If changing the chassis ground wire made a difference, there may be a ground loop or ground fault.
Check for any loose pot nuts, check all the pot mounting brackets (wings) solder connections as they are used to make ground connections in some places.
Check for any non-stock parts, like pots missing their 'wings' or plastic jacks that have been replaced by metal (which can cause ground loops).

Quote from: phatt on November 27, 2022, 08:12:52 PMHum,,My thought, What about Bridging R and T?
Find them at Test point 8 after IC4A also at J21

R and T are normally connected. (through insert jack, as J21 goes to J32)
However, dirty contacts in the insert could make noise, so bridging R and T would be a good test, like we normally jumper fx loop send to return.

The reason I suggested cutting the traces to the DSP was in case noise would still bleed into the signal, if the DSP is at fault.

Can you read schematics ok and translate to the board traces?
If so, cut the traces where red x's are shown.  Then connect the 2 traces shown with the blue link.
Reconnect any boards you disconnected.  Amp should mostly work.

You cannot view this attachment.

Are you asking where the power supply connections go to on the circuit board?

It's not possible for that mod to make your amp 'always on', so somehow you have made a mistake.
Pictures of how you did it will help.

Many Boss/Roland schematics seem like fabled unicorns if you are not authorized to have them.    They are very protective of their IP.
I'm a bit surprised to see the MkII is leaked and available online.  The MkI is not though.
Here is a thread with a couple bits of the schem and some info about the less common pcb mount fuses:
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/963156-need-help-repairing-a-boss-katana-50-mk1

Quote from: Montywitha-Y on October 08, 2022, 07:16:13 PMGrounding out the very tip of the plug switched it to channel 3. Grounding the ring made it switch between 1 and 2.

Do you have an ohm-meter to check your work?
put one probe at the plug tip and one at the sleeve (ground)
When you push the switch it should read 0 ohms, when not pushing switch it should read 'OL' or similar.

If you don't have a meter, post some pictures of the wiring at the plug and switches.

When you were doing the tests to see if you needed momentary or latching switch, how did you get it to engage channel 3 ?

joecool85 explained how to test whether you needed momentary or latching type switches.  What were the results of that test?

If you don't find DC on the output you may have blown a ground trace.
If you do find DC, probably a blown output transistor as mentioned above.

Quote from: mandu on June 19, 2022, 10:23:46 AMI would suggest you experiment with those old electrolytic capacitors before throwing out to learn something and acquire some knowledge out of it. Regards.

Yes this is a good idea.  And running the charger with a limiter lamp will avoid the overheating that you experienced, and sometimes bring them back to life if you are patient.

Quote from: BenGunn on June 03, 2022, 06:22:21 AMMaybe I'll wait for an answer from other members of the topic?

I sent a PM to bajaman for you.  Hopefully he will respond in this thread.

So it seems the problem is on the -48V rail.  I'm not sure why it didn't show up when you measured the AC volts there before.

Check that the solder is good and there are no broken pads at C48.  Sometimes the pads break near the cap and you can't see it because the cap is glued down.  Measure resistance from the (-)lead of the cap to Q2 collector just to be sure.
If no solder or pad/trace issues, replace C48.

So there is excess 100Hz hum in both the preamp and the power amp.  That pretty much narrows it down to the power supply.
Recheck again the DC and AC volts on the power supply rails.

That is a substantial difference.
I searched for "100Hz squarewave" on youtube and it sounds pretty much the same as your soundfile in post #41.
Are you on 50Hz 240V power?

Can you run the 'preamp out' to another amp and see if it has the same sound?