Sorry, no help on your problem, but I'd agree that I wouldn't do repairs, at least not through Sony. If they even still worked on it, I'd bet it would be a minimum of $250.
I have the Sony D50 recorder, which I find a very nice recorder, been very robust for me, takes a digital input (optical), and sounds very good from its analog inputs. I mainly record in 24bits, but if you want to record at 16bits, the D50 offers the SBM circuitry.
Given the advances in circuits, amp chips, and A/D chips, the D50 with or without its SBM circuitry probably sounds as good or better than the original SBM1. You could probably pick one up for ~$350. I don't know that I'd really recommend recording at 16bits with so many 24bit recorders, but even if you're set on SBM-dither down to 16bits, I'd probably put my money towards a D50 and replace your SBM1 and MT, rather than spending more money to fix a pretty dated SBM1.
:2 cents: