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I suspect the design rationale for the 2225 being how it is was is that it supports a low voltage DC input as option 07 (not fitted to mine) as well as line voltage. Ergo they picked an intermediate DC voltage which was easy enough to derive efficiently from line and DC voltages and was usable for further derivation of some bias voltages inside the unit. The DC input option uses another switching converter to step up the voltage from 11-30v and therefore supports common 12v auto/mobile and 28v avionic/military voltages. ?Rather clever really.?My old Philips PM3217 had a very similar power supply arrangement but there was only one switching supply. Very few parts on that supply considering the scope of the design. Picture of the board here:

I must admit to sitting there for several hours with a service manual occasionally. There is a lot to learn from looking at other designs. Sometimes I'll even clone bits of them to get an understanding.?This has no purpose as I'm a software engineer now, but it sure beats doing a crossword!

- Chris Smith