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9. How to "HOT ROD" a Marquee
Here we go. Now for those of you who like bending the rules, and putting 8
cylinder engines on motorcycles, 200 watt amps on 6" speakers, etc.
Here is how you "Hot Rod" the video chain. As you may already know, with the
exception of three coupling caps, the entire video chain in the Marquee is
directly coupled (DC) from the BNC connectors to the CRT's.
The front end of the VIM is where you'd find these coupling caps. There is
one on each input (RGB). Direct coupling is better, but it has it's drawbacks,
and that is that if the DC offset is off on the source (input) device, it will
effect the offset in the video chain of the projector. When the DC level
(offset) is off, you'll then effect the blanking level (this is the reference
for black). And from there you can forget about the great black level feature of
this CRT.
So what they do is add these coupling caps and make the front end AC coupled,
meaning that the caps will block DC, and only pass the AC signal. But it's not
that simple, because once you AC couple you'll have to get and keep the DC
offset at zero, so they add a circuit called a "DC Restore Circuit" and this
circuit would keep the video front end DC level to zero, and if the following
circuits are designed properly, the entire video chain would have a balanced (0)
DC level. Therefore with proper DC level, we will have proper blanking level and
black levels. So our goal is to not effect this feature in our quest to improve
the projector.
To enhance the performance of the video chain, change the value of these
three caps (C33, C49 and C65). The value is 470N or 0.47 ufd. If you increase
the capacitance the picture would get hotter, but don't use electrolytics
(especially polarized electrolytics). If you use electrolytics, you'll force DC
reference into the video chain whenever there is bright scenes. The DC restore
circuit will correct this, but it may not do it fast enough because of longer
charge on the higher value cap. For instance, if you double the value of the
coupling caps (0.94 ufd), you'll may also be putting the circuit outside of the
speed of the DC restore circuit, because there is also a cap in that circuit
that holds a charge. So what you may experience by doing one thing, you would
more likely be effecting something else. So the "hot rod" effect of changing out
C33, C49 and C65 would not be a miracle improvement, though this works well in
high end audio (there's no need for DC restoration). It could be a problem in
video.
I'm of the opinion that you cannot drastically improve on the performance of
a Marquee, and that's mostly based on the fact that the video chain has a very
wide bandwidth, and the components that were used (upgrades) were very high
performance devices. Still there's room for improvements, but I would think that
the end results would be more subtle than drastic.
The video chain in a marquee is very well designed. And this means of hot
rodding could cause effect to the video chains DC offset level. You'll
experience a brighter and falsely more dynamic picture, but pay close attention
to the black bars from the test disk.
Now, here's a nugget. You can improve on the video chain, by replacing that
cap with the same value (0.47 ufd), go from a 50vdc to a very high quality 100
vdc poly. The change would be subtle, but noticeable if you're using a high
quality input source.
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