How about slowly varying focus? I have a SuperVGA monitor that
won't keep its focus. If I focus it when I first turn it on, it
slowly goes out of focus as it warms up. If I try to keep it in
focus as it warms up, I can keep it in focus, but some of the other
adjustments peak out (like horizontal and vertical size). Is this
a power supply problem?
Dave
Tony Duell wrote:
cdrmool(a)interlog.com wrote:
Would someone be kind enough to direct me to a
good FAQ or URL for
diagnosing/repairing monitors? There has been some excellent information
passed around on this list since I've subscribed but I need something a
bit more basic. For example, descriptions of different types of problems
(flickering, only 2 colours) and the possible causes and interventions. I
realize that its not quite as simple as I'd like it to be but I have to
start somewhere. I keep finding SVGA monitors that need some work and
would like to get them up and running.
Sounds a lot like a bad flyback transformer, getting ready to die. Anyway,
Flickering could be due to the flyback, could be due to a lot of other
things. A missing colour is certainly not the flyback, though.
Here are some simple-ish faults and possible causes. There are plenty of
other causes for these faults, but this is what I'd check first.
Missing colour (only 2 of the 3 guns working)
----
Bad cable or PC plug (bent pin)
Dry joints on the CRT base panel (Where the video amplifier normally is)
Defective components (transistors, LM1203 chip, etc) in the video amplifier
CRT (unlikely)
Horizontal line (only)
----
Vertical output chip
Capacitors near the vertical output chip open/shorted
No power to vertical output chip (maybe fusible resistor O/C)
Vertical line (only)
---
If EHT comes from the horizontal output, then O/C yoke, connector, or
coupling components.
If separate EHT generator (fairly common in monitors with multiple scan
rates, including even EGA), then horizontal output transistor, drive to it.
Blank screen, power on light normal
---
Look for glow from CRT heater. If missing, check supply (maybe from the
flyback, maybe from the main PSU) and heater continuity
Check EHT with a special EHT meter. Expect about 25kV on a colour
monitor, about 12kV - 15kV on a mono one. If missing, check flyback,
horizontal output transistor, etc
Check other CRT voltages. If any are missing, trace back to the source
(probably from the flyback).
It's possible, although rare, for a video amplifier fault to bias the CRT
beyond cut-off. Sometimes turning up the 'screen' control will get a
raster in this case.
Blank screen, power light flashing about once a second
---
The main PSU is tripping (assuming this isn't some infernal power-save
mode). Most likely cause is the horizontal output transistor shorted,
probably due to a shorted flyback.
Main fuse blows violently
-----
Short on the primary side of the PSU :
Chopper transistor, bridge rectifier diodes, degaussing posistor, chopper
control module, etc. Sort this out like an SMPSU
Flickering, especially when case is tapped
----
Dry joints (could be just about anywhere). Often a complete resolder is
the quickest way to solve this.
www.repairfaq.org is down at the moment (I believe the amount of data
pulled from that site exceeded the limits set by the ISP). There are
mirrors, but I forget where. A quick search of sci.electronics.repair
will find them, though.
FInd that site and read the safety warnings before you pull the case on a
monitor. There are dangers in them - high voltages (not just the EHT to
the CRT, but also mains), the high-vacuum CRT that could implode
(although it's unlikely), etc.
you need and unless you have a high voltage probe
to test the voltage going to
the cap on top of the picture tube you'll have to guess-timate as to whether a
flyback is at fault. MCM Electronins is a good end user source for many of the
Normally, when a flyback fails, it develops shorted turns and kills the
horizontal output transistor as well. You don't need an EHT probe to find
that. A ringing tester will find shorted turns, but you need a 'scope.
parts used in monitors as well.
-tony