750P

The 750P was IBM's first attempt to combine the Stylus pen input of the 700, 710 and 730 Tablets with the conventional notebook workstation form factor of the 720/750 Thinkpad. As with many machines of this era, it had limitations - a greyscale STN screen in a world where bright, fast, colour TFT panels were becoming a huge selling factor. 

Here are some choice photos of a refurbished 750P:

I spent a weekend working on two 750P machines which had a variety of issues. Most of these will be commonplace in anyone looking to run these machines in the 2020s. The base of these machines is extremely similar, if not the same as the rest of the 750 series. The real difference is in the lid and it's ribbon cable connectors to the base. The lid has a seperate set of arms which allow the screen to be rotated to work on both the inside and outside of the lid. This would mean the machine can be carried in one arm and used in portrait-mode with a stylus whilst walking around. It also means the machine could revert back to a standard landscape notebook computer. 

Here is a couple of shots of the top bezel removed before I completely disassembled the machine. If you've dealt with other 720 ,750 or 755 machines, it's very simple and one of the easier machines to take apart and rebuild without missing a screw.

Display

The display is completely unique to the 750P and whilst quite clever, it's not all that complicated. However it's worth noting that whilst the 750 (mono) and 340 (mono) machines look like they have the same display, they definitely do not.

Here is the display with the front bezel removed:

The display and touch system and is made up of 3 key components:

1) The LCD display itself which incorporates a touch panel into the frame.

The LCD itself has a wide PCB down one side which has 7 small though-hole electrolytic capacitors, which were not leaking but may well have failed. This board is relatively easy to de-couple and remove to work on. Please note in the picture below, the Diachu touch PCB in the centre is connected facing the wrong way around!

This is a close-up of the removable PCB on the back of the LCD display, with the larger connector wrapping around to the other side and the smaller connector going to the ribbon cable into the base. Note the 7 little caps on the board.

2) The inverter along the right side, which controls the high voltage supply to the twin CCFL tubes along with brightness and contrast wheels

The inverter can be removed by unplugging the CCFL cables top and bottom and then disconnecting the ribbon connector in the centre. There are 2 through-hole electrolytic capacitors and 3 surface mounted electrolytics on this machine and they were all failed and leaking. In one case, the failure had causes corrosion to seep up within the PCB itself and inside the various traces nearby.

Here is a shot of the corrosion which has found it's way within the PCB board tracks:

And here you can see how the PCB panel has turned brown where it has soaked up leaking elecyrolite:

Also on the inverter is a small fuse with "20" written on it. It's worth continuity testing yours because my one has blown and I swapped with with a "15" fuse pulled from another thinkpad:

Before:

After:

3) The Diachu KS-112 PCB which controls the touch sensors in the panel translating a stylus movement and touch into mouse control.

This is a flat board which sits behind and centre of the LCD display. It does not have any electrolytic caps but you should be careful to reconnect it the same way up as it came out, as it is not keyed and can easily be flipped and plugged in the wrong way. Which leads me to a silly incident. On the first of the two 750P machines I worked on, the Diachu PCB was loose and I lost track of which way around it went in. I initially connected it the wrong way around, which did not prevent the panel working in DOS as a normal panel but did cause a boot-up error within the CMOS of 4101. This is described as a 'System Board' (motherboard) issue likely related to the BIOS and the data held within it. I wasted several hours trying to determine what the issue could be, until I finally realised it was this board the wrong way around. So, if you get a Boot error 4101 then it's this little PCB not even inside the base! Here is a picture of the Diachu card:

Here is the display working, albeit with the LCD bezel removed:

Here is that error if the touch PCB has a problem:

I did some updates to my machines and it's worth noting that usually a BIOS update for this era of thinkpad would require an (increasingly hard to find working) battery fitted. However the 750 did note that a working, charged battery should be installed, it did not require it and did update the firmware no problem.

And finally, here is a money shot of both machines I was working on booting up again. Almost booting up again.