Looking for Atari SC1224 monitor schematic

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Apr 21 17:19:56 2004

> Hi Tony,
>
> I've located a set of rather poor schematics (can't really read most of the
> component values), but it did include 10 pages of troubleshooting information.

Right.

>
> According to this, the filaments do run off a winding on the flyback.

This does not suprise me. Most colour monitors seem to do this. Watch
out, because the heater supply is anything but sinusoidal, and most
multimeters -- even those that claim to read true-RMS, get really
confused by it. The RMS value is, of course, what matters here.

>
> The power supply has three outputs, a main one which is supposed to be 115v,
> and two smaller (thin wires/connectors) which are supposed to be 12v and 14v.
>
> Under no load, I'm measuring 140v, 15v and 17v, shich is probably about
> right. With the monitor connected, I'm reading about 55v on the main supply,

OK...

> I have not measured the other two as I'd have to remove the PSU board and
> tack on wires, however since it's a single-switcher design, I'd guess that
> they were proportinally lower as well.

Yes. Now, what if one of those lower voltage lines is heavily loaded.
Does either of them feed a 3-terminal regulator (78xx or similar)? If so,
a short on the output of that will pull the input down.

Since the main output is not at 0, this doesn't sound like a shorted line
output transistor. Let's hope, anyway :-). I would think a short circuit
on any of the rails produced by the line output stage would kill the line
output transistor, but maybe not.

I would check those other 2 outputs when the PSU is connected to the
monitor. Then find _all_ the derrived rails (from the flyback, from the
lower voltage PSU outputs) and check them for shorts to earth. Your
problem could be as simple as a shorted electrolytic capacitor.

-tony
Received on Wed Apr 21 2004 - 17:19:56 BST