Diagnosing PLL that won't lock

MisterBill2 said:

It almost looks like some power supply ripple on the tuning voltage line. Have the DC supply voltage lines been checked with a scope to verify that they are free of ripple and noise? That should have been done quite early in the diagnostic sequence.

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No, would have been a good place to look.

We 'fixed' it to the posters satisfaction by increasing the value of the loop filter cap from 0.33 uF to 1.0 uF. In my research, I have run across other receivers that use the same or similar PLL/loop filter and use the 1 uF value. Has to be some reason that 0.33 uF worked before - almost has to be some leakage current in to/out of the loop filter (integrator). The JFET and NPN were tested with a transistor tester and seemed good. That left leakage from the phase detector. I was a bit hesitant to ask the poster to procure and replace the PLL chip, as many times the only source of these parts is either very expensive or from ebay which may be counterfeit.

Anyway, with the 1.0 uF cap in place of the 0.33, the poster says the unit tunes cleanly, sweeps properly and autostops properly. Good to go.