HP-97 Charging circuit not working

[teenix]

Attached is an image of the resistor positions. It seems, as you mention, R3 and R4 reversed in the parts list as the 8R2 resistor is across Q1 and is shown as R4, but R3 in the parts list.

The resistor you mention may not have been required. It would have been for the cathode driver and low battery detection.

The lower image half also shows a change to a mistake in the original schematic. This monitors the current through the displays and when required shunts R4 by turning on Q1 to allow more battery current.

If the 8R2 resistor is open circuit, that will stop the charger current reaching the battery. A 500mA fusible 8R2 resistor seems feasible.

cheers

Tony