Keeping Vintage Technology Alive

Antique TV Radio

Variable Tuning Capacitor

What we see in the picture is a 35pf Variable Capacitor. It is labeled as the Fine Tuner. Notice that the knob selected is a good match with the other knobs on the front of the Television Set. Also notice the small screw in the knob. It kept coming loose and ended up slipping on the shaft. The way to correct the problem was to remove the knob and drill a small dimple in the shaft. At first the intention was to drill all the way through, however it made more sense to just drill a small crater for the set screw to go into. That worked really well. It never came loose again. Maybe a drop of Loc-Tite might be considered as well.

Here it is being mounted inside the Chassis. It is grounded through the mounting shaft attached to the chassis, however an additional ground lead was attached to assure a good solid ground connection.

Oscillator Coil

Notice the Oscillator Coil in the picture. It looks a bit tattered. It started out as 5-Turns 24 AWG Magnet Wire wound on top of a 1/2 inch form. You can use anything such as a Socket Wrench Extension or the socket itself, just as long as it measures 1/2 inch in diameter. While the wire loops are still on the form, it is possible to take some sandpaper and clean off a spot at the second turn. A wire is soldered at that location as a Tap. The reason the coil looks so deformed is because it was being stretched out more and more in order to get the proper Oscillator Frequency. You need approximately 88 MHz for Channel 3 to work. You might need to run the oscillator a little bit higher or a little bit lower depending upon the tuning of your IF’s. The Fine Tuner should give you plenty of range to dial it in. It’s the sum and difference of the Oscillator Frequency and the Carrier Frequency which needs to be calibrated to the IF’s and Sound Coils.

It’s nice having an Oscilloscope with a Frequency Counter. It’s so much easier than counting each graticule and multiplying by the Time Base and calculating the reciprocal. We see 86.32 MHz indicated, and it was tuned to 88 MHz with the Variable Tuning Capacitor. All we really need to concern ourselves with is channel 3. Unless you want to be adding more channels. Having just Channel 3 together with external video and audio jacks is probably good enough for most of us. It was relatively easy to make it work as a Three Channel Tuner.

Nylon Washers

Notice the Nylon Washers on the back of the Chassis. They are needed as High Voltage Insulators. They are available at any good Hardware Store. The metal of the chassis is drilled out to accommodate the width of the washers. On the other side of the Chassis, those three (3) Potentiometers are mounted on a small piece of Plexiglas. You cannot mount these three (3) Potentiometers directly on the chassis, because there is insufficient distance between the wiper of each Potentiometer and the outer case.

Within the Red Rectangle is the small piece of Plexiglas mounted to the inside of the chassis. And the three (3) Potentiometers are mounted on the Plexiglas and not directly on the metal portion of the chassis.

Don’t Make This Mistake

Don’t make the mistake of accidently drilling this to the wrong size. It can be really annoying patching it afterwards. That location gets a Local Tube Socket which is a little different from Octal. Making it the wrong size is an easy mistake to make, and very annoying to find out afterwards. The socket will be populated with Tube 7X7. The XXFM Detector. The other thing to keep in mind is to remember to point all the Tube Sockets in the same direction. The notch is pointing towards the front. The speaker is in front. So that’s the way the tubes should be pointing. You don’t want the Tubes pointing every which way. All the resistors and capacitors have leads long enough to reach that way, although a few of them have a long stretch to make. It might just barely reach in a couple of circumstances, however if one of the Tubes had been inadvertently turned the other way, in some cases the component leads would not have been long enough. The other Tube you need to be careful with is the 6J6 Tuner Oscillator Tube which has 7-Pins. If you look closely, you can see that it isn't pointing perfectly towards the front. The screw was being used as a reference, and it doesn't line up the same way on a 7-Pin Tube Socket as compared to an 8-Pin Octal Socket, which in that situation it had the screw in the center, and lined up with the positioning notch. On the 7-Pin Tube Socket, the mounting screw is not in the center of the gap, but rather instead, a little bit off to the side from center. They vary more or less from one brand to another. It doesn’t have to be perfect, but it does need to point approximately towards the front.

Getting The Chassis Made

Call 3 Sheet Metal Fabrication Shops on the phone and describe the Chassis as a box with the dimensions of 17 x 11 inches. The prices can vary drastically from one vendor to the next. Try to get the best price. My local Sheet Metal Shop was the cheapest. Take the picture shown and print it out on a sheet of Card Stock. Cut it out with a pair of Scissors and fold it on the lines to create a model of the Chassis. Take it with you to the Sheet Metal Shop and show them what you want. Make sure to mention the fact that you are showing them a smaller model of the box, and the actual dimensions are larger as indicated. A lot of these shops use galvanized sheet metal, and plain Sheet Metal is usually priced a lot higher. So if you get it made from galvanized sheet metal, than you will need to find some way to remove the galvanized coating.

Here is a scan of the original cutout made from cardstock that was taken to the Sheet Metal Shop.