The AM Forum

Hi
I have a Globe 500B that wants to drift on crystal. If I key it up on say 3.885 it will start out at 3.88485 and then count up to 3.885 and sit there. If I UN key and key up again it does it all over. If I use the VFO it is solid as a rock. I have replaced all the surrounding parts in the crystal circut. I thought it might be a voltage change but I can't see any change. After using the shotgun method of parts replacement I am stumped. Could I have a bad rock?

73 N8QPC

Some xtals drift more than others.  The only way to make it drift-free is to hold the temperature constant.  That's why broadcast transmitter xtals are often housed inside a thermostatically controlled oven.

The old General class exam used to always have at  least one question where the frequency tolerance and temperature coefficient of a xtal were given, and you had to calculate how close to the band edge you could safely order a frequency.

Some cuts, or the way the quartz plate is ground in relation to the crystalline structure, are inherently more stable than others.  Usually they are a compromise between stability and ease of oscillation.  The older handbooks fully explain the details.

Sometimes, excessive drift may be caused by too much crystal current, which generates heat, and when carried to extreme, may actually cause the xtal plate to fracture.

Some have a positive temperature coefficient and others have a negative.  I have a pre-WW2 3619 kHz one that drifts worse than yours.

I am confused as to why you are think this is out of spec. VFO section and oscilator are off when its not keyed. When you key it, power energizes this section of the radio. All my globes behave this way. Nobody can hear you being off that much. I dont even worry unless I am a KC out and even then, Most dont even care. 

I think its ok...

Clark