CRT based RPTV coolant problems - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 5 Old 12-16-07, 10:16 AM Thread Starter
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CRT based RPTV coolant problems

The contamination of CRT coolant has been so pervasive in many Philips made sets that it seems useful to discuss the matter. It has been a subject of concern and debate among techs for a while, and now, with many of these sets reaching the age where people are not willing to spend the money to pay a tech to repair them, threre seems to be more interest in DIY solutions. While Philips sets have been the worst by far, other brands have had problems as well. We have seen contaminated coolant in virtually every brand, but mostly in sets made by Philips, Thomson, and Samsung.

Another problem that is related to CRT coolant is the problem of leaks. This is more common on Mitsubishi and RCA sets than most others, but is a serious concern for anyone changing coolant in any set.




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post #2 of 5 Old 12-16-07, 10:17 AM Thread Starter
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The Coolant

CRT based rear projection sets have for many years used a system for cooling the face of the CRT that includes approxmately 8-16 ounces of liquid in a chamber between the CRT glass and the first lens in the optics. This coolant has typically been a mixture of 70% ethylene glycol and 30% glycerine. It serves the dual purpose of cooling the CRT to avoid excessive aging and cracking of the glass, and providing optical coupling to the lens. The coolant should be very clear, not yellowed nor cloudy.

The coolant can be purchased in small quantities from most distributors that sell electronics service parts, such as Tritronics, Andrews, or Vance Baldwin. It does not seem to be critical which brand is used, though there are going to be slight differences in the optical properties of the diferent brands. The big difference you will see is that there will be slightly different alignment between tubes using different coolants, or even old coolant vs new. The bottom line is that they all work, but you WILL need to adjust focus, convergence, and perhaps size after a coolant chage. Don't sweat the difference in the coolant brands, as long as they are a 70/30 mix described above. Do have the alignment documentation for your set ready, however, regardless of what you use.

The coolant contains ethylene glycol, so be aware of your local environmental regulations regarding disposal. For the most part, it seems that it is safe to dump it down a sanitary sewer, as it breaks down within days in the environment. Some may differ with me on this view, and some areas prohibit this, but after all, they spray the stuff on airplanes all the time for de-icing the wings. Some will be hsyterical about the hazards...I say just be careful not to ingest it of get it on your skin, be aware of local environmental laws, and use good sense. Be aware and careful like with any DIY project, and be responsible.




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post #3 of 5 Old 12-16-07, 10:18 AM Thread Starter
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Symptoms of Contamination

The typical symtoms start with a haziness in the pix, similar to the effect of dirty optics. As it gets worse, you get a halo around images in the affected tubes and it is impossible to get good focus. Eventually you get little more than a faint, out of focus image with mostly a haze of the affected color. In the worst cases, you can get cooling problems and actually crack a tube due to uneven heating.

It is most common in the Philips sets to have the problem in the Blue tube, followed by the Green tube. I have rarely seen it in red tubes in the Philips sets. I have seen the Red contaminated in other brands.

What is the contamination? I have heard various explanations, including bacterial growth, algae, and an inorganic coagulant that is a result of the coolant attacking the aluminum in the coolant chamber. My guess is that there are a couple of different answers, because the result is slightly different in different brands. The Philips sets seem to have something that looks like and organic growth in them. I suspect algae because it is limited to the blue and green tubes. Others seem to be more of a crusty inorganic stuff. Who knows, but the result and solution is the same. It may be inorganic or both, as even in the Philips sets there is a crusty buildup on the back of the lens that seems to be inorganic, and very hard to remove. The rest of it looks like dirty toilet water when you come back from months of vacation.

An example of what the symptom looks like can be seen in this post:
http://www.hometheatershack.com/foru...html#post67421




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post #4 of 5 Old 12-16-07, 10:18 AM Thread Starter
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Fixing the Problem

REPAIRING THIS PROBLEM INVOLVES SERVICING AREAS THAT USE, TRANSMIT AND CAN STORE VOLTAGES TO MORE THAN 30,000 VOLTS. THERE MAY BE RESIDUAL VOLTAGE ON CRTS AND RELATED CIRCUITS, AND CONNECTIONS AND WIRE ROUTING ARE CRITICAL TO SAFETY AND PERFORMANCE. PROCEED WITH GREAT CARE AND WITH THE UNDERSTANDING THAT YOU ARE DOING SO AT YOUR OWN RISK. BE SURE THAT THE SET IS UNPLUGGED AND HAS BEEN OFF FOR SOME TIME BEFORE SERVICING.

There are two schools of thought on this repair. The first is to simply suck out the old coolant while the tubes are in the set, then refill with new coolant. This is a poor technique because it does not get all of the contaminant out, it leaves the crusty residue on the back of the lens, and it risks spilling coolant in the set. Many techs do it this way to minimize the time in the set and thus the cost of the repair, but in my opinion, it is too risky and ineffective. I have gone behind these repairs and found coolant on boards that causes (sometimes unrepairable) damage and tubes in virtually the same condition as before the repair after a matter of months. The proper way to repair the problem is to remove the tubes, completely dismantle them, clean all the parts thoroughly, re-seal the coolant chamber, refill, and re-align the set.

