I wanted to write back here - thanks so so much for your clear, detailed and simplified strip down. I actually came here to try and fix my keyboard - I suspected the bluetooth board was defective. It may well be I have damaged the board with batteries in for a long period of time without using - no visible damage but I did wonder. The keyboard was appearing on bluetooth devices for a few seconds at start up but only for a few seconds and then disappeared. Anyway kindly found a replacement board and hooked up the component and even for a novice person like me it wasn’t that hard just by being methodical and taking my time. Here I am writing from the said keyboard (it’s the A1255 with the transmitter board at the end as opposed to the one where the opening is in the middle which is I think newer (i.e mine is the older one). Anyway, just wanted to say how grateful I am and also nice to recycle something and continue using it rather than disposing and buying new (amazing all my keys still work)! Thanks again.