cctech Digest, Vol 96, Issue 5

That PSU is one of the most complex ones you are likely to come across. It's actually 3 SMPSUs in one box, with aditional complications. All run off the same rectifier/smoothign circuit, the capacitros for which are the 2 coke-can sized ones at the left hand side. There's a small, and relatively convientional SMPSU to provide power for the rest of the supply. This one is nasty in that much of the circuitry is not isolated from the power line. It also has no overcurrent protection, so a fault in one of its loads can caus the chopper transitro to fail. The logic supply (+5V @ 125A and +/-15V) has its control circuity on the ouput (isolated) side of the supply, powered by the above SMPSU. The memory supply starts with a 36V SMPUS which is similar in design to the logic PSU. The rail can be battery-backed, and powers non-isolating switching regulatrors to provide +-5V and+/-12V for the memroy boards. It also powers a full-H driver circuit to operate the unconventional 35V 70Hz AC fans. There is a ltitle control circuitry to enable the appropriate supplies for each postion of the console switch. Note that when mains is applied, the first PSU I mentiond will be running (irrespectvie of the position of the cosole swtich), and the large cpaactiros will be charigng from the mains. There is a relay in the mains inptu circuit, but it's not an on-off switch. Its function is to short-circuit an inrush limiter (either a resistor or a thermistor depending on the version) once the PSU has got going. Repairing it can be easy or difficult depending on the fault. The first thing to nore is that the 350V DC from thos large capacitors, which is fed to the logic and memory supply boards by wires on barrier strip terminal, is letjhal. Very lethal. Take great care if you have to work on the unit when it's powered up. The first PSU, the little one, is probably the worst to repair in that it's mostly on the mains side of the isolation barrier, so an isolating transformer is very useful. -tony