Sam Kelly, W6JTT
12811 Owen Street
Garden Grove, California 92641
Tuning in an RTTY signal is virtually impossible unless some form of tuning indicator is used. The most common indicators are the tuning eye tube, meter, and cathode ray tube. Of these, the most flexible by far is the cathode ray tube. In addition to providing tuning information it can be used to identify interference and check on proper transmitter operation.
For some reason, most amateurs shy away from building oscilloscope indicators. Actually, they are no more difficult than any other electronic project. This little indicator is about the ultimate in simplicity. This is due to the availability on the surplus market of the 902-A cathode ray tube, his tube is available for about $3.00. It has a deflection sensitivity of 90 V/inch making it
Schematic for the Simple Scope.
Top view showing water pipe shield instaf-led. Note the small foam rubber cushion at the front of the tube.
possible to obtain adequate deflection without amplifier stages. In addition, its high voltage requirements are modest, allowing the use of a small cheap power transformer (300-0-300 v) in a half wave rectifier configuration.
The scope was constructed on a 6 x 9 x 2 inch aluminum chassis. The circuit (Fig. 1) was divided into two sections. The power and oscilloscope control components were mounted on the panel and chassis. The input transformer - tuned filter networks were constructed on a 2 x 5 inch piece of vector board which was mounted underneath at the rear of the chassis on stand-offs. Leads to this board should be long enough to allow the board to be slid out of the chassis for tuning.
Due to the small size of the cabinet it was
Bottom view showing filter board removed for tuning*
necessa: y to mount the transformer with the core parallel to the axis of the cathode ray tube. This produced an un-wanted deflection. The cure was a tube shield made from a 4 inch length of 2 inch diameter galvanized water pipe. After cutting, the shield was de-burred and painted black.
All my receiver output lines have an 8 ohm impedance whether they are from the hf communications receivers, or the vhf FM system, A standard 88 mH toroid is used for a combination tone -liter and step up transformer To match the 8 ohm line a primary winding of 35 turns of No. 22 enameled wire is wound over the existing turns of the toroid.
After checking the wiring, turn the scope on, allowing it to warm up. Advance the brilliance control until a spot appears, then sharpen the spot with the focus control. Center the spot with the centering control Connect an audio signal generator to the input- Remember, most audio oscillators have a 600 ohm output impedance, so a matching transformer should be used.
Now you are ready to tune the filters. There are several ways to do this. The results obtained by using a counter to check the frequency of your audio oscillator are well worth the trouble of obtaining the use of
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