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draganm
Joined: 08 Mar 2006 Posts: 8990 Location: Colorado
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Link Posted: Mon Oct 26, 2015 8:26 pm Post subject: |
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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.
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gjaky
Joined: 05 Jun 2010 Posts: 2789 Location: Budapest, Hungary
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Link Posted: Mon Oct 26, 2015 8:33 pm Post subject: |
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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. |
What about the heatsink is that remain cold (with no signal)?
_________________ 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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barclay66
Joined: 27 Jun 2011 Posts: 1291 Location: Germany
TV/Projector: Marquee 9500 Ultra
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Link Posted: Mon Oct 26, 2015 10:22 pm Post subject: |
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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
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Link Posted: Fri Mar 25, 2016 3:43 am Post subject: |
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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: 2789 Location: Budapest, Hungary
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Link Posted: Sun Mar 27, 2016 8:35 am Post subject: |
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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, VIM-DB
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draganm
Joined: 08 Mar 2006 Posts: 8990 Location: Colorado
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Link Posted: Mon Mar 28, 2016 3:42 pm Post subject: |
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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. |
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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