow-up the processor.

There was a thread where a guy had a PSU fault on a Boss PH-3. Boss OC-3   He did a good job documenting the problem and while he fixed the power supply the unit as a whole was still dead afterwards.

IIRC, some of those DSP based Boss pedals use the same boards with different software.   Despite that, the PSU ckt is very similar across the models.
------------------
Actually here's the thread.  It was in fact an OC-3.

https://www.diystompboxes.com/smfforum/index.php?topic=121371.0

[duff2]

My guess (as I did not analyse the expected voltage) is 6.8V is a bad voltage due to a fault.  That voltage could blow-up the processor.

There was a thread where a guy had a PSU fault on a Boss PH-3. Boss OC-3   He did a good job documenting the problem and while he fixed the power supply the unit as a whole was still dead afterwards.

IIRC, some of those DSP based Boss pedals use the same boards with different software.   Despite that, the PSU ckt is very similar across the models.
------------------
Actually here's the thread.  It was in fact an OC-3.

https://www.diystompboxes.com/smfforum/index.php?topic=121371.0

Awesome!!!  Thanks for bringing this to my attention!!

As posted in that thread, I am going to replace the IC and MOSFET with a conventional 3.3V voltage regulator.  If that doesn't return things to normal, I'll know to not waste any more time on it.

[Rob Strand]

The output diode might be worth checking as well.

[duff2]

The output diode is fine and there are no shorts on the MOSFET.

However, there is no periodic square wave signal on Pin4 of IC6.  It appears that this is the source of the problem, so I have ordered a replacement for IC6. 

With regards to replacing the MOSFET and IC6 with a conventional 3.3V regulator, I found this locally: 

https://abra-electronics.com/ics-semiconductors/voltage-regulators/l4931-3.3v-250ma-linear-voltage-regulator-l4931-3.3-to-92.html

I'm planning to give this a try while I'm waiting for a new IC6 to show up in my mailbox.

[Rob Strand]

However, there is no periodic square wave signal on Pin4 of IC6.  It appears that this is the source of the problem, so I have ordered a replacement for IC6.

Agreed, looks like IC6.

With regards to replacing the MOSFET and IC6 with a conventional 3.3V regulator, I found this locally:

https://abra-electronics.com/ics-semiconductors/voltage-regulators/l4931-3.3v-250ma-linear-voltage-regulator-l4931-3.3-to-92.html

I'm planning to give this a try while I'm waiting for a new IC6 to show up in my mailbox.

I'd probably go for a much larger 3.3v regulator.   That small package is only going to give about 70mA on the 3.3V rail; due to heat limitations.

The OC-3 is spec'd at 50mA on the *9V* rail so if a good proportion of that is going down the 3.3V rail it's going to be drawing upto 180mA on the 3.3V rail.

In other words there's a chance it won't work.  If you use a larger regulator then you at least know it's not going to shut down due to overheating.  Since it's only a temporary solution you don't have to skimp.   The downside is the larger regulator will have a higher short circuit current and so will have more chance of taking something out in the case of a fault - with 6.8V on the 3.3V rail that might be the last of your worries!
 

[duff2]

However, there is no periodic square wave signal on Pin4 of IC6.  It appears that this is the source of the problem, so I have ordered a replacement for IC6.

Agreed, looks like IC6.

With regards to replacing the MOSFET and IC6 with a conventional 3.3V regulator, I found this locally:

https://abra-electronics.com/ics-semiconductors/voltage-regulators/l4931-3.3v-250ma-linear-voltage-regulator-l4931-3.3-to-92.html

I'm planning to give this a try while I'm waiting for a new IC6 to show up in my mailbox.

I'd probably go for a much larger 3.3v regulator.   That small package is only going to give about 70mA on the 3.3V rail; due to heat limitations.

The OC-3 is spec'd at 50mA on the *9V* rail so if a good proportion of that is going down the 3.3V rail it's going to be drawing upto 180mA on the 3.3V rail.

In other words there's a chance it won't work.  If you use a larger regulator then you at least know it's not going to shut down due to overheating.  Since it's only a temporary solution you don't have to skimp.   The downside is the larger regulator will have a higher short circuit current and so will have more chance of taking something out in the case of a fault - with 6.8V on the 3.3V rail that might be the last of your worries!

Good advice!  Thanks!

[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.

[Rob Strand]

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. 

Yeah bummer.  At least you gave it a shot.   

I guess not many 3v3 devices will survive 6v8. Back in the day, circuits used crobar protection to stop a failed regulators taking out the digital stuff.