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Technical Tips from Mark W. Persons
RCA Crystal Oscillator Frequency Change

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.   
A back and bottom view shows the oscillator was built to last many years.
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. 
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. 

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.
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.

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

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