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An Unusual Failure |
R. W. Stuart I recently got a letter from Al Masters from which I quote. "It was back in August, while flying one of my non-generic models at the LCRC field, that I totaled the model----- but good! When I got the debris back home, I checked and found all the servos were undamaged, but when the complete four- servo system was set up with the receiver, switch, and Nicad supply, there was no response from any channel (control function- Stu) when the transmitter was pulsed! Was it a bad switch? Bad receiver? What? Changing the servos to another receiver setup proved that all the servos were okay! Also the switch harness checked out fine. The receiver must have been creamed during the bash ! I was about to write-off the receiver when I took the channel 49 crystal out of the defunct receiver and replaced it with a channel 49 crystal from a working receiver. The non-working receiver now worked fine! The receiver has been range-checked , and flown in other models without apparent loss of reliability.-------The purchase of a new crystal (Tower Hobbies) means purchasing a crystal "set", one for the receiver and one for the transmitter. Nuff said!" --AM-- I totally agree with Al's logic and approach for solving the problems of a non-working system; a nearby channel xtal and associated xmtr would work in modern systems if he did not have another Ch 49 xtal. They are called crystals because the are made from either natural or man made quartz crystal. The crystal is sliced at a specific angle to the axis of the crystal, ground to a precise thickness, and cut into about 1/4" squares. Depending on thickness and size, the crystal has a natural vibrating frequency which may be excited by either electrical or mechanical tweaking. When plated, top and bottom, the crystal may be used in a circuit to produce a specific radio frequency. Years ago in the "old" ham days we used to slightly change xtal freqs by either polishing away quartz (increase freq) or penciling on graphite (lower freq). Frequency is dimensioned by cycles per second (cps- old way) and hertz (hertz new way). I prefer cps because it directly describes freq, where hertz makes one think of rental vehicles and must be defined as cps anyway. Also, many users will talk hertz per second, which comes out as hertz per second second and that describes rate of change of freq, not freq. When two freqs are mixed in an appropriate circuit the two freqs, the sum of the freqs, and the difference of the freqs all show up in the output. By selective band-pass filtering all except the difference in freqs may be removed from the output. In a typical double-conversion RC circuit, the difference between the incoming radio signal freq and the local xtal freq is 8.500 mc (called first intermediate freq or 1st IF). The 1st IF is then mixed with an electronically produced freq to produce a difference in freq (called second intermediate frequency or 2nd IF), typically 455kc. The 2nd IF is then demodulated or detected to the pulse signals put into the transmitter and the pulses are then presented to the servo logic in the rcvr. The above, as a generality, is used in both AM and FM (my preference) systems; and much of the above is accomplished with one chip. I would add to Al's logic an inversion of cause and effect- a broken xtal may have caused the crash instead of the crash causing the xtal problem. Catastrophe is hard to analyze. |
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