Fuse between rectifier and cap = BAD.
The crowbar is a thyristor across the unregulated DC rail. In the event that a regulator pass transistor fails hard short from collector-to-emitter (the most common failure), the FULL, UNREGULATED (30V or so) voltage is then conected to the output.
The overvolt sense cannot shut down the transistor, because all it does is close down the transistor base voltage... which does nothing because the transistor is SHORT fromo base-to-emitter.
Recognising this, there ie a 4-amp thyristor across the rail. On paper it looks like the incoming power from the bridge rectifier goes -through the fuse- to the thyristor, and will blow the fuse, making everything safe.
But no...
The thyristor is rated at 4A, and probably explodes abotu 10A peak. The reservoir caps -when fully charged- supply a peak current of about 40Amps, utterly destroying the thyristor, and therefore removing the last protection that you had against overvoltage. (The recifier/secondary doesn't respond to an increase in current demand anywhere NEAR as quickly, since the secondary has too much inductance, and a SUDDEN change is seen as a high frequency event... iductance increases impsdance at high frequencies, so less current as a result). Now, you have a hard-short transistor, and no way to shut it down.
Move the reservoir caps to the OTHER side of the fuse, so that now the COMBINED current from the rectifier AND the reservoir caps goes through the fuse, and you have a design that works as intended.
That's all you have to do to makeit work. -It WILL NOT WORK as built at the factory.
You can safely test it with the console disconnected by brushing two wires together, one wired to the unregulated voltage, the other to the output. The way it's built from the factory, the thyristor will explode, and as long as the wires are touched, the full output passes unprotected. once modified, the fuse will pop and the thyristor will live, and the supply is safely shut down.
the blue sockets were pretty terrible, and aged poorly with heat, becoming crumbly. If they're not disturbed they hold the chips okay, but if they are over-handled, they tend to crumble and break. replace any that break, but don't go LOOKING for trouble... a "shotgun" replacement will tend to lift traces, since I remember that the PCBs tended to do that, although when I was working on them, my jedi soldering skills weren't as good as they are today. :wink:
Keith