>>>>Frank's design also seems to show IC15:7 as NC. On my FDC it is GND.
That's' one of the power pins, def wont work without that being grounded
>>>Frank's design seems to show IC15:14 as NC. On my FDC is it +5V. (Probably irrelevant as IC15:13 is also +5V).
That's the other power supply pin
so yes very relevant.
So IC15 has no power connections at all is one problem.
>>>>>Frank's design seems to show IC16:15 as connecting to C3(-) and GND. On my FDC it only connects to C3(-)
As shown here pin 15 should connect to C2. It looks to me as though it does connect there in Franks layout but also it (looks) like it connects to the ground via the decoupling capacitor to the left of IC16. Can't really tell without seeing each separate layer.
>>>>>Frank's design seems to show IC16:1 as GND. On my FDC it is NC.
From the datasheet its supposed to connect to one side of the timing capacitor (of the second monostable in the IC).
And indeed on a NOS FDC controller PCB it is indeed NC as you say.
Its a bit difficult to read but those numbers on IC16 don't look right - pins 2,3,5 are for the monostable outputting on pins 5 & 7
however the pin used for output as shown in the schematic detail above is (hard to see) but looks like 10, the other trigger pins for that mono are on 12,13,14 .
So as it stands I'm a bit confused by what's shown, one half of IC16 isn't used, but the schema shows the wrong pins being used.
I just checked each pin on the schematic against a fresh FDC PCB. The schematic shown above is almost right... or the board is definitely wrong...
Pins 3,4,5 on the schema above are shown tied to +5 and as this is a cmos chip that is indeed where inputs should be tied (or grounded, depends on the chip) and not left floating.
In the actual PCB these are left NC which is frowned on so I suspect the actual layout of the original board didn't follow the schematic.