Monitor repair

From: David C. Jenner <djenner_at_halcyon.com>
Date: Thu Sep 24 16:57:36 1998

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_at_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.
>
> > try http://www.repairfaq.org/~filipg/HTML/REPAIR/F_IBMmon.html for the info
>
> 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
Received on Thu Sep 24 1998 - 16:57:36 BST

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