A600 PSU on a C128. OMG

[Phil]

I got a flat C128 from ebay which powers up fine. I was going to use the PSU to troubleshoot a faulty 128D when I decided to check the output voltages. It's then that I found it's not a C128 PSU but an Amiga 600 PSU. WTF! This has been powering the 128 for about 3 days now with the only noticeable fault being very faint grey horizontal scrolling bars. How the hell is this PSU even powering the 128!? Obviously I won't be using this PSU with the 128 again and am now in the market for a 128 PSU brick - anyone in the UK selling one? Have I done any damage to the 128? I just can't believe the 128 even booted with no +9VAC. A lesson very well learned when buying from ebay, even reputable ones.

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Phil.

 

[carlsson]

An Amiga power supply will deliver 12V DC on the exact same lines a C128 power supply delivers 9V AC. Inside the C128 there will be a rectifier to convert the AC to DC, but since you supplied with DC to start with, I suppose the rectifier will have nothing to do.

I have also heard that for a limited amount of time you might power your C128 from 12V DC, but it definitely isn't recommended.

 

[jthiemann]

While I agree that it's not a good idea to run a 128 with a A500 PS long-term, I doubt that any damage has been done. The worst effect may be that the RTC doesn't work, I think that is updated by the line frequency, though someone feel free to correct me on that.

For the voltages, remember that 9VAC is RMS voltage, not peak voltage - assuming a perfect sine, it'd be over 12V for a significant portion of time. If insufficiently loaded, a full-wave rectified AC signal will easily be above the RMS value.

For short times, 12V DC input to a 9VAC system should be well within tolerances, especially for a line-powered consumer item, else every home computer would have blown up once the fridge starts.

Dont try powering a A500 from a 128 supply though - the 12V is needed only for the floppy drive, and you find yourself thinking "I was _sure_ this workbench floppy was still good! Maybe I need to replace the drive!". Not that *I* would ever make that mistake. No siree! Now, where's that screwdriver?

 

[Phil]

Yeah, I've been looking through the schematics of the A600 and C128 PSU's. It appears I've been feeding the +9VAC lines with +12VDC and -12VDC. The rectifer was converting this to +8.10VDC. It looks like I got off lucky this time. Had this been the other way round with a C128 PSU powering an Amiga 500/600/1200 it would be a whole other story!

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Phil.

 

[Phil]

The worst effect may be that the RTC doesn't work, I think that is updated by the line frequency, though someone feel free to correct me on that.

Thanks jthiemann, that would explain why the on-screen clock of the Diagnostic Cartridge wasn't advancing. Doh.

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Phil.