Just remembered one more thing from something I noticed before, but didn't look closely - had to go back to the original HP circuit to be sure. Take a look at the rather elaborate decoupling circuit they used on the other input (pin 8) to the prescaler. On close inspection, you can see the extra damper R10/C23, and R6 to ground, apparently a factory selected value - that's to shift the DC bias just like I was talking about, except using the opposing input. You can picture it as a differential input. They used ground though, to move it, which works the same way, while I'm more of a purist and go with Vcc, by habit. And of course, R6 had to be determined and selected somehow, to go with the particular IC U5.
Ed
