Hello.
I have just changed the battery on my A3000T to a coincell and im having a bit of a problem.
Pin 18 on 190 is only getting 0.5v
Do you have about 5V on pin 18 of the RTC with the computer powered on? If so, the RTC and C190/C191 are likely to be OK.
Check that you have about 470 Ohms between the battery + terminal and pin 18 of the RTC, do this without the battery fitted and the computer powered off.
And yes, you'll need a series diode to prevent the lithium cell from charging. Lift one side of R192 and fit a Schottky diode for lower insertion loss compared to a silicon diode, something like a BAT85 should do the job.
