I managed to track down the Service Manual for the PR307 which is very similar to the PR305. I’ll upload it here soon.
In the Service Manual there are some instructions for running some basic PBITs which is pretty cool. Most of them involve turning on the keyboard while holding down two particular keys, but for some of the tests you need to make a “Checking Device”. The manual describes it, but either the manual is wrong, or the PR307 is backwards to the PR305.
I grabbed a low-power LED and a 1kΩ resistor to make an indicator, and some component lead off cuts to make a ‘switch’.
Pretty rough and ready, but worked a treat. NB the manual has the pins backwards to the PR305 main board.
PR305, Main PCB QJBG2173, Connector CN3 pinout for ‘Checking Device’:
It seems I can’t insert images to this reply for some reason, so I’ll attach instead.
Here is my PR305 main board, QJBG2173:
Corrosion doesn’t look to be too rife. I have +15.2V, -15.2V, +5.0V, and +3.3V, so all the power rails I can find look to be good.
The LCD connector is on the far left. I have done some preliminary fault-finding, and it seems that the LCD is not getting sent any data or commands.
The SED1330FBA is a CMOS low-power dot matrix LCD controller. Its CS (Chip Select) line is supposed to be active low, but is held high. Meaning that the LCD driver is effectively turned off, and the LCD is not being told to display anything.
I’m flying blind here, but educated guessing is better than giving up.
Anyone with any pointers feel free to chip in … In the meantime I’ll keep posting …
My next move is to find out WHY the CS is held high.