Building a power supply for a Vic 20

Question

I recently acquired an early model Vic 20 computer but don't have a power supply for it and thought I might be able to make one. The Vic 20 is the early model with the two pin AC connector - not the later model DIN plug.

From what I've read it needs 9VAC at about 3A. I'm in Australia where mains is 240VAC, so I'm looking at this transformer:

http://jaycar.com.au/productView.asp?ID=MM2014&keywords=mains+transformer&form=KEYWORD.

It doesn't say on that site but in their dead-tree catalog it's got two sets out secondary outputs each multi-tapped 0->9V->12V, 2.5A each.

Since I've not done much with mains, just want to check somethings:

1. Wiring the two secondary 0v + 9v outputs in parallel will give 5A output - enough to power the Vic and some accessories.
2. Will I need a fuse on the mains side? They recommend one on the page so I presume so.
3. What sort of case would you recommend for something like this - it's just the transformer and fuse, with a mains and 9VAC leads coming out. Should the case be ventilated, plastic, metal, how to best mount the transformer etc...
4. Have I missed anything?

Answer 1

1. Check!
2. There's discussion whether the fuse should be placed before or after the transformer. I place it before. I'd use a 500mA, slow-blow ("T") version.
3. I would use a metal case rather than plastic. Not that there would be a risk of the plastic melting — it doesn't get that hot — but metal transfers the heat better to the environment. I would use a ventilated enclosure if I could find it, though it will cost a bit more.
   Mounting should be straightforward; if it's a classic EI transformer use a bolt and nut for all 4 mounting holes. Use spring washers so that the nuts don't come loose because of vibrations. A toroidal transformer often comes with mounting material, so use that.
4. I don't think so. Success!

Answer 2

The Commodore VIC-20 makes 9VAC available on the user port so that applications which need higher-voltage can employ a voltage doubler, but the VIC-20 would otherwise be just as happy with a DC supply of either polarity. Figure a drop of 1.4 volts for the rectifier and 2 volts for a 7805, an 8.4-volt DC supply would probably be ideal, with anything up to about 12 volts being tolerable. At least on my VIC-20, the 7805 generated a fair bit of heat; using a lower-voltage supply would help mitigate that.