On Fri, 25 Jan 2008 23:13:47 -0600, you wrote:
>OK, I've started replacing all electrolytic caps that I have matching
>values for, and I guess I'll have to call Bob Roberts for the rest. The
>X and Y deflection have settled down somewhat, and I'm not getting
>random bad draws anymore.
I had a feeling that might be part of your problem. :) Old ELs are the
root of many evils, electronically speaking; a lot of times I'll just
shotgun them all as a matter of course.
>But the display still seems a bit dotty - Literally - I peered closely
>at the image and noticed that the deflection wasn't ripply at all; but
>the intensity of the line is! It's like the beam is constantly
>fluctuating in brightness, producing a granular appearance on what
>should be a straight plot. It's especially apparent on dimmer lines.
>
>Questions: where is the overall monitor brightness controlled? Is the
>beam intensity dual level like a Cinematronics game or is it more complex?
It's more complex than that. If you refer to pages 102 - 103 of the
schematic: there are four intensity signals on the Vector Output Bus,
INTEN0 - INTEN3, coming from the 74LS273 at C2 of the Mother Board.
(Although they originate from the 2114 RAM chips as signals SEQD4 - SEQD7
on the Vector Data Bus.) The INTENx signals are fed to the base of Q204
via R244-R247, effectively making a crude but reasonably workable D/A
converter, and from there the analog signal passes through Q206 and becomes
the Z OUT signal that controls the vector intensity. Since these
transistors are fed by the +5V from the logic board, any ripple or
instability in that supply will affect the Z intensity also.
A stuck bit on the 74LS273 could also cause incorrect vector
intensities.
If none of the above are the case, then IMO the next most obvious
suspect would be the monitor itself, since it generates its own supply
voltages from the 60VAC feed it gets from the second power transformer and
thus has its *own* crop of potentially fussy electrolytics. Suspect C101
and C102 on the monitor chassis, since those are the big filter caps across
the B+ and B- rails just after the bridge rectifer; also suspect C504,
since it's associated with the input stage of the Z amplifier. The
capacitors at C500 - C503 *might* also be worth checking, since they're
associated with the spot-killer circuit which in turn controls the Z amp,
but I'd say C101, C102, and C504 would be the prime suspects.
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Received on Sat Jan 26 01:55:57 2008
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