Please suggest a working power supply (12 Volt) for the internal CPC 3" drive.

[Velktron]

A Darlington pair should survive having 12V put across it the wrong way around.Bryce.

Perhaps....however I overestimated Amstrad when I thought they would use a Darlington (or even a power MOSFET) for the job: the 2SC950Y transistor (component W303)  is a plain NPN transistor with a nominal Ic of 0.1 A, so it's questionable even how long it would survive in protracted operation/strain. Some sources claim that a reverse voltage of 5V is enough to cause complete junction breakdown and cause uncontrolled reverse current flow through an unprotected NPN transistor...

...then again, as electronic "old farts" often say, if the Magic Smoke doesn't escape, then it's probably OK 

[Bryce]

I've never checked the schematics... let me go and do that.

A small NPN with reverse 12V would probably die without letting any of that magic smoke out.

Bryce.

Edit: As far as I can see from the schematics, the reversed 12V current has only one path to the "now" 12V ground. It would have to travel backwards through the KTA950Y. This won't happen because the transistors breakdown voltage is 30V. The KTA950Y can pass 800mA by the way.

[TFM]

Thanks to everybody helping with the issue. I resoldered one of the 12 V PSUs yesterday and it seems to work. Means I can hear the drive spinning. Now I need to wait until my new old Monitor arrives to actually have a picture.