[Boatanchors] Crystal Holder Pressure

[Gary Peterson]

Around 50 years ago, I was responsible for maintaining a Collins 300G, licensed as an auxiliary transmitter.  The FCC required that it be tested, weekly.  We also checked the frequency, monthly, via a frequency measuring service in Lee’s Summit, MO.  The 300G had two separate crystal oscillators, with ovens and crystals.  
Each crystal was mounted in a brass and ceramic cylinder with a screw-in pressure plate.  Coarse frequency adjustments were made with the adjustable pressure on the quartz blank.  Fine adjustments were made with a variable capacitor.  Getting the frequency close (plus or minus 20 cps) with the pressure adjustment was a real challenge.  Once close, adjusting the capacitor was a piece of cake.  I don’t know what cut-type the crystal was.  Maybe some cuts are more sensitive to pressure than others.  I do know that some cuts have much smaller temperature coefficients.

Gary
KzeroCX
Old BC engineer, somewhat retired




It;s interesting. I have been dissecting various crystals for many years.
Especially the FT243 ones.

You woul dbe amazed at HOW many different internal designs there are!
The SPRINGS have a HUGE variety.  Th epressures must be ALL over the map. Not only from the spring but MANY other factors. That being said. You can move a blank from one crystaol holder to another without much change in frequency with spring pressure etc.

What seems to have REALLY put the whammy on older crystals was to put a SCREW through the front of the top plate and start stressing the quartz crystal with it. This was a technique used by Bliley and others who would commercially supply these monstrosities to hams so they could VXO their quartz crystals I understand that this owuld quite often bring a very sad look to Junior's face whenever he would try to VXO "Too Far" in the hopes of working that "Rare One!!" Of course some guys made their own too, by drilling and tapping a hole. Not as fancy looking as the commercial ones in appearance. Probably equally disastrous results for many(!)

Bry AF4K