BEFORE Wiring The Power Supply Transformer

The power supply board was intentionally designed to allow a wide variety of transformers to be used. As such, no transformer will be a perfect mechanical fit. While the mechanical footprint will vary, any transformer that can provide 35V at 220ma will suffice (that's 10VA to15VA).

The transformer specified in the parts list is a good, economical, torroidal variety and if chosen you will need to attach wires to the legs as shown below. (Again, our Power Supply Board does not provide holes to accept the legs for this type of transformer.)

Look at the top of the transformer and note the voltage markings. The two 115V windings are the PRIMARY and will be connected to the AC mains via fuse and power switch. On the SECONDARY side are two 22V windings that will be connected to the rectifier via J1 on the power supply board.

NOTE: Since there are two windings, this transformer can be rewired for 220-volt operation. Fuse and switch connections are external to the power supply board and will be mounted to the chassis. YOU MUST USE A FUSE for safety and economics reasons. We all make mistakes. Mis-wiring on the Secondary (low voltage) side of the transformer could cause irreversible damage. 

For 120 volt operation the two primary windings will be connected in parallel. The secondary windings are connected in series with the center tap going to ground. Begin by taking 3 different colors of 24-guage stranded wire. A good choice of "standardized" colors would be Black (hot), white (neutral) and green (chassis / ground). 

The type of power cord, switch and fuse are left up to you. You may want to put a chassis mount fuse holder and IEC type removable AC connector on the back. The power switch should be mounted on the front panel for easy access. 

Assuming the above, a white wire will connect the neutral side of the IEC AC connector to the transformer primary. Safety ground from the IEC AC connector should be attached to a star point on the chassis. See the drawing below for details on wiring a fuse and switch.

A black wire will connect from the hot leg of the IEC AC connector to the fuse and then from other fuse terminal to the front panel power switch. (Each is in "series."). The other side of the front panel power switch will be attached to the transformer primary (but don't do that yet).

ONE WIRE TRICK

The black wire from the power cable / IEC socket will feed the fuse and then the power switch. 
 

  1. Cut a piece of black wire, about 18" long, to go from there to the power transformer. Strip and tin one end of the black wire, loop it over the appropriate leg on the transformer as pictured. Make sure you are on the high voltage PRIMARY side of the transformer (but donít solder it yet). See Figure-1.

FIGURE-1: The first step, connecting the HOT wire.
 
Make a measurement over to the other leg of the transformer and mark about an 1/8" inch on each side of that leg with a sharp x-acto knife or razor blade. Take the wire off and cut the insulation at those two marks by rolling the wire on your workbench or board. The goal here is to cut away a section of insulation exposing the wire, while keeping the wire in one piece. 

NOTE: You may want to practice on a piece of scrap until you get a feel for the amount of pressure needed to cut only the insulation and not the wire. A sharp blade is the trick here because it needs the least amount of pressure. 
 

  1. Repeat Step-1 with a white wire (See FIGURE-2)

FIGURE-2: Step-2, connecting the NEUTRAL wire.
TWO WIRE OPTION: Primary (line voltage)

After cutting through the circumference of the insulation, cut horizontally across each vertical cut and then peel away the insulation. If you canít get the insulation to cut away cleanly without breaking the wire strands underneath, skip this step and using two wires instead. Keeping the wire in one piece is a neat way of making this connection but has nothing to do with function.

Now bend the exposed section of wire around the transformer leg and solder each end in neatly. Trim any excess wire that may be sticking out so it does not come in contact with the adjacent legs of the transformer. Continue with a 6" piece of white wire in the same fashion as the black wire and solder to the remaining two primary legs as shown.

SECONDARY (low voltage)
Wire the secondary side of the transformer by attaching a 6" piece of green wire to the two legs as shown below. Make the long piece extend out the left side of the transformer as you look at it from the bottom. The green wire is the center tap and will be attached to ground near the rectifier via J1. Pictured Below.

To differentiate the mains voltage from secondary voltage, itís a good idea to use at least one different color wire on the secondary. I used red, and white for the AC connections to the rectifier D-1 via J-1. Cut 4" of red and white wire. Solder them to the outside legs. Put red on the right and white on the left as shown below.  Cut the secondary leads to fit into J-1ís holes and solder them in the order as pictured.  From this view, the red wire goes on the left outer hole, the two inner holes are ground (for the green center tap wire ó either hole is correct) and the white wire goes to the outer right hole. 


Wiring the Secondary (low voltage) side 
of the power transformer.

Wiring the secondary (low voltage) of the transformer 
to the PCB.
MOUNTING THE XFRMR TO THE PCB
The power supply board has a hole in the center where the transformer mounts. Put a 1/2" inside diameter, 9/32" thick rubber grommet under the transformer (available from your local hardware store). Take the M4 x 12 bolt with washer and thread into the bottom of the transformer to mount the transformer to the board. The rubber grommet is used to keep tension on the bolt threads and to keep some weight off the transformer legs. If you canít find a rubber grommet, you can use a variety of items such as weather-stripping or pieces of rubber as long as they donít fall out. 

 

The Bolt and The Grommit
(Sounds Like a Soup Opera, eh?)

Put the Grommit here 
and the bolt goes there...

Mount the Tranny and tighten the bolt...

Drill a hole in the PCB 
for the Primary (line side) wiring
It is a good idea to drill an 1/8" hole on the primary side of the board near the black and white leads you added, as shown below. Thread the leads through the hole's top and out the bottom. This will add some strain relief to the leads.

Finish stuffing the power supply board with the components as listed in the parts list.