Build your own TV

Installment Five

Materials you will need for this installment:

IR LED and Sensor. (See Text)

Misc metals, wood, RTV, cemets, materials, (See Text)

Four Potentiometers, (See Text)

One of the special parts you need to make, is the sync pickup that detects the circle of holes located about one inch in from the edge of the scanning disk. This is accomplished with an "optical fork" that you will build. It will generate pules which the motor control circuits need to control the motor speed. The fork has two main components; an infra-red (IR) LED is one and the other is an integrated circuit device consisting of a Schmidt-trigger circuit and an infra-red sensitive photo-diode. Figure 1 (A) shows the integrated circuit syncforkwhich looks very much like a transistor with three leads, except for a round bump on one of its surfaces. The bump is the light sensitive part of this device. The IR LED, Fig 1(B) looks like a TO-18 transistor with only two leads and a glass top. Pin 1(cathode) is next to the tab.These parts cost about a dollar each and are available from Digi-Key Corp. (1-800-344-4539) If you have an old IR remote from a TV or VCR, you can use the LED from it instead of the 55BQT. These remotes sometimes have two LEDs, in which case you will have a spare. The series resistor for the LED is a 1K ohm, 1/4 watt.The 156BQT needs no additional parts. Figure 1(C) shows a "U" shaped part made of wood, metal or plastic with small holes in each leg. The LED is mounted in one of the holes and the sensor goes in the other, in line and facing each other. Hold them in place with contact cement, RTV or epoxy.The gap in the "U" is about 1/2" to 5/8", to allow adequate clearance for the scanning disk. The depth of the "U" is about 2" to 2-1/2" so that the sensor and LED can be in line with sync holes, without the base of the "U" scraping the edge of the disk.sync fork top view

Here is how you deternine the length of the tines on the fork. The center of the scanning disk is 2-3/8" above the surface of the rear top cover, which is 3/8" thick. The sync holes in the disk are 1" from the edge of the disk, therefore 5" from its center and 2-1/4" from the bottom surface of the top cover. Since the side panel is 4" high, the sync holes, at their lowest point, are 2-1/4" above the surface of the cabinet bottom. That also puts the edge of the disk at 1-1/4" from the bottom. If any of your numbers are different from these, set up the LED and sensor so that their line of sight passes directly through the sync holes. If you do that, it will work.

The wires and series resistor for the LED are run along the outside surfaces and under the "U" and held in place with cement, RTV or pieces of shrink sleeving over the legs. Record the colors of the wires and their function and leave these three wires from the fork, about eight inches longer than necessary. They will be trimmed to length later. The picture on the right shows the fork mounted on a structure that allows it to be moved about one inch, side to side, with a knob on the front panel. This added feature is not a necessity. It was incorporated in this design as an experiment and for those just starting out, it is better to not include it. This monitor will work just fine without it. For you, the fork will be in a fixed position, to be set later.

fork, close upHere is a closer view of the fork. It happens to be painted black. (Sorry about that) It is made of 1/8" plywood and has shrink sleeving on both legs. When the scanning disk is in place, the disk is located in the center of the "U". The LED is on the front leg (lower) and the sensor in on the back leg (upper). The reason for the extra space between the legs, is to allow easy removal and installation of the even closer view, forkdisk of the hub without damage to the fork. Here is another view of the fork that might be a bit more revealing. That business to the left in the photo is a connector to the printed circuit card that controls the scanning disk drive motor. It does that by comparing the incoming sync pulses, which are a part of the video signal, with pulses coming from the fork.


monitor, top viewHere is another view of the top of the monitor with the rear at the bottom of the photo. It shows the placement of the various parts. All of the main parts seen here are mounted on the bottom of the cabinet. The transformer in the lower right (rear) supplies low voltage AC to the two printed circuit cards at the center and left rear of the cabinet. The smaller transformer is actually mounted on a printed circuit board, which is the audio amplifier portion of the set. The speaker at the top right is mounted on the bakelite front panel.

fork and controls up closeThere are four evenly spaced controls with 1" long shafts, mounted on a 6-1/2" length of .060, 1" x 1/2" aluminum angle. This could just as well be made of wood. Here is a better view of that area. Note the extra loop of three wires from the rear of the fork , to allow the fork to move from side to side approximately one inch. The control bracket is located about 1/2 inch behind the front panel. The sliding mount for the optical fork is made up of various pieces of .060" aluminum sheet. Wood can also work.

fork motion controlIf you are interested in making the fork movable as I did, here is a photo of the part that accomplishes that. It is a 1/4" shaft passing through short length of aluminum channel with shaft collars with set screws on both sides. On the end of the shaft is a 1-1/4" disk with an off center screw connected to a rigid wire to the fork assembly.

monitor rear viewThis is a rear view of the monitor with the front and rear top panels removed. The rear top panel contains the scanning disk drive motor and the LED array. A cable from these connects to a plug mounted on the side panel on the left. The rear panel is a suitable length of .060, 2" x 1/2" aluminum angle. It supports the line cord, main power switch, two RCA female jacks for the video and audio inputs and three test points. There are also identifying labels for each of these items. This panel too could just as well be made of wood.

This time we have more of a front view. The four knobs starting at the left, are for the audio volume and amplifier power switch, contrast, brightness and motor speed. The last knob, which is a lever type knob adjusts the vertical centering of the image. The motor speed control will do that too.

The speaker used here is about a three inch diameter. Most any kind of speaker with a 4 to 8 ohm voice coil will do. The volume, contrast and speed controls are each 10K pots, with the volume control also having a switch. The brightness control is a 1K pot.

End, Installment Five

On To Installment Six

Peter Yanczer



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