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Discussion Starter · #1 ·
I bought a Marquee 8500 that had a cracked CRT face on the green tube, but the other two tubes are in excellent condition. My question is, what could have caused the green tube to crack? Is it just a stress fracture from rough handling or is it possible that some electrical event could have caused the problem?

Thanks
 

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From the reading i've done over the years it's possible it came from someone pushing the green tube to hard (i.e. turning the contrast way up). Eventually the high heat causes the face to crack because the heat cannont be dissapated fast enough. Im sure someone else can expand on this more, but that is a start for you.


Alex
 

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Does the green still work?
 

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Look at the Boards.

Is there any residue of the leaked Glykol/Glycerol?

This stuff is corrosive, so it may have eaten some copper traces...

Roland
 

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Tubes can crack if you don't bleed them, the glycol absorbs moisture and expands and then -crack-. Putting a little bubble in your tubes is the way to go apparently...
 

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Guys!


Eight inch Marquee tubes do not often fail this way. Mr. o2, or an experienced technician, should disconnect the anode lead (the pink high voltage wire) of the green CRT from the anode splitter block (the gray block device which is screwed to the green tube cover) then isolate the splitter block from any metal of the chassis with layers of plastic bubble wrap, stand back and away, and see if the projector fires up with red and blue. This is 35,000 volts there, and it deserves your total respect. Run it for only long enough to see if high voltage fires up. The noticeable crackle of high voltage firing up at power-on would be very welcome, and might suggest that the cracked tube happened in shipping. If high voltage does not come up, then, very likely, the green neck board may have failed for the previous owner, and allowed runaway green beam current to crack the green tube face; this is ugly and suggests you need to replace the green CRT, the neckboard for green, and the high voltage power supply; $4000 USD or so in all. Remember that CRT neck boards, or final amplifier stages, apply a higher voltage (+85v to +130v) to the tube gun cathode to shut it off; lack of voltage means that the entire high voltage supply current meant for all three tubes funnels to the face of one CRT, and the heat can crack it in moments. E.mail me if I can offer further advice on what things to inspect or replace.
 

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Discussion Starter · #8 ·
Tim,

Good call! I followed your procedure and disconnected the anode lead, turned the beast on, heard the crackle of electricity, and then the red and green tubes came on. I was able to move through and see several different menu items on the face of the two tubes.

So, can I assume from this that all that is needed to be replaced is the green tube or is there other items I need to look at?


Thanks Tim for your assistance!!!

Brian
 
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