GE Clock Radio Model 660

[old_coot88]

[QUOTE=MadMan;3177106]

HERE
Aha. L'arn something new every day. All the battery portables I ever serviced used the B+ dropper for the 50ma. filament string.

The PSU wiring was actually extremely confusing for me. They could've just had the double transformer, two selenium rectifier bridges, and maybe a capacitor or two, but instead they came up with this crazy contraption. Before I got the diagram, I actually drew my own just to see if I could make sense of it! The really weird thing is... they have a proper rectifier bridge for the filament power, and then they just use a tiny single diode for the (what I would consider much more important) 67.5v B+. I just... don't even. And actually, despite what the diagram says, the power coming 'out' of the transformer for B+ is actually 110v. It really looks like they put one diode in there to roughly halve the power, and cleaned up the oscillations with a capacitor and called it good. Come to think of it, maybe that's the reason that capacitor went bad in the first place.

Actually the half-wave B+ supply is quite adequate for the low current requirement (no filament string to supply). The 'A' supply on the other hand, is full-wave with fairly massive filtering to deliver ripple-free current (at 100 ma in this design). Very snazzy and makes total sense.

If you felt like raising the B+ a bit, you could play with values of the voltage divider R13/R14.