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Re: A sick computer monitor



Regarding bc's endorsement:
...
This fits bw's suggestion to check the amp.

brian whatcott wrote:
...
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.

I agree that it is more likely than not that the horizontal amplifier
output transistor is shot, and will probably need to be replaced if
Ludwik decides to try to fix it. But I also strongly suspect that there
was a reason (external to it) as to *why* the horiz. output stage blew
out. If the reason is not removed then any fix brought on by just
replacing the horiz. output amp. transistor would be quite short lived
and the problem would return. That reason is probably an overload caused
by a short or other malfunction in the devices that load that stage.
Among the things that could be usual suspects for causing it could be (as
suggested by bc) a blown high voltage rectifier and or damper (if it has
one), but such a situation would presumably be caused itself by some
other overload to the high voltage circuit. Such an overload could be
something as mundane as current leaking or arcing along a moist dust
laden pathway. It is also very possible that a short has developed in
the flyback transformer. I've had personal experience with two different
TVs where a leaky (or was it open?) flyback transformer *was* the culprit
and which did fry the horiz. output stage (once it happened on a tube set
and once on a solid state one). Although it would be less common, it is
not unheard of for the deflection yoke to develop a short that would
overload the horiz. output stage.

I suppose it is always possible that some higher power solid state device
(hi-V rectifier, damper rectifier, and/or the horiz output transistor)
spontaneously burned out on its/their own without any help from an
overload external to them. But if that is the case it would be a signal
that the circuit design was not very conservative or robust. It may
suggest, then, that the circuit's normal operation had little extra
margin to allow for expected aging of circuit components (e.g. resistors
that change their values over time), thermal degradation of performance
characteristics and component life span and/or the like.

David Bowman
David_Bowman@georgetowncollege.edu