Sentinel 416 Restoration Notes

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I bought the 1950 Sentinel 416 TV at a VRPS swap meet in 2002 for 5 dollars. At the time, I was only in the radio collecting hobby for about 3 years and the TV was a far more complex repair project compared to most tube radios I had encountered. I spent 13 years on and off trying to repair the TV and making small progress each time. The following pictures show the TV chassis, which was fairly clean and rust-free.

The tuner, IF amplifier stages, video and audio amplifier, and power transformer are located on the right side of the chassis.

The high voltage cage containing the flyback transformer, 1B3 HV rectifier tube, horiziontal output and damper tubes, and the vertical oscillator tubes are located on the left side of the chassis.

The chassis is fairly bulky and heavy when the CRT is mounted on it, but it is safe to turn it over to the side and use a plastic cup to prop up the CRT neck supports when doing repairs. I placed the chassis on top of the TV cabinet when I worked on it the first few years I had it. In the above photos when I worked on the TV most recently in 2015, the chassis was placed on the kitchen table since my workbench was not well suited for handling a chassis of this size. Since I did all the repairs with the CRT in place, it is important to note again that because the 16GP4 CRT has a bare metal conical shell connected to the high voltage, it poses some hazards when working on the TV. After turning off the TV, it is a good practice to always take a screwdriver jumpered to chassis ground and touch the metal CRT shell to discharge any residual charge stored in the CRT since it is essentially a large low-pF capacitor. While the TV is in operation, I avoid coming anywhere close to the CRT shell since it is energized to at least 15-20kV relative to chassis ground.

The TV originally had many problems with the power supplies including a history of fire in the high voltage section and bad B+ rails. All the electrolytic filter capacitors had to be replaced. One notable issue in particular was the main B+ filter capacitors were 475V rated and most new electrolytic capacitors tend to be rated 450V at the most. I had located 500V replacements from a radio swap meet, but after installing them, the 5U4 rectifier tube instantly put on a fireworks show when turning on the TV. It took me several weeks to realize that the filter capacitors I had put in were dead shorts even though they were correctly rated for the voltage and installed properly. Just because they were "new" from a radio swap meet doesn't mean they were good! I ended up having someone make me a custom filter capacitor replacement that worked.

Also, obviously all the paper capacitors had to be replaced. This process was fairly straight forward for the most part. The chassis undereath the high voltage cage was all charred black as a result of an exploded paper capacitor and a broken power resistor. At the time, this was a challenge for me to repair because in order to determine the original component values to replace them, I had to learn how to trace the connections in the chassis and correlate it to the schematic. Also several wires in the charred high voltage area had their insulation burned off and needed to be replaced. Many power resistors were replaced with new higher wattage ones and several other out of tolerance resistors were replaced.

The next major problem was the vertical positioning potentiometer was bad. It is a 20 ohm wire-wound pot with a center tap supplying power to the vertical deflection yoke. The slider portion of the pot would adjust the DC voltage on the other side of the deflection yoke that the vertical ramp oscillator drives. Unfortunatley, the pot went open from the main B+ input rail resulting in all the other rails not coming up. Several other TVs of similar design from this era tend to be afflicted by similar problems with failed potentiometers (not necessarily a vertical positioning pot) when used in the power supply section. Replacement potentiometers with a center tap are as rare as hens teeth to find nowadays, so I replaced it with a standard 40 ohm potentiometer and used two 20 ohm resistors connected in series and placed in parallel across the 40 ohm potentiometer. This produces an equivalent 20 ohm resistance as the original potentiometer did, but the center tap now comes from the series connection between the two 20 ohm resistors.

Repairing the vertical position control pot restored power to most of the B+ rails and the high voltage was functional once again. A quick way to verify the high voltage is functional is either by bringing a neon bulb near the transformer and see it glow without making contact or using a screwdriver to draw an arc off the plate of the 1B3 HV rectifier tube. However, the TV still refused to produce a picture. After fiddling with the controls and slightly tweaking the ion trap, I noticed a very dim raster visible in a dark room as shown in the following picture.

In 2005, I obtained a CRT tester to test the 16GP4. The tester indicated the 16GP4 was weak at 20% emissions, so I decided I could not make further progress without finding a replacement CRT. Now the search for the unobtainum 16GP4 CRT began. I had the opportunity to buy a replacement from a well-known TV collector at the time, but balked at the $75 price tag to purchase and ship the CRT for a TV that only cost me $5 at the time. I have regretted that decision not to buy the CRT at the time for literally a decade because 16GP4 CRTs seldom turn up and when they do, they find a home fairly quickly and often for a higher price. In 2015, I was fortunate to come across a good 16GP4 CRT tested with 80% emissions in the Early Television Foundation parts listing for a similar reasonable price that I had turned down 10 years earlier. After the CRT had arrived in the mail, I was curious to use the same CRT tester to compare the new tube to my existing one. To my surprise, the tester claimed the new tube was at 15% emissions and the one I had to begin with was at 20%. So in an interesting twist of events, after searching long and hard for a replacement 16GP4, I ended up with 2 good CRTs and a broken junk CRT tester! I decided to keep the second 16GP4 CRT as a spare considering how hard it was to find. The TV still did not produce a picture so I was back where I left off 10 years ago.

