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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