Re: A sick computer monitor

[brian whatcott <inet@INTELLISYS.NET>]

At 16:12 7/15/00 -0400, David Bowman wrote a worthy summary
of likely causes for a failing line scan.

> ...apparently your sick monitor has a problem in its horizontal
> deflection circuitry that has gotten worse with time.  At first the
> problem only seemed to affect the amplitude of the horizontal deflection
> pulses and this seemed to mess up the pincushion correction modulation on
> those pulses.  But as time has worn on the problem has gotten so bad that
> the horizontal deflection pulses have stopped altogether (i.e. their
> amplitude went to zero).  Presumably, the problem is in the horizontal
> deflection oscillator or the horizontal deflection output amplifier or
> the flyback transformer or the horizontal deflection yoke (the 2nd, 3rd
> or 4th possibilities being most likely).  Since the horizontal deflection
> pulses power the primary of the flyback transformer, and the CRT gets its
> high anode voltage from a step-up secondary winding on that transformer,
> this means if the horizontal deflection signal is lost the screen will go
> black.  Another secondary winding on this transformer powers the
> horizontal deflection yoke.

> My guess is that maybe a short developed in
> the flyback transformer or the horizontal deflection yoke which then
> overloaded the horizontal deflection output circuitry and reduced its
> amplitude.....

> David Bowman
> David_Bowman@georgetowncollege.edu


Sometimes you can replace the line output transistor and it will hold up.
 The line output transistor is never overspecified. But it is not
expensive to try a replacement. I seem to remember a down home recipe for
diagnosing a failed output transistor of this kind: the forward resistance
collector-emitter resistance should be greater than 100 Ohms (if I remember
rightly).

Sometimes it fails again. Then David's line output transformer
suggestion will need your consideration.

brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!