sorchard wrote:I've been tidying up the mess in my roof this week and I'm hoping to find my PAL CoCo. Assuming it still works and the power circuit is similar I will measure the voltages on the board and let you know.
I found it hidden behind some bags of old clothes. The PSU circuit looks completely different to your one. I tried to find some PAL schematics online without any luck but I took some measurements anyway:
The transformer has outputs for 0V, 2 x 9 VAC and 2 x 17 VAC.
The 2 x 9 VAC outputs get rectified/smoothed to +10.5 VDC
The 2 x 17 VAC outputs get rectified/smoothed to +/-21 VDC
Protective earth is connected to 0V.
The +/-21 VDC supplies get regulated down to +/-12V by 7812 / 7912 regs.
The +10.5 VDC is regulated down to 5V by a linear circuit based around a 2N6594 PNP transistor. That's a lot of power to get rid of so the transistor gets pretty hot. After about 5 minutes, it's too hot to keep your finger on for more than a few seconds. If you plugged a disk controller in I imagine the transistor would really start to cook. That's commercial circuit design for you
The 10.5 VDC in my PAL coco is significantly higher than the equivalent voltage (8.5V ?) in your NTSC coco, which explains why the transistor and heatsink are much larger in the PAL coco. As long as the transistor doesn't get much hotter than 80 deg C then it shouldn't be a problem. Of course, it will last longer if it runs cooler so lowering the voltage from the transformer will be of benefit.