Sorry this is very much unclear to me, there are more than 300 messages in total and the search is not really helping. I will go through the 300 messages, for now I found a photo with some jumpers plug near the bottom module contrary to the point T in the unbrick thread I found earlier.
I will try to make sense out of this and summarize the procedure after some rest (already spent ~16h buildings several kernels and learning a lot about bootloader, android toolings, EDL, SBL, etc)
I bought a USB to TTL adapter to hopefully get UART readings.
I am making progress, I realized on the git repository there is a Pull Request from chaosmaster that explicits the UART TX point.
I have open my device and check some connections/voltage, I can see there were some mistakes (as stated in the introduction) so for now I have fix the volume/power buttons and added some notes : https://github.com/Corentin-pro/fp3-notes .
I have a problem to power the phone while opened, maybe it just works but I am not familiar with UART to check it. I would have like to feel the vibration or see something on the screen but if I just put the MB (motherboard) and screen together it won’t power on. I think the big metallic shielding that can be easily removed (2 special screws) is connecting the grounds together but even with it I couldn’t make the phone on (trying by shortening BTN 3 and 4).
I will try different ways and contacting people. If anyone from Fairphone reads this I would be very pleased receiving some guidance (I am trying to port PostmarketOS, a already compiled and flash several kernels but lack feedback from the phone as it is not booting properly).
The UART on the FP3 is 1.8 Volt.
A 3.3 V TTL adapter will probably work fine (that’s what we used as well) if you just want to connect to the FP3’s TX.
Just be sure, not to connect to RX or VCC.
If you want bidirectional communication you’ll need an 1.8 Volt adapter, or build yourself a level-shifter.
I am sorry, after reading your EDIT, I was under the impression you already found the location:
I don’t remember if the battery needed to be connected for the device to be powered on.
You will need the bottom-module and the mainboard.
Then short the correct pins for the power-button to turn the device on.
Alternatively if you are planning on using the UART for development, on @basxto’s device we removed the speaker-module and replaced it with a female pin-header to have access to UART from outside on the completely assembled phone.
@k4y0z thank you very much, the pin-header is very clever.
I got the boot log from UART, indeed the bottom module is very helpful (I am not sure if it is needed yet) for the vibrations. I didn’t solder anything as my only cable are jumper cable (way too thick in my opinion here) so I went with an open-case solution.
My adapter can select different voltage (from 1.8 to 5V) so I didn’t had any issue about that.
I will try to give information here, feel free to ask more if needed.