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Bleeding CRT Tubes

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What exactly is the problem and which CRT tubes are affected?

Tubes in air coupled CRT projectors have a cooling chamber filled with glycol in front of the tube face. This cooling chamber is supposed to have an air bubble in it, to allow for expansion of the glycol (due to heat) when the projector is operated.

Unfortunately glycol is hygroscopic. It absorbs moisture from the air (even through the silicone seal) and then its volume increases. If you heat up the tube regularly the moisture desorbs and is pushed back into the air (through the seal), but this does only work very slowly. Once the glycol has absorbed enough water to increase the pressure, the desorption is not fast enough when you warm up the tube and so the pressure increases too much and the glass breaks.

A NEC technician out of Sydney Australia has also confirmed that they’ve seen this problem. When they investigated they noted that it happened more in humid cities than dry ones. 

All air coupled tubes are affected, independent of manufacturers. This includes Barco, NEC, Sony, Electrohome Marquee, among others. If you have a projector with air-coupled tubes, you’re best to use the procedure outlined in the following pages of this section and check to make sure you're safe.

How do I know if my projector is air-coupled or liquid coupled?

Air-coupled and liquid-coupled tubes look completely different. Remove one of the lenses and look at the tube. The glass surface in front of an air-coupled tube is flat while the glass in front of a liquid-coupled tube (called the c-element) is curved (see pictures below). An experienced user will be able to see the different without removing a lens at all.

Air-coupled tube (flat glass surface) Liquid-Coupled tube (curved glass surface, aka the c-element) Note: Half of the coolant has been removed to better show the curvature.

Alternatively, find your projector in the CRT Projector Specifications pages and check the "CRT Size/LC" column in the tables.  Liquid coupled projectors will be marked by the characters "LC". Air-coupled projectors will only be listed by the tube size.

Liquid-coupling is usually reserved for high-end 8" and 9" units (though there are some exceptions) as liquid-coupling increases contrast ratio.

My projector uses liquid coupled (LC) tubes. Am I affected?

No. You’re safe. In LC tubes, the face of the tubes is cooled by glycol in the c-element. The c-element has a rubber expansion chamber to allow for increases in internal glycol volume.

Is there anything you can do to prevent the pressure build-up in air-coupled tubes?

If you run the projector regularly, the glycol inside the cooling chamber will usually get warm enough so that any water that has been absorbed will desorb again.

However, in certain regions with high average humidity, this self regulation might not work as desired.

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