The last version of the original PlayStation 1 design was the SCPH-900x. Most of these consoles have PU-23 boards inside. The most notable difference between the 9000 series and earlier consoles is that Sony removed the parallel IO port on the back. They also shrunk down the size of the board significantly.
I sell Mayumi v4 modchips in my store.
Above is the diagram for the PU-23. Pin four is located towards the top left side of the board.
Since the diagram isn't the clearest I'd like to mention that the second pin connects to the top most pin on the left row of pins on the chip. Pin three connects to the left contact on the middle resistor, and pin six connects to the top contact on the left capacitor.
Here are some tips I have for you when you are soldering your chip into the PU-23.
Here's a list of people who have successfully modded their console with this diagram. Leave a comment and I'll add you to the list.
This section has photos of some successful installations which you can use to get a better understanding of how everything is wired and positioned.