Pilot TV-37 Rebuild Project
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The chassis was missing all the tubes, except the rare 3KP4 CRT! I paid an extra five dollars at the swap meet to obtain about half of the tubes I needed for the Pilot TV. The chassis was a little dirty and beat up, but I was willing to take up the challenge of bringing the chassis back to life. Below is a picture of the Pilot TV-37 on my workbench with the 3KP4 CRT removed before cleaning and repairs.
Once I got the Pilot home, I inspected the chassis closely, and then looked on the Internet for pictures of complete Pilots. Mine was missing the high voltage cage, a small shield that went behind the field coil speaker, and obviously the audio output transformer. I had to figure out what to do about the field-coil speaker because it was missing the cone and was not easy to repair. The brown-black panel on the front of the TV that holds the CRT and speaker in place was broken at the upper right. I made an identical front panel using plexiglass to replace the broken one. Below is a picture of the new plexiglass front panel below the original front panel.
I replaced the field coil speaker with an 8-ohm permanent magnet speaker. A tiny audio output transformer salvaged from a parts radio was mounted under the speaker. Below is a picture of where the audio transformer was placed. At the time the picture was taken, I had the filter capacitor removed for restuffing.
The rusty part of the tuner was repainted with hammered gray paint. Since I was working with only a chassis, I had the freedom of modifying the Pilot a little. The original Pilot TV-37 basically had two knobs on the band switch shaft; one for the variable tuner and the other to switch between low (channel 2 to around 6) and high (around channel 6 to 13) bands. I did not want to bother searching for the original knobs for a TV chassis that was missing the original cabinet. So I mounted another shaft that was salvaged from a potentiometer on the middle of the chassis and strung the tuner to the shaft using fishing line.
Once the chassis was cleaned up, I proceeded to the most tedious task in the entire Pilot TV-37 rebuild project, replacing all of the capacitors. All the paper capacitors were replaced with their modern equivalents. The high voltage capacitors used in the vertical coupling are 0.005uF 6kV paper capacitors, and I replaced them with 0.0047uF 10kV film capacitors that I salvaged from a 1970s Tektronix display board. There were two other high voltage capacitors of the same capacitance used for the high voltage filter, but rated at 3kV. I replaced those with two more of the 10kV capacitors from the Tektronix display board. The last high voltage paper capacitor, a 0.001uF 4kV capacitor that also serves as a high voltage filter, can be found on top of the chassis next to the 1B3 high voltage rectifier. I could not find any 0.001uF paper capacitors of that voltage rating on any of my Tektronix display boards, so I used another 0.0047uF film capacitor rated at 8kV. Below is a picture of the Pilot chassis at this point, all clean and shiny.
After reading other repair notes on the Pilot TV-37 at the antiqueradios.com forum, I learned of one potential problem with the series filament string. This issue is a result of varying filament resistances even in identical tubes. Some tubes have a lower cold resistance than others. When the TV is powered up, there is an inrush surge and some tubes will warm up faster than others then for a few seconds suddenly face a lot more voltage than their datasheet ratings. This momentary stress can cause some tubes can burn out, and perhaps the reason the 3KP4 CRT is so rare is because it burned out often. Pilot TV-37 restorers have various approaches to this problem. One approach is to use some sort of surge limiter or a globar resistor. The globar resistor is a resistor that has a higher resistance when cold, then the resistance drops when it heats up. Another approach is to take the CRT filament out of the series string and put it on a 6-volt transformer. Some restorers use a combination of these methods.
I found a small 6-volt transformer and placed it in a vacant space in the chassis between the 25L6 high voltage oscillator socket and the 1B3 high voltage rectifier socket. The transformer was a precise fit for the space. I disconnected the filament leads from the CRT socket and connected it to the transformer. Additionally, I put a 10 ohm 10 watt resistor in place of the CRT filament in the series string. I found some globars and experimented with them, but eventually I settled on using the transformer for the CRT. The rest of the tubes are easier to replace than the CRT if they burn out.
After replacing most of the paper capacitors in the TV, I had to tackle some more serious problems. The B+ power supply section and audio output section both were tampered with. Some wires in the power supply were left floating. I had to ensure they were the B+ leads by tracing the connections and comparing them against the schematic. Also the field coil of the speaker was used as the choke for the power supply. The resistance of the field coil is 150 ohms according to the photofacts, so I put in a 160 ohm 15 watt resistor.
