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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 6th May 2020, 8:18 am   #1
Outrun_uk
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Default Commodore 64 Power

Hi all,

Iíve been busy playing about with some of my old computers and was chasing my tail a bit with a C64 which turned out to be a marginal 5v from the PSU. Iíve since replaced all the bits but was thinking about building a power supply using new components.

Just a quick question - for the mains transformer I have a choice of PCB or toroidal, is there any advantage or disadvantage using one type over the other? I need 2 x 9v at 35vA or more. One tap straight to the computer and the other via bridge rectifier/smoothing and a modern switching regulator is my plan.

Also if anyone can help out with a MOS6526 CIA chip for a reasonable price (seem to be £20+ now!) please let me know.

Iíll also be building some C64 PLA replacements when the bits arrive (from China so may be a while) so if anyone is in need of one drop me a message!

Cheers,

Kev
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Old 6th May 2020, 11:09 am   #2
SiriusHardware
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Default Re: Commodore 64 Power

The main advantage of toroidals as far as I can recall is that they have a lower external magnetic field around them than conventional transformers, so if you're thinking of leaving cassette tapes lying around of top of your power supply that might be a consideration.
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Old 6th May 2020, 6:57 pm   #3
Outrun_uk
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Default Re: Commodore 64 Power

Makes sense! I’ll have a dig around and see if I’ve a suitable transformer but if not then toroidal are about £10 from CPC.

Cheers,

Kev
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Old 10th May 2020, 12:58 pm   #4
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Default Re: Commodore 64 Power

Additionally Toroidal transformers are a little more efficient and don't hum as much as transformers with laminated cores. That's why they are usually more expensive.
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Old 14th May 2020, 10:03 pm   #5
Outrun_uk
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Default Re: Commodore 64 Power

Hi all,

Construction is now complete! I had to add an additional capacitor after the photos were taken, the switchmode regulator I used needed additional smoothing. Pleased with the end result though!

Cheers,

Kev
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Old 14th May 2020, 10:39 pm   #6
SiriusHardware
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Default Re: Commodore 64 Power

Are you seriously telling me that fingernail sized SMPSU can source the 5V needs of an entire C64? I'm impressed. Although....

As with linear regulators, that basic sort of SMPSU regulator can suffer from short-circuit failure of the main power handling device which is usually placed between the input and the output.

If that ever decides to go short-circuit, just like a linear regulator in the same situation, it will pass the full input voltage to the output.

For that reason, I would suggest putting some sort of overvoltage protection like a chunky 5.6V zener across the output rails so that if the regulator ever does fail straight through, the zener will clamp the voltage to no more than 5.6V (and get very hot, and probably go short-circuit) - but at least the C64 won't get killed. We have exactly those sorts of SMPSU regulators and that overvoltage precaution on a few of the products we make at work, along with semiconductor fuses to limit the current in the event of regulator failure.
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Old 15th May 2020, 12:37 am   #7
Outrun_uk
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Default Re: Commodore 64 Power

This is the regulator I used :-

https://uk.rs-online.com/web/p/switc...2132/?sra=pstk

Rated 1.5 amp which is the same as the original PSU. Good idea about the zener, I’ll have a look tomorrow and see if I’ve anything with a decent power rating - possibly have some 5w ones but if not will put some on order.

I had it running for a couple of hours earlier and it seems happy enough, nothing getting too warm and the computer was stable. I would have bought one of the newly manufactured PSU’s but it would be around £40 posted, this has cost me £15 for the transformer and regulator as I had everything else in stock. Going to have to add 50p to the total for the zener now

Cheers,

Kev
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