15GP22 Rebuilding Report
Rebuilding the 15GP22 CRT: A Trip Report from John Folsom
DAY 1: JUNE 10, 2008 HAWKEYE PICTURE TUBE CO. DES MOINES IOWA
The day begins early at Hawkeye Picture Tube Co. when owner-operator Scott Avitt arrives for the days work at 3:30 a.m. A somewhat bleary-eyed and sleep deprived John Folsom, Bob Galanter, and Pete Deksnis arrived at 3:30 a.m. to find Scott already at work.
The plan was to rebuild the two CRTs I had sent Scott, a 15GP22 and a 19VP22. The previous afternoon Scott had prepared the CRTs by giving them a thorough cleaning to remove the external aquadag coating, dirt, and debris, which might tend to foul the oven.
The first task was to let air into the CRTs, both of which were still under vacuum. Scott used a compressed air drill with a smooth pointed carbide bit. The bit is held against the neck of the CRT near the base, and the high-speed rotation and pressure causes enough friction to melt a very small hole in the glass. Once the pointed tip of the bit penetrates the glass, it is withdrawn and air begins to enter the tube. It takes 10 to 15 minutes for the tubes to equalize with the outside air pressure. Once at ambient pressure, Scott scores a line on the neck of the tube with a diamond file and places a nichrome hot wire around the neck of the tube. When the wire is heated ,it causes the scratch made by the file to propagate around the neck of the tube, resulting in a clean cleavage of the glass. Scott extracts the old gun and then washes out the neck of the tube to remove the old getter material.
SCORE THE NECK
THE OLD GUN
Next, the CRTs were placed ina warming oven at approximately 180-degrees F to prevent condensation while Scott prepares to weld a new length of neck glass onto each tube. This welding is performed in a vertical lathe, where the CRT is held stationary and the gas flames rotate around the neck of the tube.
THE VERTICAL LATHE
The CRT is suspended from above, and the new piece of neck glass rests on a Teflon necking pin, which sits atop a chuck. The steering wheels in front of Scott allow him to move the chuck or rotating flames up and down.
ALIGNING NECK GLASS IN LATHE
POSITION GLASS FOR WELDING
As the flames rotate, Scott raises and lowers them to slowly heat the glass above and below the area to be welded in order to prevent thermal shock and breakage. As the glass warms, Scott slowly raises the temperature of the flames, and decreases the up/down travel.
When the edges of the two glass pieces reach the right temperature, Scott raises the chuck until the edges touch, and the weld is made.
MAKING THE JOIN
The joint is slowly cooled under the flames, annealing the glass to relieve stresses built up by the temperature gradient.
THE WELD IS COMPLETE
Next, the new gun is positioned in the neck of the tube. Scott made measurements and marks on the CRT prior to removing the old gun to let him know where to position the new gun in the neck. The gun is held rather tightly in place by little springy fingers mounted around the gun. Scott folds the wires on the button tight against the stem so they will fit inside the chuck on the vertical lathe.
The CRT is placed atop a hotbox at about 550-degrees F with the neck and gun inside the box. This thermally prepares the neck and gun assembly for sealing the button into the neck. The button is the round piece of glass through which the wires extend. The stem is the hollow piece of glass at the center of the button through which the CRT is evacuated.
THE HOT BOX
Now back into the vertical lathe to weld the new gun in place. The end of the chuck is a hollow conical shaped metal piece, which Scott raises to be flush with the bottom of the button. The evacuation stem with the wires pressed against it fits down inside the center of the chuck. The cone supports the button and shields the wires and evacuation stem from the heat of the flames. In a manner similar to the procedure for the neck extension weld, the glass is slowly heated until it reaches the proper temperature.
READY TO SEAL
As the neck glass continues to heat, it eventually reaches a temperature where it softens to the point that it begins to stretch out under the influence of gravity. The glass below the heated area falls until it is stopped by the spring wrapped around the chuck. As it falls, the neck glass stretches out, thins, and pulls inward until it makes contact with the edge of the button. The amount of "fall" is critical and is controlled by the placement of the spring on the chuck. Scott continues to pour on the heat, and the neck glass fuses to the rim of the button. The thin curtain of glass below the button is broken as Scott blows puffs of air up through holes in the chuck. This, along with the action of the flames, cuts off the curtain of glass.
Once again, the glass is cooled slowly under the flames.
THE BUTTON IS SEALED INTO THE NECK
Next, Bob Galanter applies a bead of VACSEAL to the glass-to-metal seals on both sides of the metal flange at the face of the CRT.
The CRT is placed on a rack in the exhaust cycle oven.
THE OVEN SETUP
The CRT is suspended in a holder in the oven above the exhaust port. The port is connected to the vacuum pumps below the floor of the oven. A mechanical pump draws outmost of the air from the CRT, while a diffusion pump achieves the very high vacuum required for CRTs to operate. The stem of the CRT fits down into the open hole of the exhaust port. A rubber bushing slides tightly onto the stem, with a knurled threaded cap above it. When screwed down onto the port, the cap compresses the rubber bushing makes a vacuum-tight seal with the glass stem. The whole port assembly is cooled by a water jacket. Above the port on the stem is a pinch-off coil. This is an electric heating element that is energized at the end of the exhaust cycle. Heat from the element softens the glass stem until air pressure causes the stem to collapse, pinching off the vacuum in the CRT. Note that the button wires are now flared out to allow the pinch-off action to be close to the button. Around the neck of he CRT is the so-called bomber, a RF induction coil used to heat the elements of the gun during the evacuation cycle to drive out more of the stray gas molecules.
