I have an MB 121H combo, bought second-hand about 5 years ago. After a couple of years it changed sound and seemed to distort sometimes. Took it to Real Electronics who changed the pre-amp board. Total cost was about £130, including VAT. The board only £30, the rest was diagnosis and repair and VAT. This all seemed ok for an amp which costs £750-800 when new.
Now it makes a whistling sound (1120 Hz) with all controls at zero and no guitar plugged in. Sounds like a power supply problem. Having removed the amp head I can see there are only two circuit boards, the pre-amp and a bigger board with everything else! If the bigger board could be supplied for, say, £100, then I could fix it myself, which would be a reasonable outcome.
I will contact Real Electronics and see what they say. I still have the original pre-amp board so can prove the pre-amp board is not making the whistling sound
