Fig. 5. Block diagram of the entire operating set-up.
inator, (Incidentally, this same converter, when combined with a rectifier and dc amplifier at the output, can be used to run a teletype machine.) The reason for using commercial RTTY signals is that they are far more numerous and easier to recognize, and their pulses are of longer duration than a station sending slow scan TV pictures or a weather map on facsimile-
When you have noted the correct positions of the tuning dial and the BFO for sucking out mark or space of the R1TY signal, you can then look for one of the reliable facsimile stations, such as Navy (NSS) on 3357, 4975, 8080, 10865, 16410, 20015 KCS or Air Force (KWAF) on 4502.5, 10185, 14550, and 19955 KCS (frequencies subject to change), These stations (when sending a picture) can be identified aurally by periodic tick-seratchings they make -like the sound of a phonograph record needle left running and repeating in the end-of-record groove of an old 78 RPM record with the volume turned up.
Often, when they are not sending fax pic-
cures, they send "limit signals," one-half time black, one-half time white, at 800 Hz apart. These are ideal for tuning in the station. These limit signals are followed by phasing pulses, "ticks" only. They are then followed by a tone, then the picture. At the conclusion of the picture, another tone is sent.
rhe picture, as seen on the scope will be a series of pulses of audio at random times. (They will look like noise pulses.) Sweep of the scope should be set as slow as possible. If you are tuned to the wrong sideband of the signal, then you will see a kind of almost continuous signal with many dropouts. (This would give you a negative picture with white lines.) To reverse the situation, go over to (he other sideband of the receiver and readjust the BFO. The tuning which gives you the cleanest baseline and maximum pulses is the correct one.
You will find this little converter every bit as satisfactory as the CV 172 and a lot cheaper! More fun too!
Was this article helpful?