Pilot Model TV-37 "Candid" Television (1948)

           

           

The Pilot TV-37 "Candid" is an unusual vintage television. With a tiny 3-inch picture tube, it is barely watchable by modern standards, but the TV-37's compact size and unusual design make it a favorite of collectors.

Pilot introduced the TV-37 in 1948, at the then-amazingly low price of $99. Most 1940s TVs sold for hundreds of dollars, the cost of a decent automobile. Pilot made this set for only a few years, but sold a lot of them during that time. Many TV-37s still survive.

Pilot marketed this set especially to college students, and it would have been a lucky student who got such a gift. The owner's daughter recalled neighbors crowding into their living room in 1948 for a look at this marvelous new invention.

Description

I bought this TV-37 from the daughter of the original owner. It had been well cared for and the owner partly restored it during the 1960s, leaving detailed notes.

The TV is housed in a streamlined cabinet with a wooden base and an upper case made of pressboard. The entire TV is only 14 inches wide, 13 inches deep, and 9.5 inches high, the size of a large table radio. Most of the upper case is perforated for ventilation.

The front controls are simple: power/volume, brightness, tuning, and contrast. The tuner control includes a high/low switch, which lets you select two tuning ranges. The low range is channels 2-6 and the high range is channels 7-13.

The knobs have Pilot's classic ship's wheel design. If your TV-37 is missing a knob or two, reproductions are available.

My TV-37 came with the original owner's manual:

   Pages 2-3
   Pages 4-5
   Pages 6-7
   Pages 8-9
   Pages 10-11
   Pages 12-13
   Pages 14-15
  

The next photo gives you an idea of the screen size. That's a half dollar coin!

My Pilot came with a magnifying lens to place in front of the picture tube. The second photo is animated to demonstrate the magnifier's effect.

  

Magnifiers were an extra-cost accessory, as was a pressboard carrying case with built-in antenna. Magnifiers are harder to find the TVs themselves, since not everyone bought one and some were lost over the years. Magnifiers typically hooked onto the horizontal grill bars; these photos show that sort of lens.

  

My magnifier stands on little metal legs that slip under the cabinet. The owner's notes include a pencil mechanical drawing with dimensions for the magnifier and legs. Evidently, he built the legs and painted them to match the trim on the magnifier. This is an improvement over the original scheme, since the heavy magnifier tended to warp or break the thin grill bars.

On the bottom of the wooden base is a label showing the tube layout. The TV's serial number (204462) is written there in yellow crayon and also stamped in the rear of the aluminum chassis.

  

The next photo shows the TV-37 with its hat off:

The TV-37's electronic design is similar to other electrostatic-deflection TVs of that era, such as my 7-inchers, the Admiral 19A12, National TV-7W, Motorola VT-73, and Hallicrafters 505.

The television uses 21 tubes, including a 3-inch electrostatic-deflection picture tube.

Tube Type Function
V1 12AT7 RF amplifier
V2 12AT7 Mixer
V3 12AT7 Oscillator
V4 6AU6 1st IF amplifier
V5 6AU6 2nd IF amplifier
V6 6AU6 3rd IF amplifier
V7 6AU6 4th IF amplifier
V8 6AU6 Video amplifier
V9 6AU6 DC rest./Sync. separator
V10 6AU6 Sound IF amplifier
V11 6AL5 Ratio detector
V12 35B5 Audio amplifier
V13 12SN7GT Vertical multiplier
V14 12SN7GT Vertical amplifier
V15 12SN7GT Horizontal multiplier
V16 12SN7GT Horizontal amplifier
V17 25L6GT High voltage oscillator
V18 1B3GT High voltage rectifier
V19 35W4 Negative rectifier
V20 25Z6GT Positive rectifier
V21 3KP4 Picture tube

The TV-37 has DC restoration but it lacks AGC (automatic gain control). Its Contrast control could more properly called a manual gain control, since it varies the gain of the RF and IF amplifiers. As with other TVs using manual gain control, such as my RCA 630TS, you'll need to adjust this when switching from one source to another (or sometimes, from one program to another).

Unlike most TV tuners, the Pilot's is continuously variable. Instead of clicking from one station to the next, it turns smoothly like a radio tuner. This was a cost-saving measure; the Pilot tuner contains far fewer parts than other TV tuners. Its construction is similar to a radio tuner, using air variable capacitors, pulleys and strings. (Don't confuse this type of tuner with the robust—and vastly more expensive—continuous tuners used in my DuMont RA-103 and RA-113 televisions).

Another cost-saving (and weight-saving) design tactic was to eliminate the expensive and heavy power transformer. This is a transformerless "series string" TV, in which the tube filaments are connected in series. As in series-string radios, the voltages of the tubes add up to the voltage of the AC power supply (around 120 volts). Given the number of tubes, it actually uses two parallel tube strings, a design repeated in some other TVs such as my Philco Predicta.

