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Marquee Modifications and Performance Enhancement
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draganm



Joined: 08 Mar 2006
Posts: 8776
Location: Colorado


PostLink    Posted: Mon Oct 26, 2015 8:26 pm    Post subject: Reply with quote


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gjaky wrote:
draganm wrote:
Did anyone ever figure out why some MArquee video neck boards run hot on the big 400V power resistor? I thought at first it was leaking bypass caps, the two AL can 22uf/85V factor ones are terrible (and under-rated), but putting in new 22uf /160V doesn't help.

Bad diode further up the chain?


Does the board working otherwise?
yes, and that's the weird part, the board otherwise runs fine and will continue to run for hours on end. It just gets hot, I mean the resistors are too hot to touch. On other VNB's the resistor is just warm. Confused
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MARQUEE HD MODS: Bring your Marquee CRT projector (or clone) back up to spec and offer even greater performance!
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gjaky



Joined: 05 Jun 2010
Posts: 2650
Location: Budapest, Hungary


PostLink    Posted: Mon Oct 26, 2015 8:33 pm    Post subject: Reply with quote

draganm wrote:
gjaky wrote:
draganm wrote:
Did anyone ever figure out why some MArquee video neck boards run hot on the big 400V power resistor? I thought at first it was leaking bypass caps, the two AL can 22uf/85V factor ones are terrible (and under-rated), but putting in new 22uf /160V doesn't help.

Bad diode further up the chain?


Does the board working otherwise?
yes, and that's the weird part, the board otherwise runs fine and will continue to run for hours one end. It just gets hot, I mean the resistors are too hot to touch. On other VNB's the resistor is just warm. Confused


What about the heatsink is that remain cold (with no signal)?

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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
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barclay66



Joined: 27 Jun 2011
Posts: 1251
Location: Germany

TV/Projector: Marquee 9500 Ultra


PostLink    Posted: Mon Oct 26, 2015 10:22 pm    Post subject: Reply with quote

Hi,

I think the heat is due to tolerances between the NPN and PNP Motorola transistors. The VNB amplifier section is designed a cascode current amplifier which produces some amount of quiescent current. This 'waste' current is lower the better symmetry the amplifier stage has (less heat). I don't think they did a preselection process on those Motorola transistors. So it's pure luck if each transistor in the complementary pair has a near to equal hfe (amplification parameter) or not...

Regards,
barclay66
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cmjohnson



Joined: 03 Apr 2006
Posts: 5180
Location: Buried under G90s


PostLink    Posted: Fri Mar 25, 2016 3:43 am    Post subject: Reply with quote

Does anyone know how much gain is in the video amp chain on the VIM board?

I'm...investigating something. It may be useful. The answer to my question will help... a lot.
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gjaky



Joined: 05 Jun 2010
Posts: 2650
Location: Budapest, Hungary


PostLink    Posted: Sun Mar 27, 2016 8:35 am    Post subject: Reply with quote

cmjohnson wrote:
Does anyone know how much gain is in the video amp chain on the VIM board?

I'm...investigating something. It may be useful. The answer to my question will help... a lot.


Theoretically VIM has less than unity gain, since the input signal can be 1V, but the VNB needs 0.7V input signal for full drive.

_________________
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
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draganm



Joined: 08 Mar 2006
Posts: 8776
Location: Colorado


PostLink    Posted: Mon Mar 28, 2016 3:42 pm    Post subject: Reply with quote

gjaky wrote:
draganm wrote:
gjaky wrote:
draganm wrote:
Did anyone ever figure out why some MArquee video neck boards run hot on the big 400V power resistor? I thought at first it was leaking bypass caps, the two AL can 22uf/85V factor ones are terrible (and under-rated), but putting in new 22uf /160V doesn't help.

Bad diode further up the chain?


Does the board working otherwise?
yes, and that's the weird part, the board otherwise runs fine and will continue to run for hours one end. It just gets hot, I mean the resistors are too hot to touch. On other VNB's the resistor is just warm. Confused


What about the heatsink is that remain cold (with no signal)?
well you can't run the PJ with no signal, there is some kind of default "internally generated video" that has to run all the time to keep the beam moving. I don't know off the top of my head what this on start up but it's pretty low, maybe 15khz?
I guess the question is "does the problem VNB get hotter" when a higher scan-rate is introduced. I don't think it does, the VNB's that run hot will do so just sitting there idling with no external video present.

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MARQUEE HD MODS: Bring your Marquee CRT projector (or clone) back up to spec and offer even greater performance!
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