I found Service Bulletins (SB) 2-5 for the 250M not the 250 in the digital database Marantz (page 10) section of AK, they are free for download: http://akdatabase.com/AKview/index.php M-250-2 Mar 26, 1975 New part numbers for specific transistors and rules for beta selection M-250-3 Aug 16, 1976 Increased reliability by replacing pre, driver, and output transistors. Add flyback diodes, bias and test unit. M-250-4 Sept 24, 1976 Correction to bias setting in M-250-3, sets current to 1/2 of previous. M-250-5 Nov 15, 1977 New part number for power transformer and how to replace a failed one. The 250 manuals can also be found there but much earlier in the database. Anyone have SB M-250-1 ?