I found that there were quite a few dry joints on the main board. It's worth going over every joint to check it visually, and for sake of a second or so, re-solder it. Also, part every ribbon cable connector and plug in socket that you can see and spray with Servisol 10.
I found the following notes also useful:
https://www.oocities.org/grundigradioboy/650adj.htm. It makes mention of dry joints associated with loose pot to PCB soldered joints. Scroll down the left hand side of the screen page.
Poor connections and dry joints are major areas of concern when restoring a 600/650. Check that when you plug in a mains connector that it operates the battery/mains change over switch properly (clean the contacts).