The Plus/4 square power connector is not available anywhere, so if you don't
already have the power pack that came with the computer, you only have a few options to
make your computer work. 1. fabricate a connector yourself by poking some wires into the
computer and soldering to them, 2. modify an existing connector so it will fit the
Plus/4 socket, and 3. change the socket on the computer so it can use a standard C64
power supply. I have considered these alternatives and will give you the information I
have for each. Let me say now that if you value that computer, you should be using a
better supply than the old C64 black brick. 'Nuff said about that.
     1. The quickest way to get "up and running" is cut the plug off a C64 power pack
and attach some heavy buss wire to the four wires so they will fit the Plus/4 socket.
You need to know which wires go to which pins. Commodore didn't make the job any easier.
I have several CBM power packs and each has different colored wires inside the power
cable. To determine which is which, you will need a volt-ohmmeter to test the power
pack. Since there are only four wires, it's not too difficult. First, with an ohmmeter
(and the power supply unplugged from the wall outlet!), find the two wires that show a
low resistance (less than 10 ohms). Note the color(s) of those wires. Those are the two
wires for the 9VAC output. They go to the two upper terminals of the computer socket
(seen facing the rear of the computer). Next, with a voltmeter and the power pack
connected to the wall outlet, measure the 5 volts DC output. Be careful not to short any
of the pins together when the supply is running! The plus or positive meter lead
(usually red) will go to the positive 5 volt wire of the supply, and the negative meter
lead (usually black) will go to the negative of the 5 volt supply. The polarity is
correct if the meter pointer deflects upward (or in the case of a digital meter, it will
indicate positive voltage). If the meter deflects backwards (or the digital meter shows
negative voltage), reverse the leads to the meter. Now you know the polarity of the 5
volts DC output of the supply. Those two wires will be different colors. Write down the
information for reference. Again, facing the rear of the Plus/4 computer, the +5VDC
(positive lead) of the supply goes to the bottom left terminal of the socket, and the -
(negative lead) of the supply goes to the bottom right terminal. By the way, if the
power supply measures over +5 volts DC, don't use it. That would indicate the regulator
IC in it is failing. It could damage the computer.
     2. There are two types of existing DIN connectors that can be modified to fabricate
a working plug for the Plus/4. Actually, that computer was made with one of two types:
the four pin square beast that is not available anywhere, and the same seven pin DIN
that Commodore used for the C64. The power packs are identical for both. Note that
although the computer power socket has seven pins, only four of them are used. Most
power packs only have four pins installed in the plug. The users manual for my Plus/4
shows the seven pin DIN, but my computer is the square four pin type. With a bit of
experimentation, I found that the pin spacing for the two types is identical if several
of the pins are cut off the 7 pin, leaving four pins. The following diagram shows what I

    O         O           O         O          O        O

  O             O       X             X  

    O         O           O         O          O        O
         O                     X


That doesn't mean you can just take a C64 supply and cut some pins off and make it work.
The pinout for the Plus/4 is slightly different. Here are the pinouts for the C64 and
the Plus/4 for comparison:

 9VAC ---7           6--- 9VAC   9VAC--- 3         4 ---9VAC

        3             1             

+5VDC ---5           4          +5VDC ---1         2 ---GROUND
     GROUND ---2                                       

        C64 POWER SOCKET             PLUS/4 POWER SOCKET

Notice the pinout is offset by one pin. Another obvious problem is that the 4 pin
connector is square. Fitting a round one to the Plus/4 requires cutting the front half
of the outer shield off. This exposes the pins, so you must be careful not to allow them
to short if the connector is not plugged into the computer, but the supply is connected
to the AC outlet. The last problem is that without some kind of shell on the modified
plug, it's possible to connect it incorrectly since there is now no keyway to prevent

     3. Probably the best way to fix the problem of the four pin square DIN connector on
the Plus/4 is to replace it with a standard 7 pin DIN, as is used on the C64. Then, a
standard C64 supply will plug right in. Since some versions of the Plus/4 already have
the 7 pin DIN socket, I examined my four pin version and discovered that the board will
accept either type of connector with no modifications. If you have an old dead C64
motherboard, you can rob the power connector from it. If not, those 7 pin DIN right
angle sockets are still available from several sources. One that I know of is:
http://www.digikey.com. There are several Digikey part numbers (because of different
vendors) that you can use to order the 7 pin DIN 270 degree sockets:  CP-2370ND,
275-1021-ND, CP-7070ND, and 275-1026-ND. Each costs about US $1.50.

     I can give you the information of other parts outlets and part numbers if you want
to try and make up a four pin plug for your Plus/4. Even Radio Shack has 7 pin DIN
plugs. The modifications will involve a bit of work and the results will be less than
optimal, but you will not have to dig inside your computer. Changing the socket requires
removing the Plus/4 motherboard from the case and removing the bottom shield
(unsoldering). It's similar to the C64 in the way it is put together. That's how I would
do it.

Ray Carlsen CET
Carlsen Electronics