Thats a great help, I'll see how I go. Thanks for the tips.

wrote:
James Sweet wrote:
fredrickew wrote:

Sorry, just noticed that there are two more rails also, a -12v and an
AC3.9v. I'm not sure if the AC is refering to Alternate Current or
accessory. I'm pretty sure it would be a dc rail though.

The AC voltage is for the heater in the VFD display. You'll spend a lot
more time re-engineering a PC power supply to work than you will just
fixing the original.

3v could possibly come from 5v via 2 power diodes, which will drop apx
1v each. However the exact drop depends on the diodes and current draw,
so feeding a cpu from this would be chancey. A 3v reg would be rather
safer. Easier though would be a regulated 1.7v dropper, basically a
transistor multiplying resistors/diode on the base side. Easy to
implement.

The vfd filament is a single fixed load, so this can be run from the
smpsu transformer output via a cap or resistor. IOW you need to either
measure the vfd filament current draw, or start with a high value R and
move down until you get 3.9v on the vfd. IIRC with vfds start by
assuming 20mA, then adjust upwards as required.

VFD -27 rail will be very low current, so you can get this by voltage
tripling the ac output from the smpsu transformer. It does not need to
be 27v, 24-36 is fine. Because the smpsu runs at high freq you should
be able to get above 27v no problem. Capacitor size determines how much
of the notional '36v - diode drops' you get, so again start small and
increase them to get enough V.


NT