Asimov wrote:
"E P" bravely wrote to "All" (12 Nov 04 12:55:17)
--- on the heady topic of " Video output transistors for Sylvania Superset"
EP From: (E P)
EP For a while there were big red shadows appearing on the right side of
EP objects on the screen; they were intermittent at first but now they
EP seem to always be there, and they're growing. I don't have the proper
EP tools to diagnose the TV, but I figure I can't do any worse than the
EP TV repair place I took it to did (they charged a big pile of money and
EP didn't actually do anything). There's another (presumably unrelated)
EP problem: when I turn on the TV, it's very fuzzy for about 30 seconds.
EP I figured I'd start with the color thing, and then tackle the focus
EP thing with a can of air and see if anything reacts badly to cold.
EP Then the third thing that bugs me is that the screen overscans about 1
EP inch too far all the way around. But one thing at a time...
Any change in horizontal scan width sometimes indicates a problem with
the high voltage. If the high voltage is too high the beam gets very
stiff and the set underscans. If the HV is too low then the beam gets
soft and overscans a lot.
The CRT either has a biasing problem or it has reached the end of its
life. The very fuzzy CRT at startup sort of confirms the latter. The
reason is that most modern CRTs use a dynamic convergence system which
depends on the proper beam biasing. The convergence and focus gets
upset as the emission weakens. However it could be a G2 or Screen
problem as well, maybe a leaky bypass cap for example.
The red flames to the right of objects is caused by severe
overloading. Overloading may be a sign of insufficient cathode
emission. Try swapping another gun for the red. If the blooming
problem moves to the new colour then it is a biasing or video problem
but if it remains red then the crt is likely a gonner. You would have
to test the CRT emission to be sure and maybe even measure the HV with
a proper HV probe.
It's sounding more and more like the tube is going bad, I think. When
you say "swap another gun for the red" does that mean just swap the
signal wires coming in to the neck board? The only diagnosis I've done
so far has been to unplug all of them at once, and the screen had a
definite red tint to it, but I didn't try anything else after that.
I don't have any HV test equipment, and frankly that stuff scares me a
lot so I don't even want to mess with it ... if that's the problem, I'm
getting a new TV
At any rate, I'm in the process of putting stuff back together; when
that's done I'll flip the TV on and try some of the safer checks that
have been suggested.
Thanks
--
Elliott