The AM Forum

After finding a schematic for the Mackay 2003 transmitter, I was able to build up the various power supplies and get it to
producing RF. 

The transmitter was designed for 2-24 MHz with crystal control in the lowest frequency and multiplying for the higher ones.
I have found that if a 160 meter crystal is used the transmitter will automatically double the frequency, e.g. 1985 doubles to 3970 on the
output.  If a crystal above 2 MHz is used, it transmitts the fundamental crystal frequency.  I don't understand this.  Why does it automatically
double on any freq. below 2 MHz.?

I'm still looking for info as to tune-up/operating procedures on this Mackay 2003.
 

One possible answer is that the crystal above 2 MHz was cut as a fundamental type; the crystal you happened to use below 2 MHz was cut as an overtone type.