After reconnecting the floating wires and replacing the filter capacitors, I took a look at the audio output section. The audio output section may have been modified for a different audio output tube than the 35B5, or there were possibly some late-version changes made by Pilot that were not documented in the photofacts. What was obviously incorrect was that if I used a 35B5, the plate and cathode would be connected, which is not a good thing. I redid the connections to use with the 35B5 tube. The primary of the new audio transformer was connected to pin 5 and 6 of the 35B5. I also put in a missing wire that supplies the B+ to pin 6 of the 35B5, otherwise there would be no sound at all. Pin 2 of the 35B5 was connected to one side on the audio output transformer secondary, and the other side of the secondary was connected to chassis ground.
After I obtained the remaining tubes for about $15, I decided to try out the TV for the first time. The filament in the 3KP4 had continuity, so I was eager to see some results after replacing all of the paper capacitors and electrolytics in the Pilot. I plugged it in an isolation transformer and powered it up slowly with the variac. All the tubes glowed, no smoke, but no picture either. I brought a neon bulb near the high voltage transformer and it glowed, so the high voltage supply was working fine. However, I noticed the 3KP4 did not seem to glow and there was internal arcing in the CRT. My worst fear was realized. I shut off the power and took a closer look at the 3KP4. The getter was grayish-white when it should be shiny and silvery, an indicator of a loss of vacuum. The internal arcing indicates that there is air in the CRT, rendering it useless. I wondered why the filament did not burn out with the air present in the CRT, so I hooked up the filament to my DC bench power supply, and cranked up the voltage to 6-volts. I scrutinized the innards, searching for some glow, then, as I spotted a faint reddish orange glow, the meter on the bench power supply suddenly spiked. The CRT filament finally burned out due to the air inside the CRT. I asked myself whether I could have saved the rare CRT by having it rebuilt, but later concluded that the CRT was pretty much beyond hope due to the internal damages caused by the loss of vacuum.
A search for a 3KP1, the green phosphor equivalent of the white phosphor 3KP4, was unsuccessful. However, I had a spare 3RP1 CRT, so I went to the antiqueradios.com forum and asked if a 3RP1 could work. I was told that it is a potentially good candidate. After pulling up the datasheets on the 3RP1 and the 3KP4, I learned that all of the specifications were practically identical except for the deflection voltages. The reason for the different deflection voltage is because the 3KP4 is roughly 2 inches longer than the 3RP1 in length, so the 3KP4 requires less deflection voltage than the 3RP1 because it has a smaller deflection angle. The most I could get was a smaller picture on the 3RP1 because the TV was designed for a 3KP4. I used jumper wires and hooked up the 3RP1 to the 11-pin socket on the TV and fired it up again. The CRT glowed, and I got a small half-inch line in the middle of the screen.
When I measured the voltage on the B+ supply to check if it met the specifications, it was way too low. The photofacts specifies a B+ voltage of approximately 120 to 125 volts DC, but I read about 70 volts. Next I replaced the rectifier with a NIB 25Z6 rectifier tube, but the voltage only increased to about 80 to 90 volts. I searched through the TV to see if there was some kind of heavy and undesired load, but found nothing obvious. I checked the DC resistance of the audio output transformer primary, and it was about 150 ohms. The DC resistance of the original audio transformer was 330 ohms. I placed a 200-ohm resistor between the audio output transformer primary and pin 6 of the 35B5. The B+ rose to about 100 volts, but was still not enough. I placed a diode across the rectifier tube to provide some aid for the tube and the B+ rose to 140 volts, but when the Pilot fully warmed up, the voltage dropped down to about 125 volts so that problem was solved. Finally I got a full-size picture on the 3RP1, but there was a lot of noise and interference as you can see in the picture below:
I decided to manually inject the video by feeding video into the TV at point A in the schematic from a video camera to see if perhaps the raster was just sloppy. I got a picture of myself below, but there was obviously terrible interference in the picture.
Several resistors in the TV had drifted off value, so I decided to replace every single resistor in the TV to minimize the possible problems. All of the resistors were replaced as well as all of the 0.0015uF capacitors throughout the I.F. stages. These capacitors appear to be similar to ceramics, but I decided to replace them with .0011uF film capacitors. All of the true ceramic disc capacitors were left alone. There are more resistors in the Pilot TV than capacitors, and the color codes on many resistors were unreliable because the color of the stripes changed over the years. Some of the worst color changes were when the yellow changed to light green, and purple changed to gray. I had to follow the schematic to double-check the value of the resistor that was being replaced.
The interference problem in the picture still remained. I thought maybe the jumper wires from the socket to the CRT could be picking up undesired interference, so a better and more permanent solution was necessary. Below is a picture of the setup I had with the jumper wires from the TV to the 3RP1.
