Need help repairing a Boss Katana 50 MK1

[heekyet]

I am having a similar fault with my Boss Katana Mk 1. The earphone output is working but the speaker is not sounding. It looks like I the TDA7293 may be the problem too for me. I'll start with that based on @icefire's experience.

[Hanger-18]

Hey I'm hijacking this slightly. I just got a used KTN-50 and it wouldn't come on. I checked the glass fuse and it was OL. I jumped it with a different style fuse and got power and lights. Still NO SOUND. I ordered new glass fuses and the circuit board ones. I checked the output at the headphones and no sound. I will try to use the aux input to verify the power amp. Anyone have this happen??

[MatsEriksson]

I don't know if this is appropriate to have as a first post in here ever, but I'll try.
Since this is the topic "need help with a Katana 50 mk1" I thought this would fit in here as well. I've bought this one new in 2016, and it served me very well at home as a practicing amp at low volumes. Not ever brought it out live, because it couldn't cut it. So I have not taxed it to the max and cranked it.

Now here's my recent problem, it has - sort of - "died" on me in the following way:

1. Whenever I turn it on, just by flicking the on/off switch, a loud tone (not ground failure note with buzz) at about 50-60 Hz is heard although the switch selector is set to standby and every other knob, volume, eq's are set to zero. No output from headphone jack or auxillary at the back.

2. The tone is heard about full volume, as I can't stand next to it without using headphone like ear protections (like those used by construction builders). I can rotate all knobs without anything changing. Stale and consistent volume and tone.

3. The tone is more sinus wave like than the "ground failure" one with added harmonic buzz. After a minute it starts to tick, and slowly fades away until it becomes dead silent after say 2 and a half minute. No smell at all, as indicating overheating.

I've downloaded the Service Manual (for the Mark1) as displayed here and tried to fail search it to no avail. I just wonder if someone else has had the exact same problem? I have tried to do a search in here first but no one seemed to have the exact same problem. As it is, I think I should change out a couple of components, as suggested and done before in this post, but I really think something is under-dimensioned for it all, because I think this would happen again.

Maybe this is a "Boss service center only fix" or? About the cost as a brand new amp again?

EDIT: I have soldering skills, and multi-meter skills.

[The Dude]

It would be best to start a new thread for your amp. Not all amps fail in the exact same way. That said, your problem could be caused by many things. I would first unhook the speaker to avoid damaging it. Then measure the output and see if the noise is AC or DC. If it's DC, there's likely and output device failure. If it's AC, my first suspect would be filter caps or cracked/cold solder on their connections. As always, start by checking all power supply voltages and make sure they are up to snuff.

[MatsEriksson]

OK I'll start a new thread. See if I can make a sound file of the "noise" from it.