Thursday, June 07, 2018

Plus 4 Power Supply

A departure. This is not about an Amiga but it IS about a Commodore machine, or at least, a part of a Commodore machine. On the way to me (eventually) will be two Amstrad CPC 464s and a Commodore Plus 4. None of these machines have power supplies or other bits but that's no big deal these days. Except for the Plus 4.

The Plus 4 is an odd machine. It started life as a 'budget' series of computers from Commodore that were intended to compete with the likes of Sinclair's Spectrum. The boss of Commodore, Jack Tramiel was determined to get that part of the market but, sadly, his sudden departure following a disagreement with Irvine Gould (Commodore's main investor) left the ship without a captain.

What followed could have come directly from a Dilbert cartoon. The 'low cost' range was 'got at' by the marketing department. Changes were made to the design. All of a sudden, instead of a budget range of computers, Commodore released a parallel range of computers that were incompatible with its flagship computer which happened to be selling very well - you might have heard of it, the Commodore 64 - and cost almost as much. Genius.

Anyway, the machines were in themselves OK. Not great but solid 8-bit machines, the top of which was the Plus 4 (others in the range are the 116 and Commodore 16 which is not a cut down Commodore 64 - thanks CBM marketing!). The name came from three plus one utilities that were built into the machine, requiring no disks or cartridges.

Plus 4
Image from Wikipedia
So, on the way (at some point) is a Plus 4 but without a power supply. This machine uses +5v DC and +9v AC on it's supply. Not only that, but at some point early in its production run, Commodore changed the design of the power socket in the machine from the standard Commodore 64 type, to a square DIN style - similar to the Amiga but with only four pins instead of five. These DIN plugs are impossible to source now whereas round DIN sockets/plugs are still widely used. Commodore strikes again...

The Infamous 4-pin DIN. Grrr.

Fortunately I managed to source a Plus 4 supply which came to me as untested. Naturally, I was a bit reluctant to just plug this in and switch it on without looking at it a bit. Getting into it was tricky a they are sealed, but given its age, it has clearly been through some punishment and the bottom came off surprisingly easily...

A Potted History - geddit?
The first thing to notice was that the transformer is potted i.e. the case is filled with epoxy to prevent 'meddling'. The thing I was interested in, though, was the little board at one end. This board contains the circuit to convert 9v AC to 5v DC, including a bridge rectifier and 5v regulator. Unfortunately, the centre pin of the regulator had a bad solder joint and, in the process of poking it, the bit of track with the hole in came off and disappeared. Crap.

I also noticed that the circuit board under the diodes was darker than the rest of the board. This indicates that the board has seen some heat. This is most likely just a cumulative affect of being on and off over the previous 33 years - this supply was made in 1985! I actually went as far as to remove the diodes in the rectifier from the board, test them and then put them back. All were OK.

Diodes - All checked OK
To be sure that the regulator was working I removed the main circuit board and soldered wires to the three legs.

Regulator - Temporary Wires
9V Battery
On pin 1 and 2 I applied 9v from a simple 9v battery. On pins 2 and 3 I connected my multi-meter to check the output voltage. And, joy of joys, it read a rock steady 5v.

5V From the Regulator - Nice.

After putting the board back and putting in a repair on the, ahem, missing bit of track for the regulator, I realised I hadn't tested the fuse. Trying to get the fuse out proved harder than I expected. It took a lot of twisting and pushing and releasing and more pushing and twisting before the fuse holder would come out of its housing. Even then I had to pull it out with pliers. For some reason it was caked in a white residue, possibly grease of some sort? It took a bit of effort with some isopropyl alcohol and cotton buds to clean it all out. Now it's clean, the fuse pops out when requested, just like it should.


Finally, I put the case back together, temporarily stuck with super glue and cable ties to prevent any accidents. Then I tested the output voltages at the plug end and, to my relief, there is a steady 5v DC and 9v AC on the correct pins, so, as far as I can see, this power supply is ready for (limited) action. I don't intend to use this permanently as there are easy DIY ways to create a more stable, modern supply. But for now, this will be OK.

I just need the Plus 4 to plug it into now...

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