Many land-mobile transceivers (such as Uniden and Midland) had clip-on crystal-heaters. These have "posistors" on them, which maintain a constant temperature well above ambient temperature. The posistor connects directly to a 13.8VDC source.
A crystal-heater is clipped onto the reference oscillator crystal, and connected to +13.8VDC and ground. Then after a warm-up period, the reference oscillator is realigned.
Basically, this is the same as Icom's High-Stability Crystal Unit, but at less cost.
Here are three views of a Murata crystal-heater element:
Here is a crystal-heater installed in an IC-746PRO:
NOTE: I no longer have any of these crystal-heaters. And I do not know of any sources, other than surplus Uniden and Midland land-mobile transceivers.
CLARIFICATION: I often see folks refer to Icom's High-Stability Crystal Unit as a TCXO. It is not a TCXO. It is merely a crystal in an oven. A TCXO is a temperature-compensated crystal oscillator, which is a totally different animal.
DISCLAIMER: Attempting these modifications on your own equipment is at your own risk. If you have any doubts, don't try it.
Mike Nadeau - N1EQ
www.n1eq.com
Last updated: March 16, 2009