5v might be too high. Let's use ch20 as an example. If the VCO needs to be at 35.005MHz, the VCO's varactor needs enough range to make it go down to 34.765 ( for ch1) and up to 35.205MHz (for ch40). For this reason, the radio is set to ch20 and L13 is adjusted so that 35.005 happens at the middle of the varactors tuning range. When you get the upper channels to drop out, it is because the voltage is already too high, and by the time you get to the higher channels, it falls out of lock (either because the PD output is already maxed out and can't go any further or the crystal sees too much capacitive reactance). Edit to say that with the trimmer, this latter case is not possible, so its maxing out the phase detector for sure in your case)
Here is the tuning chart from the datasheet for a varactor (SVC251S, couldn't find chart for the actual one, but they all behave similarly):
View attachment 75551This chart may lead you to believe that 5v is in the center, but the pF scale on the left is logarithmic. Going from .5v to 5v changes the capacitance from about 40pF to about 12pF. Going up to 10v from there only changes that another 5pF. The most valuable part of the varactor is the first 5v. If you already have 5v at ch1, that's no good. I would recommend setting it for about 3v at ch20 and see how it behaves.
