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Marquee blurring at high contrast

 
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tschaeikaei




Joined: 08 Apr 2013
Posts: 489
Location: Germany/Saarland


PostLink    Posted: Sat Aug 13, 2022 11:25 am    Post subject: Marquee blurring at high contrast Reply with quote


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Hey there,
how have you been all these years? I have been starting a new business since 2020 and never had the time to even think about projectors. But then i tought after two and a half years it could be a good idea to take a week off, and you know... i remembered i had two marquee under the ceiling and a superduperspecial graphics card, that silently died last february. Managed to replace the card, did a new setup and now i would like to adress some problems with image quality i ignored for a long time.

I am absolutely sure that this problem must have been talked about many times over the years, but wasn't able to find a solution for it.
Let me describe: if you crank up the contrast setting way up (normally about 80 or more), vertical lines tend to smear to the right. i know, this is normal on a stock marquee, but can it be solved or minimized?
I use the color correction module in my blend to archive higher contrast in the edges and corners (which are in the middle of the screen when using two pjs side by side of course), this makes the problem even worse.

We managed to track the problem down to somewhere between Q21 (VNB ) and the cathode. The signal on G1 is not affected.
My test settings: grey background, white 10 pixel wide field near the upper left corner, contrast 50% and zone contrast at maxmium. Measurement is taken directly at the cathode.

Same settings on the scope, master contrast set to 100%:


Of course, these are extreme settings, just to be able show what happens. My everyday use is contrast=65, some corners are bumped up to 90% zone contrast.

I remember reading about frankenyokes, but since i can clearly see that the video-output-amplifier is over its limits, i am sure, this can be solved without doing anything to the focus coils.

My friends guess was to bypass Q21 (which is driven into saturation at these settings) B-C with a fast low Vf germanium (like 2N270) or schottky diode, which i both do not have on hand.

Let me know what you think.
Best regards, Julian

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Marquee 9500U edgeblend P43 | NEC 9PG
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Tim in Phoenix




Joined: 21 Oct 2006
Posts: 4378
Location: Phoenix


PostLink    Posted: Tue Aug 16, 2022 10:53 pm    Post subject: Reply with quote

Tell us more. What is your screen width and aspect ratio? And screen fabric. How many hours on the tubes? And your H Size setting. If your rasters are shrunk in, you lose light output and resolution and might be compensating with too high Contrast. Resulting in phosphor wear.

I am just now about to retube my pair of edge blended M9500s......after sixteen years!


Tim
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gjaky




Joined: 05 Jun 2010
Posts: 2789
Location: Budapest, Hungary


PostLink    Posted: Thu Sep 01, 2022 8:38 pm    Post subject: Reply with quote

If you follow carefully the circuit then it is obvious that the cathode and G1 driver circuits are essentially in series., ie the same current goes through the two halves.
If you want I can send you loads of application notes those are dealing with CRT amplifier design consideration.
In a nutshell, Q19,Q21 with R28 forms a so called cascode circuit.
The basic idea is that you can't build a fast amplifier by using a simple common emitter stage, because the Miller capacitance which is shunting down the gain at higher frequencies.
Adding a second transistor, a common base one on the top of the common emitter helps feeding the collector of the common emitter stage by low impedance, defeating the Miller capacitance, while the common base part keeps unity current gain, and can easily achieve frequencies up to the transistor's transition frequency, or even beyond. So to sum up the common base transistor holds up most of the voltage but does not have to be very fast, while the fast transistor in the common emitter stage can remain low voltage part and be very fast.

Q19 is a very fast transistor (2~3GHz). Q21 is "only" 1.5GHz but stands high voltage.

I don't think Q21 can get into saturation here.

What I'd try to see if what happen if you bypass the beam limit circuit for testing?

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projectors in the past : NEC 6-9PG xtra, Electrohome Marquee 6-7500, NEC XG 1351 LC ( with super modified Electrohome VNB neckboard !!!)
current: VDC Marquee 9500LC
The MOD: VNB-DB, VIM-DB
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