I've narrowed the issue to pin 5 of the SIO port (data OUT). At idle on a good working 1200XL, I get get 5 VDC on pins 3,5 and 7 on the SIO port. On this particular board, I get 5 VDC on pins 3 and 7, but an "open" on pin 5. Now to track down that run using the information Jurgen provided for data OUT.
DATA_OUT: R128 (100 ohms) and capacitor C91. Also R42 (4.7k pull-up to +5V).
O.K., now I'm officially confused. Both R128 and C91 have 5 VDC on both sides (great). Now R42 was not measuring 4.7K ohms (installed) and I got 5 VDC on the side next to the 6502C, but did not get 5 VDC on the other side of the resistor. I verified on a good 1200XL that R42 measures 4.7K ohms (installed). I replaced R42 with a new 4.7K ohm resistor, but when I put the multimeter to it, it shows 276 ohms (installed)??? I'm putting the two leads on both sides of the resistor (installed). I verified with the ohm meter before I installed the new one that it is indeed a 4.7K ohm resistor. I'm an Aerospace guy, not a EE, so my knowledge is limited here. Wouldn't there have to be another resistance in parallel to make the 4.7K ohm resistor read 276 ohms?
Also, after I removed the old resistor, I measured it uninstalled and it shows 4.7K ohms. So the original resistor was good??? So, the difference on the bad board is that something is causing R42 to show 276 ohms instead of 4.7K ohms that a good board shows.
You shouldn´t measure the value of a in-circuit resistor. At a minimum, lift one side, so that one side is floating not connected to anything. Now you can measure the resistor and should get something around 4.7K. It´s very, very rare that a small signal resistors went bad without having high current and a short-circuit. In that case you should see something burned :-)
And yes, because other connections to that signal (at least POKEY itself) are made, the result of a measure having the resistor installed is not accurate.
What do you mean with "open" at DATA_OUT? You get ~zero volts or no connection (if you use the diode tester of your multimeter)? Remember, that there´s a 100R resistor in line - most diode testers will respond up to 20R max. So check connection step-by-step from SIO to one end of R128 (100R) and the other to POKEY. The pull-up resistor of 4.7K is "behind" the 100R resistor (facing away from SIO connector), so I would examine the 100R first.