> From: chromapolaris@yahoogroups.com
>
> Well I've got it all out on a bench so no membrane tail contact..
>
> What happens is it powers on is responsive like normal for 5
> seconds then it starts going a bit crazy doing random things
> then it just freezes. If I turn it off and back on straight
> away it is still frozen but if you leave it for a minute you
> can repeat the whole thing again until it freezes. There is
> some nasty crackling happening on the output like some
> electricity bacon sizzling on a pan..
>
> I have the later secondary power board so it is a bit
> difficult as the service manual is really written for the first board.
>
> I am getting 4.80v on the DCOK when frozen.. but right away
> it is 4.76v with responsiveness before freeze.
>
> Digi +5 it is hard to get it to load up with the Lower
> function D 3 but when I can all lights are lit and I am
> getting +4.83V (now in the manual it says to adjust R9 but
> there is no R9 to adjust on the secondary board>?? I need to
> get this up to 5v?)
>
> Ref +5v - this is at +4.98v
>
> AC OK - 4.69v (measuring this from P3 4 yellow wire to the
> ground in middle of main board.)
>
> On the -12v I am getting -12.30V
> On the +12v I am getting +12.08V

If it works for a while, then quits and never comes back, I would suspect the following sorts of things:

∗ Fading EPROMs, which occasionally drop a bit. This would be my first choice, because I have a programmer and eraser, and it only takes fifteen minutes to erase the EPROMs and reprogram them.

∗ Bad ripple on the digital 5V supply, due to a dried out C3 in the power supply.

∗ A floating input on a CMOS chip, or the CPU, perhaps due to crud in a socket, allowing the voltage on that pin to drift around. That's the hardest thing to find.

The voltages you're measuring look okay. You could try goosing the digital 5V to see if that helps--it's R3 on the newer board. The schematic package from rhodeschroma.com includes both versions.

--

Ciao, Paul D. DeRocco
Paul mailto:pderocco@...