21 Apr
2004
21 Apr
'04
10:19 a.m.
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