So the original issue I left off was the raster was very dim despite restoring the B+ rails and having functional high voltage. Since a decade had passed since I last looked at the chassis, I decided to reinspect my work and check voltages. All the B+ rails were functional but a little higher than the schematic called for. In my inspection, I uncovered an old mistake in forgetting to connect a wire to the vertical centering pot so the vertical oscillator wasn't properly receiving power. After a quick correction, the B+ rails all came back within expected range with the slight added load of the vertical oscillator. After most of the voltages seemed reasonable, there was no explanation for the dim raster. I started to wonder if the ion trap was misadjusted since I don't recall ever touching it in all the time I've had the TV. I just assumed it was in the correct position or was close. After sliding the ion trap magnet back and forth and rotating it 90 degrees clockwise or counter-clockwise and seeing no improvement, I decided to just rotate it 180 degrees and suddenly a bright raster finally appeared!

I would have not guessed that the ion trap would have been positioned completely upside-down, but obviously one should not assume. It would be a good interlude to take a moment and explain the purpose and function of a CRT ion trap. The following picture shows the ion trap on the 16GP4 CRT of the Sentinel 416 in the proper position. In the picture, the ion trap is a little magnet with 2 metal brackets covered with blue plastic clamping around the CRT neck.

All vacuum tubes including CRTs are never made with a perfect vacuum. As a consequence, impurities may be emitted from the cathode of the tube along with the electrons. Since these impurities are atoms or molecules emitted with a negative charge, they are called ions. The early prewar and postwar televisions used smaller CRTs utilizing electrostatic deflection so the electrons and ions were deflected together in a single beam. As CRT screen sizes grew, much higher deflection voltages would have been required so it was impractical to continue to use electrostatic deflection. It is much easier to just generate a constant high voltage and deflect the electron beam magnetically. The magnetic deflection introduced a new problem. While ions and electrons can be deflected together electrostatically, ions are heavier than electrons and deflect less in a magnetic field so early magnetically deflected CRTs with a straight electron gun would have the electron beam deflected to generate the picture, but the ions shot straight through to the center of the CRT screen causing permanent phosphor damage. This issue has been discussed in various patents such as US patent 2569517 from 1951 for the ion trap shown below.

I will use the (number) to refer to the annotations in the patent relative to my subsequent explanation. To solve the problem of blemishes on the screen caused by ions, CRT manufacturers would purposefully misalign the cathode of the electron gun (34, 35, 36) to point to the CRT neck so the undeflected electron and ion beam would be directed off screen (45). The ion trap (11) is basically a little magnet that redirects the electron beam back to the center path (37) so the magnetic deflection can move it around to produce the raster and picture. If the ion trap was upside-down or misaligned then the electron beam is too far off that it will never hit the screen producing no picture. The dim raster I saw with the upside-down ion trap was most likely caused by the few residual electrons that bounced back off the CRT neck.

CRTs made prior to the late 1950s just had a phosphor coating behind the glass screen so light generated by the phosphor excited by the electron beam would illuminate inside the CRT as well as on the screen so picture brightness was limited and inefficient since light going into the CRT was not visible and wasted. Newer CRTs made after the late 1950s have an aluminium coating behind the phosphor intended to reflect all light from the phosphor outward from the screen to significantly improve brightness. The ancillary benefit of the aluminized coating was that it protects the phosphor coating from ions so straight-gun CRT assemblies could be used again without an ion-trap. However, some TVs in the late 1950s may have an aluminized CRT with an ion-trap because the CRT still has an angled electron gun.

Now that I was able to get a picture on the screen of the Sentinel 416 for the first time in probably a few decades, there was clearly some more work left to do. The vertical and horizontal frequencies were still far off and the horizontal had extreme non-linearity causing a vertical bar in the middle of the screen. I used a TV test pattern generator to feed a test image and proceeded to adjust the sweep oscillator frequencies. The horizontal drive strength was adjusted to eliminate the vertical bar in the raster. The picture came through alright, but the contrast control was not working very well and the audio was not coming through well. After studying the schematic, I noticed that Sentinel designers decided to use the current flow through the audio amplifier circuit to generate a 125V rail to be used as a bias voltage for the IF and video amplifiers. It appears that some video AM noise from the IF and video amplifiers can bleed onto the 125V rail resulting in a soft sync buzz noise coming through the audio that seemed difficult to completely eliminate given the way the circuit was designed. The other downfall of this strange 125V rail design is that if the 6K6 audio output tube got weaker then the rail voltage would decline resulting in the contrast control losing effectiveness.

I pulled all the tubes and checked them on a tube tester. Nearly all of them were weak or had intermittent shorts so they were replaced. I followed the IF and audio alignment instructions for the Sentinel 416 to peak the coils. Most of the coils were already spot-on. The 6K6 audio output tube was replaced and the 125V rail came back up to within range. For comparison, the Admiral 19F1 TV circuit design supplied all rails directly from the power supply without generating them in weird ways like Sentinel did with the 125V rail so it was no wonder the Admiral was way easier to repair.

With a DTV tuner box hooked up to the TV, I was able to bring in stations nicely as shown in the following pictures. One annoying design aspect of this TV is that all the vertical and horiziontal controls are in the back so if the picture started flipping, it was a royal pain to find the control in the back to correct this.

So there you have it, the TV has been resurrected after well over a decade if not decades and works well. The cabinet was in good original condition and did not require any refinishing. This concludes the culumination of 13 years of my repairs.

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