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A Heavy-Duty Power Supply for the C-64
by John C. Andrews (no email address)

  As a Commodore User for the last 4 plus years, I am aware of the many
  articles and letters in the press that have bemoaned the burn-out
  problem of the C-64 power supply. When our Club BBS added a one meg
  drive and stayed on around the clock, the need for heavy-duty power
  supply became very apparent.... Three power supplies went in 3
  successive days!

  Part of the problem was my ignoring the seasons. You see during the
  winter I had set the power supply between the window and the screen,
  Yes, outside! With the advent of Spring... well, you get the picture.

  The turn-around time forgetting a new commerical supply was not in the
  best interest of the BBS and its members. Therefore, taking power
  supply inhand, I proceeded to cut one open on my shop bandsaw. I do
  not suggest that you do this. The parts are FIRMLY and COMPLETELY
  encased in a hard plastic potting compound. The purpose of this is not
  to make the item difficult to repair, but to make the entire unit
  conductive to the heat generated inside. I doubt the wisedom of
  potting the fuse as well. However, CBM was probably thinking of the
  number of little fingers that could fit into an accessable fuse
  holder. if you want the punch line it is: the final circuit board and
  its componets are about the size of a box of matches. This includes
  the built-in metal heat sink.

  From these minscule innards I traced out the circuit and increased the
  size of ALL components.

  The handiest source of electronic parts is, of course, Radio Shack.
  All but one part can be purchased there.

        212-1013        Capacitor, 35V, 4700 mF
        212-1022        Capacitor, 35V, 10 uF
        273-1515        Transformer, 2 Amp, 9-0-9 VAC
        276-1184        Rectifier

        270- 742        Fuse Block
        270-1275        Fuses

  Note that there are only five parts. The rest are fuses, fuse blocks,
  heat sinks, wire and misc. hardware. Note also that I have not listed
  any plugs and cords. This because you can clip the cords off of both
  sides of your defunct power supply. This will save you the hassle of
  wriing the DIN power plug correctly:

        DIN PIN OUT                   COLOR
          pin 6         9VAC            black
          pin 7         9VAC            black
          pin 5         +5 Volts        blue
          pin 1,2,3     shield, gnd     orange

  The part that you can NOT get at Radio Shack is the power regulator.
  This part will have to be scrounged up from some local, big
  electronics supply house:

        SK 9067    5 volt voltage regulator, 3+ amps. (I prefer the 5 amp.)

  Radio Shack does carry regulators, but their capacity is no larger
  than that with which you started.

  The Heat sinks, (yes, more than one!) are the key to the success of
  this project. The ones I used came from my Model Railroading days.
  Sorry to say, I did just ahve them 'lying about'. The heat sinks that
  I priced at the local electronics supply were more costly than the
  other parts. The worst case situation is that you may need to drill
  out a couple pieces of aluminum sheet. Try for 12 x 12, and bend them
  into square bottomed U-shapes to save room. heat sinks should not
  touch, or be electronically grounded to each other. You can also mount
  them on stand-offs from your chassis for total air circulation.

  The Radio Shack transformer is rated at only 2 amps. If you can not
  find one with a higher rating elsewhere, it is possible to hook two in
  parallel to get a 4 ampere output. This si tricky, as it can be done
  either right or wrong!

  Here is how to do it the right way:
    Tape off one yellow secondary lead on each transformer. With tape
    mark the four remaining secondary leads and letter them A and B on
    one transformer, C and D onthe other. Hook up the black primary
    leads to a plug to your 120 wall outlet:

                                    |-------------
   Note: *'s - indicate connections |            3 ||
         +'s - indicate skip overs  |            3 || (Transformer)
                                    |            3 ||
                                    |            3 ||
                                    |   ----------
                                    |   |
          +--\  /-------------------*---+---------
        --|120|/                        |        3 ||
        --|Vlt|             ____        |        3 ||
         -|Plg|------------|FUSE|-------*        3 ||
          +--/              ----        |        3 || (Transformer)
                                        |---------

    This would now be a good time to install a fuse in your 120 VAC
    line. Now before plugging this into the wall, tie two of the
    scondary leads (one from EACH transformer) together.

    Something like this: A--Xfmr--B+C--Xfmr--D

    Plug in your 120V side. Now using a VOM meter, measure the voltage
    between A and D.
      If the meter reads 18 volts, then:
        1. unplug from the 120.
        2. tie A and C together. tie B and D together.
        3. your 2 transformers will now give you 9 volts at 4 amps.
      If the meter reads 0 volts, then:
        1. unplug from the 120.
        2. tie A and D together. Tie B and C together.
        3. your 2 transformers will now give you 9 volts at 4 amps.
 
  Below is the file corresponding to the full schematic of the power supply.
  [Ed's note: in GeoPaint format, converted by convert 2.5, then uuencoded]. As
  you can see in the picture, I used only one transformer. Because it got hot,
  I epoxied a small heat sink to it. While this solved the heat problem, it did
  not increase the capacity of the total power supply.

  Note that I used fuses on all lines.

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