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.
Joined: 05 Jun 2010 Posts: 2650 Location: Budapest, Hungary
Link Posted: Mon Oct 26, 2015 8:33 pm Post subject:
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
Link Posted: Mon Oct 26, 2015 10:22 pm Post subject:
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...
Joined: 05 Jun 2010 Posts: 2650 Location: Budapest, Hungary
Link Posted: Sun Mar 27, 2016 8:35 am Post subject:
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
Joined: 08 Mar 2006 Posts: 8776 Location: Colorado
Link Posted: Mon Mar 28, 2016 3:42 pm Post subject:
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. _________________ MARQUEE HD MODS: Bring your Marquee CRT projector (or clone) back up to spec and offer even greater performance!
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