This radio has one set of 6 crystals from 11.7 to 11.95 MHz. Each one is selected for four channels in a row. Removing one of them, and replacing it with a different crystal will change the four channels that one covered.
Only.
To simply pluck out one and get channels 36,37, 38 and 40, you need a crystal for 12.1 MHz to take the place of the one you removed.
Just one detail. It has to be the right 12.1 MHz crystal. Not an easy find, most likely. Crystals are not all created equal. The marked frequency will only be right if that crystal is calibrated for the oscillator circuit you use.
The reason it skips channel 39 is that the skipped "RC" channels are built into the other crystals. To get channel 39 you would need to put a switch on the 7.4825 crystal and add a 7.4925 crystal on the other side of that switch. This would provide a "plus ten" kHz switch that only works on channels 3,7,11,15, and 19.
40 years ago when it was still kinda new, there were vendors who stocked those crystals, along with wacky setups to swap all six main crystals at once, giving the radio 23 new channels.
The simpler solution is to hook an external VFO in place of one of those six 11 MHz crystals. Takes a VFO (slider) that puts out 11.7 MHz on channel 1.
No easy ways to do this.
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