Commodore 64 power supply repair.

[Bashstreet]

Hi all.

I been working on making a replacement power supply for my loved C-64

All is fine and dandy but i have bit of hiccup with the 9 volt AC that it requires.
I bought a 9 volt transformer but as it is unregulated even under load with the commodore it gives some 11.5 volts.. Quite a bit from where i ideally want to be...

Chematic

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/250469-rev.A-left.gif

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/250469-rev.A-right.gif

By looking the schematic i do not see anything too sensitive on the 9 volt lines.... but have a look.
I would ideally want to regulate the 9 volt line to... well 9 volt but it needs to stay AC as the 60hz is used internally in some timing functions.

How one regulates AC without effecting it's frequency ?

Cheers.
 

[anachrocomputer]

The original C64 PSU may well have been a simple 50/60Hz transformer in a box. See if you can find the original schematic.

Now, a typical mains transformer is rated at full load, and has a regulation of typically 20%. That means that a nominal 9V AC transformer is designed to produce 9V when fully loaded at its rated current. If it's unloaded, the voltage will be higher. The 20% load regulation spec means that there will be approximately 20% variation in voltage between off-load and full load.

Remember that there's also some allowable variation of the input voltage (mains voltage) and if the input is a bit high or low, the output will be too.

[stj]

the original was a transformer with several windings, combined with a 7805 on a useless heatsink.
useless because they potted it!!

[Rasz]

first half is about c64 supplies:

[Alex Eisenhut]

The 9VAC is rectified internally to feed some linear regulators. They are already running quite hot. I made my 64 power supplies with the 9VAC transformer from the 2-pin VIC-20 power supply.

However, if you have a broken C64 power supply, there's your 9VAC transformer. Simply remove the bad 5V and splice in a good regulated 5V 2A wall wart into the power connector.

That's what I do now.

[stj]

i fixed one once by mounting a heatsink on top with a 78s05 and wiring it to the exposed pcb pads inside after cutting the old pins.
i wouldnt do that now though - the caps are too old to leave in place.