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Path: utzoo!watmath!clyde!cbosgd!cbsck!wkb
From: wkb@cbsck.UUCP (William K. Brummett)
Newsgroups: net.micro.cbm
Subject: Re: C64 problem
Message-ID: <1624@cbsck.UUCP>
Date: Mon, 2-Dec-85 11:05:17 EST
Article-I.D.: cbsck.1624
Posted: Mon Dec  2 11:05:17 1985
Date-Received: Tue, 3-Dec-85 08:20:50 EST
Organization: AT&T Bell Laboratories <SCCS>, Columbus
Lines: 95


  (Sacrifice to the line-eating bug.)

  > Help!  I have a friend's C64 before me.  Upon power up,
  > nothing happens except the drive runs for a few seconds.  The
  > pilot light appears to come up to full brightness initially, 
  > then suddenly becomes dim.
  >   ...
  > Has anyone seen this or something similar?
  > 
  > Brint Cooper
  >    ARPA:  abc@brl.arpa
  >    UUCP:  ...{seismo,decvax,cbosgd}!brl-tgr!abc

        I, too, have experienced what you are desribing.  In my case,
        it was the power supply that was causing the trouble.
        Specifically, the 5-volt regulator failed and was feeding
        almost 8 volts into my 64.

        To check for this problem, unplug the power supply from the
        AC outlet and from the 64.  Plug it back into the AC outlet
        and measure the voltages on the output plug that connects to
        the 64.  You should read +5VDC and ground on one pair of pins
        and 9 V*AC* on another pair.  (Sorry I can't be more specific
        here.  I don't have access to my documentation right now.
        Apparently there are at least two versions of this plug, one
        with four pins and one with seven (3 unused?).)  If one or
        both of these readings is off, you need a new supply.

        After ascertaining that the supply is at fault, determine
        what sort of supply you have.  The newer versions are square,
        sealed, and completely filled with some sort of hardened
        resin or epoxy.  The older ones are about 30% larger, have
        tapered sides and ventilation slots, and are held together
        with screws.  If you have one of the newer versions, you're
        SOL.  Buy a new supply.  If you have an older supply, AND
        it's the 5VDC that's off, there's a cheaper way to go.

        Remove the screws on the underside of the supply and open
        the casing.  Inside you will find a transformer and a small
        circuit board.  The black object bolted to the metal heat
        sink with three leads attached to the circuit board is the
        regulator.  Unsolder it from the board and disconnect it
        from the heatsink.  You will replace it with a variable
        regulator from Radio Shack (an LM-217, I think).  Note: do
        not try to use a common 7805 regulator, as the current
        requirement of a 64 exceeds the output of a 7805.  The 217
        is soldered into the holes left by the old regulator, only
        shifted one hole to the left.  The old regulator was some
        proprietary design and had a wierd pinout.  The far left
        pin of the reglator is connected to the far right hole
        with a piece of wire as shown:

                         |   LM-217   |
                         |____________|
                           ||__||__||___
                           `---||--||--,\
                               ||  ||  ||
        Circuit board holes -> ()  ()  ()

        Connect the new regulator chip to the metal heatsink.
        To the right of the regulator on the circuit board is a
        resistor.  This must be removed and replaced with a small
        potentiometer (I used a miniature 5K multi-turn).  With the
        supply plugged in to an AC outlet (be careful!) adjust the
        pot until the appropriate pin on the output plug reads +5
        VDC.  Reassemble the case and you're done.

        If you are unfortunate enough to have one of the newer,
        sealed units, all may not be lost.  I had one originally,
        but was able to get an older, used one from a dealer's
        service department.  They told me it was broken and gave
        it to me for five bucks, but it worked just fine when I
        replaced the regulator.  By the way, I had considered just
        building a whole new power supply from scratch, but it
        turns out that the transformer used is somewhat wierd also.
        It has one winding that supplies the the 9VAC at about one
        amp and another completely separate 9V winding that is
        center-tapped and half-wave rectified to feed the 5-volt
        regulator at about an amp and a half.  I would have needed
        two separate normal transformers to duplicate this.  I don't
        really know why the two are isolated like that, unless
        Commodore plays tricks with different grounds inside the 64.
        Anyway, I hope this helps you out.


       __________________
       \_|_______________|) ________/^\________
                     | |    `,--,-\_____/-----'     W. Keith Brummett
                   __|_|_____|_/_    ~              AT&T-NS/Bell Labs
                  /__    ======||=(-                Columbus, Ohio
                     `-----------
                                          {anywhere}!cbosgd!cbsck!wkb

       "I'm not sure, but I think we've been insulted." ... "I'm sure."