20 Apr
2004
20 Apr
'04
11:15 p.m.
At 05:26 21/04/2004 +0100, you wrote:
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.
According to this, the filaments do run off a winding on the flyback.
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,
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.
I do not see any obvious physical signs of failure (discolored parts, smoke
etc). When I get a chance, I'll go thorugh the troubleshooting charts - they
list a number of voltage measurements, waveforms and components to check for
these symptoms.
I'm really hoping I can get it going, as I do not have another color monitor
for the ST's. (When I picked it up I thought it was another mono monitor as
there is no external adjustment/indication of color and it looks exactly like
my SM124 mono monitor - once I opened it I realised it was a color monitor - I
guess "SC" means "Color" (vs "SM"=Mono).
Regards,
--
dave04a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Vintage computing equipment collector.
http://www.parse.com/~ddunfield/museum/index.html
Hi Tony,
It would appear that you are correct. Once I removed the cover, I could clearly
tell that the "chirp" is coming from the switching power supply. There appears
to be no activity from the rest of the monitor, including the fact that even
with the lights off, I could not observe filaments lighting. The power LED does
It's not unheard-of for the CRT heater to run off the flyback
transformer. So if the horizontal output stage is dead, there will be no
heater glow.
light, however it's fairly dim - I don't
know how brightly it would normally
light.
The final filter capacitor in the supply is rated at 180v DC. I powered the
supply briefly under no-load and the output rose to 140v. With all the
connections in place, it is producing about 55v - I have no idea what the
normal requirements of the monitor are.
I would guess around twice that voltage at least. Does the PSU have one
output or seceral ? Have you checked if any of them are stuck at 0V?