Problem with used GK MB Fusion 500

[grinx]

Bought this used a month or so ago here on the Xfieds
Didn't try until last week
It was awful.

using A controls only, at low volumes (under 9 o clock settings) playing one note it sounds fine.
turn the volume/gain above 9:00 it starts breaking apart
play 2 notes together (power chord) at low volumes it breaks up

I plugged a cable into all of the jacks 50x each
There is no crackling from any of the knobs

Used speakon and 1/4
Used same speaker (GK Neo 112-II)
Tested against a GK 400RB-III with and without a secondary preamp. No breakup

It isn't the speaker
It isn't the cables

Next steps?

Thanks all

 

[Kevin1960]

What exactly is "this"? If you posted a pic., sorry I don't see it.

 

[Old Garage-Bander]

It isn't the speaker
It isn't the cables

Next steps?

Fix broken amp.

 

[BassmanPaul]

Quick test that cures surprisingly many such problems is to link Effects Loop, if it has one, Send and Return with a signal cable. Same for Pre-amp Out and Power amp in.

 

[Killed_by_Death]

I bought one from GC that was the same way. I returned it & they put it right back on the sales floor. Yes, I explained the problem to them, but they didn't not care, at all!
Finally it turned up at another GC & they dropped the price to less than $200 "as is".

I assumed the tubes had a problem, try changing them.

Is that what you did with the RB, bypass the preamp on the MB?
That would give you an indication if the fault lies within the preamp.

To be more clear,,,
run a patch from the RB effects Send to the MB effects Return
plug into the RB & see how the output of the MB sounds