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Technical
Tips from Mark W. Persons |
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Here is a UL-4392 RF oscillator from the RCA BTA-500MX and BTA-1MX series of AM transmitters built in the 1950's. It is complete with an RCA 807 tube. |
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A back and bottom view shows the oscillator was built to last many years. |
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The crystal is also technology from 50 years ago as well. The actual quartz crystal is the 1-inch square white-translucent piece between the two machined metal parts, which normally cradle it. This is a rather complicated assembly with lots of bolts, insulating washers, and two springs. |
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To
change frequency, I replaced the original quartz crystal with a new
crystal in a can with wire leads. You can see it is the shiny
object held in place by screws on insulated stand-offs where the
original crystal was. The company I ordered the crystal from is now out of business. I suspect that International Crystal or JAN Crystal can still make one. The original crystal needed to be kept at a relatively constant temperature in order to hold the FCC required +/- 20 Hz frequency tolerance. To do that, there is a coil of wire around the crystal can and a thermostat to turn the heater power on and off. I disconnected the heater because it is no longer required with the new crystal. |
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Here is the crystal oscillator under test on the bench. To do this, the filament and high-voltage pins on the connector were carefully marked before making temporary clip-lead connections. An oscilloscope probe is connected to the RF output, which is a bolt coming through a feed-thru insulator in the top of the oscillator assembly. |
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A look at the oscillator output reveals quite a bit of second harmonic energy. This is apparently normal. If you do this and connect a frequency counter, be prepared to have the counter read twice the intended frequency. The oscillator should run fine in the transmitter in spite of this. |
| May 11, 2012 e-mail: Your website is really informative and a pleasure to read. That is a very nice article on the RCA 807, great job done there! I really like the material on your site. Best regards, John in Dublin Ireland. |
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The stories go on and on.
Stop in again sometime. I'll leave the soldering iron on for you.
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Questions? Email Mark Persons: teki@mwpersons.com |
