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Re: A sick computer monitor



Back in the old days, I made some of my money by repairing TVs. The most common problem was in the horiz. deflection circuit and was usually either the amp or the damper, occasionally the HV
rectifier. One time a friend diagnosed a set and told me to replace the fly back so I looked it up in Sam's and replaced it; no diff, so I checked the tubes -- replacing the damper did it. I wasn't
about to put the old xformer back (lotsa connections) to see if both went! However, after that, no matter what the circumstances, I check the tubes first.

This fits bw's suggestion to check the amp.

brian whatcott wrote:

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!