[oldeurope]

Hi David,
I have a few ideas to use the AM sound section of the IC.

One way is to change the 4MHz XTAL.

But there is an other independent way I tested.

This IC uses an intercarrier sound mixing system.
You can see on the Analyzer two sound carriers one below
and one over the picture carrier.

This means the sound VCO works on 3,5 MHz not at the
high output frequency like the vision carrier does.

So you can use the 3,5 MHz sound osc without a PLL.
You have to put a DC voltage from a voltage divider
to the input SPLLFLT. I used a 10uF block and a 1K variable
resistor and 1K2 to VCCA. (ca.0,9V)
The frequency stability is much better than I expectet!

Kind regards
Darius