Date   : Wed, 06 Dec 2006 10:15:52 -0000 (GMT)
From   : splodge@... (Richard Gellman)
Subject: [OT-ish] A3020 won't turn on

Hi,

> I was curious so went looking for some info on the web and found this
> picture : http://acorn.chriswhy.co.uk/Pics/A3020C-9.html
>
> The PSU is built into the main pcb on the left hand side. The circuitry on
> the left hand bottom corner does look like it is a switched mode power
> supply. I can't see the number on the top of the integrated circuit there
> as
> the picture isn't close up enough but if you google the part number you
> can
> confirm what it's doing from the manufacturer info.

I'm inclined to disagree, but it would be the short-sighted leading the
blind. My reasoning being the apparent simplicity of the circuit and the
presence of 4 discrete rectifier diodes adjacent to the transformer. The
transformer itself also seems to have 5 output lines, which I imagine to
be +12V, +5V, 0V, -5V, -12V (even though -5V is not used).

> It's quite unusual to have a transformer along with a switcher, normally
> switchers just rectify AC mains in and use a reservoir capacitor to
> provide
> a 400v DC feed which is chopped by the switcher. Possibly Acorn wanted to
> they drop the mains voltage down first then use the switcher to provide
> multiple outputs at high currents.
>
> Maybe the transformer is providing plus and minus 12v supplies and the
> switcher is providing (or maybe not in your case) a +5 volt supply.
>
> My money's still on a dried up capacitor though if there's no blown fuses
> or
> smoke.

If it were a linear supply, I would expect the output voltage to still be
around 5/12V, but with more ripple. Or does a dry capacitor have a
shorting out effect? This would cause a current drain possibly leading to
a weird voltage level.

On the up side, I've managed to bodge a PC ATX PSU into running it, so the
the onboard PSU is completely unrepairable, the machine is still usuable
with the application of a suitable replacement PSU.