Early Electronic Television
Cossor 137T Restoration
This 13 1/2 inch direct view set has arrived from England. The cabinet is in excellent condition. We begin the restoration by taking a number of pictures of the inside of the cabinet, showing the wiring and chassis location. As the chassis are removed, more pictures are taken.
The Cabinet. (pictures). The cabinet has only minor flaws. We will take it to Old World Furniture Refinishing to have it touched up. New grille cloth has been ordered and has been installed.
The Chassis. There are three chassis in this set. The Power Unit Chassis contains two power supplies for the circuitry, one power supply for the CRT, and the audio output amplifier. It has the speaker mounted on it. The Radio Receiver chassis contains the RF amplifier, local oscillator, mixer, IF amplifiers, and detector for the video and the audio IF amplifier and detector for the TV sound and radio. The final chassis, called the Vision Unit, contains horizontal (line) and vertical (frame) sweep circuits, a video amplifier, and a sync separator. All of the chassis have now been rebuilt.
Power Unit Chassis. (pictures).The power unit chassis is in excellent condition. We will rebuild each of the electrolytic capacitors, then test it. The electrolytics have been rebuilt, and AC power applied. The transformer for the +240 supply is defective, and has been removed to be rewound. The other supplies, for +1500v and -5000v, work fine. Today I connected the CRT, and got a nice, bright spot, indicating that the tube is good. There is an occasional arc in the high voltage transformer which I hope can be corrected with corona dope. Later I will connect the Vision Unit to an external 200v supply and see if I can get a raster.
The 240 volt power transformer cannot be rewound. The iron laminates are too badly rusted. Fortunately, the rewinding company can make a new one which is almost exactly the same size, and will look very similar. I now have the new power transformer, and it does look very much like the original one.
Bad luck today. The high voltage (EHT) transformer was arcing. I made an attempt to drive the accumulated moisture out of it by baking it in an oven at 200 degrees for a day. At first, it worked. Later, it started arcing again. And, while trying to track down the cause, the filament in the CRT went dead. I later discovered that the arcing was in the winding which provided the filament voltage. In this set, the cathode (and filament) are operated at -5000 volts relative to the last anode and deflection plates. I suspect that the arcing caused the 5000 volt potential to be across the filament, burning it out. Today I removed the 10 turn filament winding from the transformer, and found that it was arcing to the primary. I replaced the windings and now the transformer works perfectly.
This CRT is extremely rare, and finding a replacement is impossible. A TV collector who contributes to Pete Deksnis's site (see my links) has been working on getting the 15 inch color CRT used in the first color sets rebuilt. In the process, he has made contacts with people who may be able to rebuild this CRT, and other Pre-1945 tubes. There is another Cossor set, with a bad CRT, in the U.S. I will explore getting these tubes rebuilt. Here is information on the rebuilding process.
After talking to several CRT rebuilders, I have determined that it is too risky to try to rebuild the tube. There is another alternative I am exploring. A few World War Two radar tubes, the CV1085, are available in England. This tube is slightly smaller, but has about the same picture area. It is electrically similar, and can be made to work in the set. The only problem is that it has a P7 phosphor. TV CRTs have a P4 phosphor, which is white and retains its image only for a short time. The P7 is a dual phosphor - a blue phosphor similar to the P4, and a yellow phosphor which retains its image for a long time.
Using this tube, scenes with motion in them will have a yellow trailing image. I think that it is possible to put a blue filter in front of the tube, and block the light from the yellow phosphor. Of course, the picture would then be black and blue rather than black and white.
The CV-1085 CRTs I ordered have arrived. After studying the pin connections for that tube and the voltages it requires, I realized that the Cossor circuit would have to be modified to work with the CV-1085. After some experimentation, I determined how to modify the set (very minor changes), and connected the new CRT.
The bases of the two tubes are different. For now, I have soldered wires onto the Cossor socket to the appropriate pins on the CV-1085 base. Eventually, I'll make an adaptor that will retain the original appearance.
The CV-1085 is about an inch smaller in diameter, but is large enough that the face fills the mask. The tube will have to be raised above the chassis by about 1/2 inch using wooden spacers to make it fit in the cabinet.
I now have a fairly bright raster, and some semblance of a picture on the screen. After rebuilding the Radio Receiver Chassis, I connected the three chassis together and attached a TV signal.
Interfacing the CV-1085 turned out to be more complicated than I originally thought. Several circuit changes were required to get proper operation of the brightness and focus controls, and to get the proper modulation of the tube. I now have a reasonably good picture on the screen, though both the horizontal (line) and vertical (frame) linearity are poor, and the resolution is not very good.
I attached a VCR to the set and played a tape. To my surprise, the image had no trailing yellow image. The CV-1085 CRT appears to have a P4 (white) phosphor, not a P7 phosphor. I tried a second CV-1085 today, and it has a yellow trailing image. I have no idea why the two tubes are different. My other two tubes also have the yellow image.
The tube had an interesting problem when I finally got the horizontal (line) and vertical (frame) sweep right. The width was fine, but the height was insufficient. No matter how much voltage I applied to the Vertical deflection plates, the picture would get no bigger. I reversed the Vertical and Horizontal plates, and the height became fine, but the width was inadequate, so the problem was in the tube itself. I discovered that placing two small magnets on the neck of the tube, near the plates, allowed the beam to sweep to the full height of the tube.
I now have a decent, but dim picture, with fairly good resolution. Here is what John Wakely has to say about using radar tubes in TV sets:
Very many years ago when I was around 13 I built a t.v. using a 5FP7 crt. That had a dual phosphor that produced a brilliant picture on the back but gave a very ' muddy ' result on its face. I think it may be the case with your Cossor tube. The screens are designed for radar use where the afterglow is important and the definition is not required, in fact discouraged ! As regards your ' cut off ' problem, this was very common with radar tubes and the ones sold to home constructors after the war were tested as ' full screen '. Courtesy of John Wakely
Radio Receiver Chassis. (pictures). This chassis is also in excellent condition. We will replace all the paper capacitors. After rebuilding all the paper and electrolytic capacitors, the video portion of the chassis worked perfectly. Next step is to test the TV audio and radio portions.
I connected the speaker and tried the AM radio. The sound was excellent. Next I tried the TV sound. It is not as good, and very hard to tune in with the fine tuning control. I have asked another collector with a working Cossor if this condition is normal. Apparently, the tricky tuning is normal.
Vision Unit Chassis. (pictures). This chassis is also in excellent condition. We will replace all the paper capacitors, then connect it to the power unit chassis and attempt to get a raster. All the capacitor have been restored, and several pieces of rotted rubber wire were replaced.