Here are the Riders and Sams service manuals for the TV-37. To download one to your computer, right-click on the icon and then choose Save Target As.

  

Pilot produced early (TV-37) and late (TV-37U) versions of this television, with some electronic changes as well as slight cosmetic differences. In this forum discussion you'll find a summary of the changes.

The 3KP4 Picture Tube

As far as I know, the Pilot was the only consumer television to use the three-inch 3KP4 picture tube. Even early kit TVs of the 1940s used larger tubes. The Pilot's 3KP4 is also notorious for burning out because it (along with the other tubes) is subjected to a stiff power surge when you turn the set on. As a result, 3KP4s are scarce and costly.

One cure for the surge problem is to disconnect the picture tube's filament from the filament string and supply it independently from a small 6.3-volt transformer (the Triad F-13X is suitable). When this is done, you should also install a 12 ohm power resistor at the picture tube's former position in the filament string.

Another remedy is to install a type CL-90 thermistor on the AC line between the power switch and the filament string. The thermistor presents a higher resistance when cold, which decreases to almost nothing after it warms up. The "soft start" moderates the surge. Thermistors were a standard feature in my Philco Predicta and other later TVs.

Some Pilot owners substitute the more common 3KP1 picture tube if their original 3KP4 is burned out, or simply for testing purposes during restoration. The 3KP1 is electrically interchangeable but it was designed for 1940s oscilloscopes with green screens.

The 3KP1 is not an ideal substitute. A green phosphor tube is darker than a white one, making normal viewing a challenge. The slow-response green phosphor also creates trails on fast-moving objects.

Since a 3KP4 cannot be tested on any tube tester, I couldn't tell at first whether mine was good. I tested the filament with an ohmmeter and found that it had continuity, so I knew it wasn't a total dud, at least. It still might have poor emission and thus be too dim to watch, however. Only restoring the TV will tell me whether it really works.

Restoration

The cabinet required no cosmetic restoration except for touching up a couple of tiny paint nicks on the base. Like all TVs of this vintage, the chassis will need electronic restoration to be made reliable.

This photo shows the underside of the chassis as found:

The previous owner had done some restoration, replacing about ten paper capacitors and a few resistors. I'd guess the work was done in the 1960s, judging by the type of components used and the fact that the owner retired in 1965. Most of the replacements were epoxy-dipped caps, usually far more reliable than paper, so I decided to leave them in place until I replaced the others and judged how the TV works.

The chassis still contained all of the original electrolytics, plus about a dozen paper capacitors, including the critical high-voltage caps. Those will be replaced by me.

Before replacing anything, I tested all of the tubes and cleaned their pins. I also cleaned and lubricated the tuner and all of the control potentiometers (volume, brightness, etc.). The tuner and two potentiometers were completely stuck on their shafts, but I freed them with solvent followed by electronic cleaner and lubricant.

Capacitors, Capacitors

After receiving some new parts in the mail, I began replacing capacitors. The new ones are smaller than the originals, making it easier to fit them into tight spaces.

Notice the tubular film capacitor rated for 6000 volts. I prefer to replace high voltage caps with this type rather than ceramic discs, which may be unstable. You can get tubular HV-rated caps from Just Radios.

I replaced all of the electrolytic capacitors, including a couple that looked like 1960s replacements. On some TV-37s, capacitor C2 was mounted underneath the chassis. On mine, it was mounted in a can atop the chassis. There's plenty of room underneath, so I mounted the replacements down there and left the old cans in place for appearance.

Note that two of the power-supply electrolytics, C2 and C3, are installed with their positive leads connected to the chassis ground. This is different than what you'll see in many radios and TVs. Don't install those two backwards—your TV won't work!

Don't forget the capacitor inside the high voltage cage.

Pay attention to the little copper collar on the midsection of the 1B3GT tube. It is important for correct high voltage output. Before removing the tube for testing, I marked its position with a Sharpie and took care to slide it back into the right place. You can read about the purpose of this connection in my Motorola VT-71 article.

First Power-Up

With new electrolytics and high-voltage caps, it was time to try firing up this TV. I connected it to a variac and slowly brought up the voltage, looking and listening for any signs of trouble.

The tube filaments began to glow, but there was no sign of a raster on the CRT. I could hear a putt-putt sound from the speaker, which became faster as I increased the voltage, but no real audio.

Time for some voltage checks! I quickly determined that the B+ voltage was much lower than specified in the schematic.

I re-tested the two rectifier tubes (25Z6GT and 35W4) and re-cleaned their pins and sockets, just in case of a bad connection. The B+ voltage was still too low. Acting on a tip from a fellow TV-37 owner, I connected a modern silicon diode across the 25Z6 tube, between pins 3/5 and 4/8, with the positive band of the diode pointing to the positive lead of electrolytic C1A.

That brought the B+ voltage up to a normal level. On the next power-up, I was delighted to hear excellent audio from the TV on its upper channel range, indicating that the TV's tuner and audio sections were basically functional. Not so good on the lower channel range, but we could defer that investigation until later.

