Hi Paul
So sorry that you had no success with IC7. I've no more idea what you could do now so far, hope another member has the special trick to bring your BM back to life.
Maybe you're owner of a signal generator and try to insert the 647 Hz directly into PIN 15 to see if the uPC comes to life....last chance and more experimental, but there is nothing to loose.
That is the point to make a decision.... I hope for you that someone has a uPC IC lying round in his workshop and that this wil help.
If not now maybe later 😴
Best regards
Chrischi