The information below is appropriate for most sets but will be focused a bit more specifically on the Philips sets because they are far more likely to have the problem.

The screen and back panel should be removed. While the set is apart, be sure to clean the mirror. Remove the plastic cover around the lenses if there is one. Mark the position of the yoke and magnet assemblies on the CRT necks. Mark them both with respect to the distance along the neck and the rotational position. DO ONLY ONE OR TWO TUBES AT A TIME, LEAVING ONE AS A REFERENCE FOR ALIGNMENT. Loosen the set screw on the yoke and loosen it on the neck. Pull the ground wires off of the CRT board and remove the board from the CRT, allowing the yoke and CRT board to hang gently below the CRT. If there are additional magnet assemblies remove them but mark them carefully so that you know what tube they went with. Remove the anode lead (large red wire) from the high voltage distribution assembly. This will usually require removing a cover or clip, or twisting a cap to release the wire. Be very careful not to damage the covers, clips, and wires. Be careful to note how wires are routed. Do not damage the anode lead. These connections must be made perfectly or arcing and failure will result. Remove the rest of the ground wires that go to the bulkhead and ground terminals. Now remove the four screws that hold the CRT mounting bracket to the bulkhead. BE VERY CAREFUL NOT TO REMOVE THE SCREWS THAT HOLD THE LENS TO THE CRT. This would result in spilling coolant into the set...VERY BAD! The CRT chould be able to be removed with the lens and coolant chamber attached. Be careful not to damage the anode lead as you pull the CRT out. Mark the lens, the CRT AND the coolant chamber so that you know where and in which direction the parts go when re-assembling. You don't want to assemble it backwards. Note that on most of the Philips sets there are two tubes oriented one way and the third, usually the blue is oriented the opposite way.

Remove the lens carefully by pulling the four mounting screws while holding the crt pointing up over a basin. When you remove the lens, you may pull the rear convex lens with it which has the coolant seal for the front. Be careful not to get coolant spilling all over the place and especially not in the lens system. Dump out the old coolant, retain the seal and continue taking the coolant chamber off of the CRT. Take everything apart in the mounting and coolant system. Wash all of the parts with soap and water and rinse. You will likely find that there is likely a crusty debris on the coolant side of the rear lens. This can be tricky to remove without scratching the lens. A very mild isopropanol based lens cleaner may be needed. Be very careful not to damage the lens.

After cleaning thoroughly, dry very well. Use a very thin layer of 100% silicone RTV sealer on each side of the gaskets and re-assemble. It does not take much sealer. A VERY THIN layer is all you need and you should not have so much that it squishes out significantly when tightneing everything down. Let the sealer set up overnight to allow all of the acetic acid to evaporate. Then when everything is dry, using the fill hole, put a couple of ounces of alcohol in the coolant chamber and swish it around to be sure it is sterile and clean. Dump it out and let it dry out. Fill the coolant chamber completely with fresh coolant. Be sure that it is completely full so that you will not have a visible bubble. Replace the fill cap and dry off any excess coolant.
To be continued...




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post #5 of 5 Old 12-16-07, 10:19 AM Thread Starter
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Leaks

Many brands of CRT based RPTV have had problems with colant leaks. This is one of the most infuriating problems with this technology, because it is so easily avoided. The reason that the leaks occur is simply a poor mechanical design that does not maintain a good seal. It is often made worse by over filling the coolant chamber, and becomes more of a problem when there is no diversion barrier to keep leaks off of circuit boards.

The coolant is usually a mix of 70% ethylene glycol and 30% glycerine. It is not itself corrosive, but becomes very much so when exposed to air. The ethylene glycol is a very good solvent and absorbs water from the air. This solution also picks up anything on the boards and creates a very effective corrosive solution that can destroy circuit traces, connectors, and the legs on components. The telltale signs are a green goo on metals or darkened coloring of the traces on a board.

Leaks often occur after a set has been moved. Problems may not show up for months, as the liquid migrates on the board and becomes more corrosive. Shorts may occur due to conductivity in the liquid or it may take a while for corrosion to finish off a part or connection.

It is VERY IMPORTANT to identify leaks and map the location of where the coolant has been BEFORE removing the boards! The reason for this is that it can be very hard to identify problems and you have to test all of the connections and traces carefully in the area that might be afected. This is very tedious and time consuming and you don't want to have to do it on any more area than needed. On some boards that are double sided, you have to test EVERY feed through grommet in the affected area (like the Mitsubishi signal boards) and some boards require repair of dozens of these connections.

I recommend that a board be mapped for areas to check and repair, then washed in isopropanol or denatured alcohol. I then run them through the dishwasher. This get any residual coolant off and dries the board thoroughly. I then replace any corroded parts or traces, repair any damaged feed throughs, and test.

Some leaks are due to sloppy assembly when changing coolant, so be very careful to make sure that seals are intact and adequate, and that no drips are left on the tube or mounting hardware. This stuff migrates very well and may not leave much trace of where it has been, so look carefully.




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