Hello,
@222Lab_Test222I analyzed the content of this thread and out of more than 450 replies, as it turned out, 25% directly or indirectly boiled down to an attempt to understand the behavior of IC23 (NEC uPD4990A) CMOS.
All those participating in the discussion sometimes came very close to understanding the truth.
All the possibilities were determined only by the presence of a two-channel scope.
But, believe me, it is very difficult to perceive the processes occurring in the product from the pictures on the scope screen, with so many components and, let's say, turning the scope into a signature analyzer or Curve Tracer.
-
I would like to offer you an idea.
-
As you can see, there are six quartz generators on the PCB, servicing various nodes.
All together they must interact - as a single organism.
Figuratively speaking, - all hearts must beat in a single rhythm.
-
Now to the point.
I want to draw your attention to the Clock 32768kHz (X1 - IC7 74ACT08).
Or, more precisely, to C10.
I doubt that it is installed for time correction (in the sense of GLOBAL/UTC).
As a rule, the accuracy is sufficient wherever the 32768kHz quartz is used.
I suspect that the C10 adjustment determines some kind of "global synchronization" that optimizes timings in conjunction with other nodes, including IC23 (NEC uPD4990A) CMOS.
The presence of checkpoints TP1, TP2 convinces me of this even more.
-
I offer you - such a scenario:
On a non-working board, you find a pin on IC7 (74ACT08), measure and write down the original frequency so that you can return to the original state ( check TP1, TP2).
And start adjusting C10, carry out the delta F deviation to a larger, and then (!), to a smaller value, fixing it all together.
That is, I suggest you find a range of acceptable values, in which IC23 (NEC uPD4990A) will work stably.
Perhaps, this will be facilitated by the LED indication and the buzzer reaction.
If everything works out, set C10 to the average frequency value in a certain interval.
You will probably have to turn the device on/off several times.
But, if we go back in time, all old PCs had a priority of devices, starting with KBD and ending with CD / HDD, etc.
If you have KBD, then the easiest way to create NMI is Ctrl - Alt - Del.
You may have a different combination.
But it should be.
Good luck.