All purpose 20V power supply modification
                  -----------------------------------------


      Here is a very quick and easy modification of a 12V power supply
available at a Long Island electronics liquidator.  However, you can use
this basic idea to modify any fixed power supply that uses any of the
LM78xx series of chips to now work as a variable supply.  The heart of
this project is the LM317T adjustible regulator.  You can pick these parts
up at Radio Shack (except for the power supply itself) if you wish.


Parts List:
-----------

                  (1) 12V DC, 1.2 Amp Regulated Power Supply (TD626-2)
                  (1) LM317T Adjustible Voltage Regulator
                  (1) 5K ohm Potentiometer, Linear Taper
                  (1) 330 ohm Resistor

                  24 Gauge Wire
                  Black Tape


Directions:
-----------

      1) Get a SHARP exacto-knife and cut along the seam of the power
supply several (thousand :-) ) times.  You may need a saw to cut through
the corners.  I used a coping saw (it was availible)....

      2) When you cut through most of the plastic around each side of
the power supply, try to pry it off with a flathead screwdriver.  Once you
get the top off, pull back the foam and use a rubber band to hold it back
on the top of the transformer while you work.

      3) Inside the power supply, find the fixed regulator chip.  Unscrew
it from the rectangular heat sink.  Be careful not to lose the little
plastic insulator between the chip and heat sink.  Yes, the white goo
is messy :-)

      4) Cut the chip's leeds off at the base of the black part of the
regulator.  You should have three metal "posts" sticking up from the
ex-leeds of the 12 volt regulator.

      5) Take a look at the schematic included with this file.  Use 24
gauge wire to solder this circuit together. Be sure to use black tape
to insulate the 317's leeds and whatever exposed solder joints there are.
A good "trick" is to insert the 330 ohm resistor between the two wires by
using a wire-stripper in the middle of the wire to create a gap of copper
wire.  Wind the resistor leeds around each exposed gap of copper.

      6) Flip the heat sink upside-down so the hole is on the opposite
side.  Mount the 317 in that hole turned sideways.  Drill a hole in the
top case for the potentiometer and don't forget the small hole for the
little tab on the side sticking up on the pot!  Make sure you fit the pot
on the left, next to the big capacitor, not on top of it!

      7) Put the cover back on and use thick TIGHT rubber bands to hold it
shut.  Use a voltmeter and test it out.


This particular transformer model can go up to 20V!  Don't ask me why it
was so over-designed.....

In case you want to use another power supply that doesn't have an input
voltage that goes up to 20V, you can calculate what resistor to use in
place of the 330 ohm value with this formula:

                        R1 = 6250 / (Vout - 1.25)


You can pick one up at:   Edlies Electronics
                          2700 Hempstead Turnpike
                          Levittown, NY 11756-1443
                          1 800 645-4722
                          NYS: 516 735-3330