97S low battery Indicator with no display

[AndiGer]

First of all: Get the extended version of the service manual here
https://literature.hpcalc.org/items/1127
There is the also the newer layout of the main PCB

The printer PCB is mounted correctly - you surely double checked (and show in the picture).

Transistor 4-xxx is NPN and equals to HP part no. 1854-0xxx
(3-xxx is PNP, 1853-0xxx resp.)