Boss Katana MkI 50 W issues and troubles, again

[MatsEriksson]

Since I've been requested to start a new thread instead of "resurrecting" a 2 year old post, I do it here then.

I don't know if this is appropriate to have as a first post in here ever, but I'll try.
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. This fault came all of a sudden, and has not starting to slowly show up, with suspicious behaviour beforehand.

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. No changes is heard in sound whatever knob I turn, the power selector (from standby to anything) or volume or else.

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 (in that other thread) 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. Some components might be too sensitive to over heating.

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.

[Mick Bailey]

Someone else may have the same symptom, but not necessarily the same problem.

The first thing is not to run it with the speaker connected. Disconnect the speaker and connect your DMM to the speaker leads and check for any DC. If one rail of the main PSU is missing, the output could be swinging to the opposite rail and put DC on the output, or you may have an output IC failure. The ticking could be the PSU repeatedly trying to restart. Check the main +/- voltages are present.

Often, if you get the full DC supply being passed to the speaker load, it places a heavy demand on the supply and the filter caps can't handle this so you get high ripple which is heard as the loud hum.

Edit: It's not a good idea to replace components unless you know them to be faulty.