Commodore 64 Power Supply

[Brent Hilpert]

Eric Smith wrote:
> 
> I wote:
> > Why does it require isolation between the 9VAC and the +5V DC?  It's not
> > obvious to me from the schematic.
> 
> Roy wrote:
> > The 9VAC supply is fed into a voltage doubler to derive the regulated +12V
> > and a separate +5VDC supply,  and I don't think that either of those 9VAC
> > lines can be grounded.
> 
> Neither side needs to be grounded.
> 
> The C64 *already* has a bridge rectifier and 5V linear regulator powered
> by the 9VAC input to produce an auxiliary 5V supply, so I don't see why
> building an external power supply using a transformer with a single 9VAC
> secondary, and a bridge rectifier and 5V regulator (linear or switching)
> would be a problem.
> 
> However, I haven't tried it, and don't accept any responsibility for
> anyone else trying it.
> 
> Eric

If I'm reading the C64 schematic (Sam's photofact version) correctly, the
ground-side diodes of the two bridges (the original low-current one in the
C64 and the new/main one in the power supply) would end up in parallel
(or almost in parallel: there's a noise suppression inductor, fuse, etc.
involved), so one may have unpredictable behaviour as to which diodes take
the current and/or issues with unintended current flow in some supply paths.

I haven't looked at what the low-current bridge diodes are rated for,
perhaps it would work for the currents involved, but I think it looks bad
from a 'design principles' perspective.

If one were willing to modify the C64 you could open the ground lead of the
existing bridge (so that you have two half-bridges for the two positive sides,
and one half-bridge for the ground side).
Or perhaps the ground-side diodes of the bridge in the C64 would just go poof and
effectively open the ground lead for you :)