How can I make an external PSU for an Apple IIc?

Question

I purchased an Apple IIc from an auction site, but it came without the external power supply unit. While this question and its answers describe how to open up the IIc and use alligator clips to power it from a bench PSU, I'd like something more convenient and portable. What are my options for this?

Answer 1

The external power input on an Apple IIc is a male 7-pin DIN jack. The standard Apple external PSU takes a country-specific input voltage (105 to 129 VAC, 60 Hz for North America) and provides (unregulated) 15 VDC.

Though on page 11 of the Apple IIc Technical Reference Manual it says that the internal power supply "operates from a 12 to 15 VDC input source," this is actually too conservative. Page 234 of that manual gives the correct specifications: the system unit accepts an input voltage of "+9 to 20 VDC" with a maximum power consumption of 25 W. I have been using my IIc with 20 V input with no problems.

Conveniently, a lot of modern laptop PSUs provide voltages in this range, and also work on any wall voltage from 100 V to 240 V, 50 or 60 Hz. I have several Lenovo ThinkPad PSUs used with older X200-series laptops (and many other models) with a rated output of 20 V 65 W, so I decided to use one of those..

Page 234 also gives the pinouts of the external power connector. Three pins are PSU ground; internally these are all connected together anyway so it's probably safe to connect just one of the ground pins if you need to go that way. Two more pins are for positive voltage and, again, these are also connected together internally. The last two pins are unconnected.

Here's the pinout using standard DIN pin numbering. Beware: the Technical Reference uses a different, non-standard pin numbering, also included that as well below. A "jack" is the connector on the computer, a "plug" is the connector on the cable, and "LI" means "looking into."

DIN Numbering       DIN Numbering    Apple Numbering
LI female plug      LI male jack      LI male jack
      ∪                  ∪                  ∪
   7     6            6     7            7     1
 3         1        1         3        6         2
   5     4            4     5            5     3
      2                  2                  4

DIN    Apple   Function
1,4     6,5    +15 V DC
  2       4    Chassis ground (AC input ground)
3,5     2,3    Signal ground
6,7     7,1    Not connected

You'll obviously need an adapter to convert the laptop PSU output to a female 7-PIN female DIN plug. Fortunately Lenovo changed their PSU connector at some point causing people to sell converter cables into which you can plug the round connector from an old PSU to convert it to the new square connector. From AliExpress I bought I this adapter (under $1), and also a 7-pin DIN plug with a female insert here (under 50 cents). (Actually, I purchased several of each in case I messed something up.)

Building the adapter was relatively simple, but needs some care to ensure you're connecting the right pins, and that pins are not shorted.

1. The DIN plug I used had a cover that came up to very nearly the edge of the metal shield. This type of plug won't fit into the IIc because there's not enough clearance, so it needs to be cut back by about a centimeter. I did this with a side cutter, ensuring that I left intact the square hole in the cover for the latching tab on the shield so that I could still remove the plug by grasping the cover and pulling.

2. I confirmed my understanding of the IIc's power connector by doing a continuity test on the power input pins. The three ground pins are shorted to each other and also to the shields of all the connectors on the back of the case (including the power connector itself). The two positive voltage input pins should be connected to each other and nothing else. It's important at this stage to ensure you have confirmed you're using the right pins, and you should re-check this on the IIc and the DIN plug after each step, ensuring both that the ground and positive voltage lines are connected and that they're not shorted to each other.

3. I cut off the square end of the Lenovo adapter and stripped back the cable jacket, exposing red (+20V) and black (ground) wires, which I stripped and tinned. After doing this you should plug the adapter into your PSU and confirm the voltage and polarity with a multimeter; it's quite possible that some adapter cables have different colour assignments.

4. I stripped some solid-core hook-up wire and used a small length of it to join the three ground pins together on the DIN plug. I used another length to join the two power pins. My plug's connector tabs had holes in them; I made sure for at least one hole for each line to leave enough space to thread through them the tinned wires from the adapter. I then confirmed from the plug side that the two power pins were connected to each other, that the three ground pins were connected to each other, and that the positive voltage pins were not shorted to the ground pins.

5. I placed the DIN plug cover over the Lenovo adapter cable (this is easy to forget!) and then soldered the adapter cable's wires to the appropriate pins on the DIN connector. I then confirmed ground connectivity (the outside of the Lenovo barrel connector) to the ground pins and positive voltage wire connectivity (the inner surface, not the inner pin, of the Lenovo adapter) to the positive voltage pins on the DIN connector.

6. I checked the wire connections to the tabs on the DIN plug and trimmed them down a bit to ensure that they wouldn't touch the shield. (This isn't a big deal for the ground wire, since ground is shorted to the shield in the IIc anyway, but it's vital that the positive voltage wire not be at risk of contacting the shield.) Then I assembled the shield over the plug, pushed the DIN cover part-way on, and confirmed that the ground and power pins were not shorted together.

7. I pulled up the latching tab on the DIN plug metal shield to ensure that it would properly seat in the hole in the cover, pushed it all the way on, and then plugged the adapter into the PSU and confirmed that the voltage between the power and ground pins was correct.

Here's a picture of the assembled adapter and the parts used to make it (click on the image to see a much larger version):

In the foreground at the left is an assembled DIN plug; to its right is a DIN plug cover and the plug itself between the two parts of the metal shield. Above these, from left to right, are the back panel of an Apple IIc (the external power jack is next to the power switch), the assembled adapter, the plug from a Lenovo PSU, and the PSU itself. Behind the assembled adapter is the original Lenovo-old-style to Lenovo-new-style adapter that I cut apart to make my adapter.