The base from the dead 3KP4 was removed by heating it up with a heat gun to weaken the bond between the socket and the CRT. I used a 12-pin socket that was typically used for 1950s TVs as the 3RP1 socket. However, the 12-pin socket only has 5 or 6 pins, so more socket pins were obtained from octal tube sockets. The pins of the 3RP1 were wired to the pins of the 3KP4 with the same elements, i.e. 3KP4 screen grid on pin 7 was connected to 3RP1 screen grid on pin 8. I glued the socket and base together using epoxy and made a nice adapter for the original 11-pin socket to function with the 3RP1’s 12-pin socket. Below are pictures of the process of building the adapter:
The interference problem still remained. I pondered the problem a short time, and then it came to me that a high voltage cage of some sort was needed. I placed a piece of sheet metal around the high voltage generator section, and the interference problem literally vanished! The high voltage generator is located right next to the vertical and horizontal sweep tubes, and the oscillations from the generator caused the mess in the picture. The sheet metal was mounted between the sweep tubes and the HV generator, shown in the picture below:
Now the TV was complete, and the picture was crystal clear with the video injected in the TV from a video camera. I hooked up an antenna and attempted to tune in some stations, but with no success. There was almost no snow or anything. I thought, perhaps the alignments were a little off since a few of the I.F. coils appeared to have been tampered with. I fiddled with some coils, and noticed the video got stronger. I finally got some stations coming in, but without audio. Then after fiddling with the alignments, I got sound but on the FM radio band. I went through the alignment procedures in the photofacts over 10 times without any success, and there were strange problems with the antenna input. I also tried using a sweep generator, but got nowhere.
The Pilot was taken to the July 2006 VRPS repair session and I worked with two guys. We went through the alignment procedures for the I.F. strip and the audio, using a voltmeter and maximizing the voltage on each I.F. coil at a specific frequency as shown in the alignment sheet. Eventually we got a very faint baseball game on the screen, and the audio seemed to match. One of the VRPS members, Cleo, offered to take the Pilot home to troubleshoot some more.
A week later, I received word from Cleo that the Pilot was working! After reading his notes, it seemed to me the entire problem was mainly due to one small grounding nut that connects the antenna input coils to chassis ground. The secondary of the antenna coil connects to ground and to the R.F. amplifiers in the tuner, so without a good ground connection there was nearly no reception at all. This explained why there were strange problems with reception at first, which led me to fiddle around with the I.F. coils. One big lesson I learned here is to never touch the I.F. coils in any TV because they are usually aligned just fine in the first place. However, my Pilot probably would have needed some aligning because one coil broke and some others appeared to have been turned from the looks of the painted dots - the factory paints a dot on the screw to indicate the aligned position. Below is the finished Pilot TV-37 chassis:
I built a plexiglass case for the Pilot. First I cut out a base for the chassis to mount onto, using a quarter-inch thick plexiglass and a table saw to cut the straight lines. Then I bought a large sheet of quarter-inch thick plexiglass and cut it in one long strip that was the same width as the base. Next I looked at the original Pilot TV-37 cases to determine the positions where I would bend the plexiglass. The plexiglass was clamped down to the table and the bend area was heated for about 3 minutes using a heat gun. After the plexiglass became soft enough to bend, it was bent to the desired angle, then cooled for another 5 minutes. I also drilled out the holes for the front controls so I could determine the position to bend the plexiglass at the rear. Below are pictures of the long strip of plexiglass as it went through several stages of bending:
A Dremel was used to make the cutouts for the speaker and for ventilation. The quarter-inch thick plexiglass can endure a lot of heat so there is no worry of warping or buckling because the plexiglass had to be heated at an extremely high temperature to bend - a temperature that the tubes couldn’t possibly achieve. Below is a picture of the plexiglass with the cutouts:
The main piece of plexiglass with the cutouts was connected to the base via four steel angle brackets. The sides are made with thinner plexiglass about an eighth-inch thick. I simply traced the shape of the side and cut it out with a band saw. I drilled small holes through the sides to the main quarter-inch plexiglass case. The holes are small enough to fit in the thin side of the quarter-inch plexiglass, and I used small metal screws to attach the sides to the main case. Below is the finished Pilot TV-37 in its plexiglass case:
Below are some pictures of the screen of the Pilot TV-37 in operation using a 3RP1 CRT. With the horizontal size turned all the way up, and the vertical turned up a little, the Pilot was able to generate more deflection voltage to fill the face of the 3RP1. The first picture at the left is a basketball game.
This Pilot TV-37 rebuild was an interesting experience, often frustrating but enlightening in the end. The plexiglass material cost me more money than I spent on the entire Pilot TV-37 rebuild, but the results came out very nicely.
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