Marshall 6100LM

[Tiny_Daddy]

Big fancy amp. The customer's complaint is: Since being dropped, the amp has a buzz, even in standby, coming from the chassis as well as the speaker. So I check it out and it's true, the power transformer buzzes, not much but it's there in a quiet room. Speaker buzz there too, 60Hz with plenty of higher harmonics. The OT sits next to the PT and sliding a steel plate between them cuts the speaker noise by half as seen on a scope. I don't know if a new PT will fix this. Seems to me the OT may need to be moved as well. Any thoughts?

[Ritchie200]

Seems to me the PT's are the most rugged parts on that amp.  If it didn't start until being dropped maybe check for a bad ground?  Any evidence of damage on the board(s)?  What about the pots?  Can you see any impact evidence to see what direction everything would have been popped on impact?  These "treasure" hunts are fun - if you can charge for it!

Good luck!
Jim

[Tiny_Daddy]

Well the owner said another shop repaired the damage and I don't see any sign of repairs etc. Will try relocating the OT, with the customer's permission of course.

[Tiny_Daddy]

It's the transformer top cover! I can feel the vibrations when the cover is 1/4 inch away from it's normal position. As the cover is moved closer, the vibrating increases until I can feel a magnetic pull from the windings. I have not observed this before. Maybe just leave the cover off? Or is something wrong with this transformer?
Come to think of it, I don't recall ever seeing a top metal cover on this style of transformer.

[Jack1962]

wouldn't this mean the OT is damaged and should be replaced ???

[Tiny_Daddy]

Leaving the "lid" off has got rid of the mechanical noise problem. But there is still the transformer coupling during standby causing an audible hum. I plan to replace the standby switch with a DPDT switch and wire it so that the speaker output is shorted by the switch when in standby.