High Voltage Testing

The absence of a raster suggested that we still had problems in the high-voltage power supply—often the trickiest section to restore in any vintage TV.

The data sheet for the 3KP4 picture tube indicates that you need a minimum of 1000 volts and a maximum of 2500 volts.

You can't test high voltage with an ordinary multimeter. I have two probes suitable for this. The one on the bottom of the photo is my favorite, a Pomona 2900A. It operates all on its own. The probe on the top is an RCA WG-284, which can be used with my old RCA Senior Volt-Ohmyst tester.

Testing indicated only about 800 volts, where you would expect to measure at least 1 kilovolt. That explains the lack of a picture!

Resistors, Resistors

The high voltage section of an electrostatic-deflection TV typically contains several resistors with values over 1 megohm, and the Pilot is no exception, using values as high as 4.7 megohms.

Old carbon composition resistors often drift upward, and several of this television's high-value resistors had drifted badly, sometimes to as much as twice their original value. I replaced several of the worst offenders, but the high-voltage level was still deficient.

A Light Bulb Goes Off

Then I remembered that a few of the 1960s replacement capacitors were not "maroon drops." Three of them were white, apparently cased in ceramic, and the fourth was a large yellow Mallory "Plascap."

I hadn't run across either type before, but I dimly remembered something about ceramic tubular caps being paper capacitors in disguise—and potentially as troublesome. I sent a brief query to a vintage TV forum and got a consensus that both types should be replaced.

Here are some of the 1960s culprits and their modern replacements.

First Picture!

After replacing those caps, the next power-up gave the first pictures I had seen from this television!

  

Now, this picture isn't perfect, but it's always exciting to see an old baby like this come back to life. Best news of all, the image indicated that my rare CRT was not a total dud.

The picture was very dim, visible only in a dark room, and the width and height were excessive. When the TV is inside the cabinet, the image size will need to be reduced to fit inside the mask.

The excessive height accentuated the horizontal scan lines, and the vertical and horizontal centering were also a bit off, but these are all things that you can adjust by twiddling controls.

These are photos of live broadcasts, received with a 1950s rabbit ear antenna. We live in an area where all broadcast signals are pretty weak, so this was an encouraging sign, showing that the TV has decent sensitivity. (2012 note: this part of the restoration was done before the 2009 changeover from analog to digital TV broadcasts.)

Another photo shows fair detail, so the tuner and IF sections are passing a reasonably clean signal. You can read "2nd & 10" in the little scoreboard near the upper right:

(The actual picture is sharper than in the photo; my camera has trouble focusing on CRTs.)

If you view the previous photo on a normal size computer screen, it's roughly double the size of the TV-37's picture. Okay, so the Pilot's not ideal for watching sports events, but other kinds of programming can still be fun to view.

Restoration, Round Two

That's where the restoration remained for a few years. I got distracted by new projects and the poor little Pilot languished on a shelf.

In 2012, I brought the TV-37 back into my workshop to complete the job. Although I had coaxed a picture out of it before, the image was dreadfully dim and I hadn't really finished the restoration.

The first trial was discouraging, with no hint of a screen image, although the audio was still working. A quick check revealed that high voltage was completely absent.

Here's an obvious troublemaker. This resistor tested open on my ohmmeter, and when I grasped one end with a pliers to unsolder it, the two halves fell apart.

Replacing that resistor brought a faint glimmer to the screen, but the high voltage was still too low and no video signal was apparent.

I began methodically checking resistors, starting with those in the horizontal and high voltage circuits. Many were far off tolerance, and so out they went. These photos show the replacement of three resistors on the 26L6GT horizontal oscillator tube.

  

Tedious work, but soon I could get a coherent image on the screen again. The next photo shows a crosshatch test pattern. The picture was rather unstable, however, with poor brightness and contrast.

Although the screen looks fairly bright in the previous photo, that shot was taken in a dimly lit room with faint backlighting. The camera's auto-correction made everything, including the screen pattern, brighter than in life.

Working through the high voltage, sweep, and video sections, I found more and more bad resistors. As I replaced them, the picture gradually stabilized and improved in brightness and contrast. Here are some photos taken during the process.

     

In the first photo, the brightness has improved but the contrast is deficient, leaving the image flat and lifeless. By the time we get to the third photo, we have a fuller dynamic range.

Final Thoughts

I put the chassis back into the cabinet and made the final adjustments for screen height, width, and so on. Then I bench tested the TV for a few hours. It performed well and looked stable, so with that, I declared victory. On to the next project!

Here's the restored TV-37 from all angles:

           

To celebrate my Pilot's return to the living, let's take another look at the animated .GIF showing it with and without the magnifier:

I didn't install a filament transformer or thermistor in this TV, after all. It won't get played often, so I'll just give it a slow power-up on the variac when I show it to visitors.

©1995-2014 Philip I. Nelson, all rights reserved