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Creativision PSU questions

Posted: Tue Apr 20, 2021 7:09 am
by Scouter3d

while working on the Creativision PS/2 keyboard interface i have removed my CV motherboard from the case.

I am talking about an MKII board.

I recognized, that (the enormous) aluminium heatspreader gets REALLY hot during operation. It cools off the 7805 and the 7812 voltage regulators

looking at the board i found an odd thing that puzzles me:
for one voltage line, there are 4 diodes (that work as a bridge rectifier)
but then i only found 2 additional diodes for the other line...

As i understand it (and found in all the PSUs i have opend) the external PSU consists of a transformer which outputs 2 voltages (9V AC and 16V AC) with 2 seperate windings...

using 2 diodes to rectify i only found solutions with a centertapped transformer... (that would only need 3 external wires, like on a TI99/4a PSU)

Any ideas to clarify this are very welcome (as i love to understand how things work :0))

Are there differnt external PSUs? for MKI and MKII?

PS: the schematic shows 4 rectifier diodes for each line (at least for the MKI)

Cheers, TOM:0)

Re: Creativision PSU questions

Posted: Tue Apr 20, 2021 7:49 am
by Scouter3d
here is a picture of the mainboard PSU section...