AW: EMS Synthi A Problem

[Haible Juergen]

	>Hints, comments and suggestions for new powersupplies
	>are highly welcome. Thanks for your patience.

The old Synthi A PSU is an interesting circuit:
It is a power *shunt* regulator !
It only needs one secondary transformer winding for
a dual dc voltaga output, and that's about all of advantages
I can think of (;->). Having said this, and that it surely was
a brilliant idea at the time, from today's pointof view it is
complete crap.

It is intended to work like this: Two power transistors
and a power resistor, all connected in series, draw
current from the rectified voltage. Each of the transistors
is regulated so that the voltage drop *across* it would
be 12V for one, and 9V for the other. So your total
current from the PSU is determined by the transformer's
secondary voltage and the value of the power resistor,
and then the two power transistors will have to eat all
the excess current that is *not* consumed by your
synth electronics. This means without the other cards
connected, the regulator must take most of the current.
This also means that the whole design must have been quite
tricky to calculate back then (remember mains voltage
has some tolerance), but now, with today's increased
main voltages, might be simply too weak to draw all the
excess current. The cure would be to increase this
power resistor. So you might replace it by a larger one,
but don't ask me for the right value, as voltage across this
one would not increase linearly with the mains voltage.

Probably the better solution is this:
I'd suggest replacing the whole stuff with an ordinary PSU. 
I have built one with an 7812 and a 7909, and 2 veroboarded
VCS-3-VCOs run perfectly from it.
You either need a transformer with 2 secondary windings,
however (a little toroidal one maybe), or you do half
way rectifiying for + and - branch, but then you'd have
to increase the capacitor values. Great question is
if this all fits into the briefcase. 
>  
	JH.