Thanks for sharing the info on the Sonicare. Based upon your photos I was able to take some measurements so that I could take a different approach. Rather than splitting the case in half along the original seams, I removed just enough of the case so the batteries could be removed and opened a small area so that I could unsolder the battery connections from the PCB. Your unit is probably beyond hope, but you might want to share the following info on your site.
The battery packda64b49f9f86eb4c111ec6478c3a1bb3 It works! Powers up. Runs for the full two minute cycle. I haven’t decided how to close it back up. Epoxy? Silicone caulk + heat shrink tubing (may not fit in charger base)? If it lasts until the new Nicads die, I’m doubt if I’ll replace the Nicads again so making the process reversible is not a concern. Prior to this success, I tried this process with an even older model (it even had a different model PCB). Failure. I used the same custom-made battery pack that was subsequently used successfully in the newer Sonicare. I also tested the older one unsuccessfully using 2.4V feed from a bench power supply. I believe the problem was elsewhere on the PCB - the unit had sat in an RV unused for over 4 years through heat/cold cycles and it had been used for several years before that abuse. Given that my cutting approach did not give full access to the PCB plus the fact that I already have the re-batteried Sonicare and a new Sonicare 9500, I decided it wasn’t worth the effort to track down the problem with no schematic.