The gas-fired oven is controlled by a simple cam and leaver system driving a sequence of relays to activate the various steps in the exhaust cycle.
OVEN CONTROL AND STATUS INDICATORS
Scott has a checklist he follows religiously for each position in the oven prior to initiating the exhaust cycle. One the cycle starts it must run to the end. Scott can override some actions in the cycle for each station in the oven, such as the vacuum pumps, bomber coil, or pinch-off coil operation, should there be a problem with a particular CRT while in the oven.
The arm rides along the spiral edge of the cam and controls the oven temperature. The cam rotates once in about three hours and slowly increases the oven temperature to approximately 650-degrees F. Scott then flips the cam over, and the oven begins to cool during the following three-hour period.
The exhaust cycle starts with activating the vacuum pumps and bringing the oven up to temperature. Then the bomber coil is activated, further heating the gun elements and improving the quality of the vacuum. As the oven cools, the pinch-off coil is activated, sealing off the vacuum in the CRT. The oven is left to cool completely overnight.
DAY 2: June 11, 2008
Scott took pity on us and told us we could sleep in until 4:00 a.m. the morning of the second day. We arrived full of anticipation. "Bad news" announced Scott as we entered the door. About an hour into the oven cycle, when the temperature had reached about 600 degrees F, Scott heard a whistle from the15GP22 station in the oven. He turned off the vacuum pumps, bomber coil, and pinch-off coil for that oven station and allowed the oven to complete the exhaust cycle. The 19VP22 and 12UP4 CRTs , which occupied two other stations in the oven, completed the exhaust cycle nominally.
We opened the now cool oven and examined the 15GP22. The failure occurred right where the stem joined the button. This was possibly caused by a shift inthe CRT position on its holder in the oven, or from a build up of lateral forces on the stem during the heating.
For the 19VP22 CRT, the next step was to flash the getters. The horseshoe shaped ring is an RF induction coil used to heat the getters, causing the oxygen scavenging material in the getter to boil off and deposit on the inside surface of the neck of the CRT. This helps to further improve the quality of the vacuum in the CRT.
FLASHING THE GETTERS
Next, high-voltage, pulsed-AC is applied to all the pins of the CRT while the anode is tied to the AC return. This is done to burn out any residual debris within the gun structure. Then the cathodes are activated by heating them using elevated voltage applied to the filaments. Last, a new base is soldered to the wires of the button and affixed with silicone RTV.
THE WAY FORWARD
The failure of the 15GP22 is both good and bad news. It is, of course, bad news that it did not make it through on the first attempt. But the stem failure is mostly good news, as this type of failure is uncorrelated to the fact that it is a 15GP22. This failure can likely be corrected by ordering stems with a stouter tubulation. Back in 2003 when Bruce Buchanan and I were first discussing this with Scott, he indicated to Bruce that a 0.25" O.D. tubulation was what he wanted on the stems. But this now seems to have been a miscommunication of some sort between Scott and Bruce. So new stems will have to be ordered. The good news, they are still available.
This photo illustrated the differences in the tubulation on our stem (right) compared to the now preferred tubulation (left).
The VACSEAL cured very nicely, and though this tube did not have a leak, this seems to offer some hope that VACSEAL may be useful in stopping leaks. But this will have to be proven when the day comes to attempt a rebuild of a leaker CRT.
In order to rebuild a 15GP22,you need a replacement gun. However, no new or new-old-stock guns for the 15GP22 are available. South West Vacuum rebuilt two 15GP22 guns for me by replacing the heaters and cathodes and attaching the rebuilt gun to the new button. But this service is not longer available from South West Vacuum. While attempting to seal in the first of these guns into the15GP22, Scott failed to achieve a proper seal between the button and the neck, so he cut the first gun out and used the second gun. This leaves us with no rebuilt guns.
After removing the failed CRT from the oven, the gun was removed and the CRT sealed with a resin tape to prevent moisture from getting in. It now awaits the next rebuilding attempt.
The two 15GP22 guns removed are undamaged, and the cathodes are still good. However, they are both now attached to damaged buttons which cannot be reused. In order to put a new button on, it is necessary to remove the heaters and cathodes and replace them with new ones, because the structure of the heater and cathode attachments to the button wires are too frail to survive any attempt at a button transplant.
Bob Galanter has taken on the task of attempting to rebuild guns for the 15GP22. Our research so far leads us to believe it is doable. In parallel, we are pursuing a solution from the French CRT company RACS.
But the clock is running….Scott will retire in about two years, so we would like to squeeze in as many rebuilds as possible.
Stay tuned for future developments!
Rebuilding the 15GP22: A Final Report
Late in February 2010, Bob Galanter and I traveled to Hawkeye Picture Tube Co. in Des Moines, IA to have another go at rebuilding 15GP22s. Bob and I each brought an under vacuum dud tube, and a known leaker tube. The plan was to rebuild the two tubes that were under vacuum, and have Scotty install dummy guns and stems on the two leakers, so we could take them to a Michigan University lab for leak testing.
Discussing this with Bob, and considering everything we have done to date, and the fact that we still have unresolved stem cracking issues, we agree that we can no longer justify the expense of additional attempts to rebuild tubes. Maybe in the future, if CRT rebuilding can be set up at the ETF museum, and the overhead cost of rebuild attempts becomes much lower, we may consider continuing the effort. But for now at least, I think we have done all that we can justify doing.