[Kenwood] TS930 PLL unlocking

[Maury Guzick]

Cliff and all other 930 experts:
 
One of my 930's has developed a problem where it drops PLL lock and the display of course blanks. This on happens on certain freqs: below about 3.680; above 25.735; and between 16.670 and 19.500 mHz. 
 
On checking the PLL board, IC13 which is the 500kHz comparitor will shift its output to about 468 kHz and unlock at the above frequencies. 
 
All the VCO's appear operating correctly and I have gone through the tech manual and done a tune up on the PLL board with no change. 
 
Any ideas?
 
73's
 
Maury