HP34C battery polarity

[jebem]

Chances are that a bad contact in the power supply area is blocking the DC volrage to reach the main pcb.
Usual suspects are the battery connectors, the power cables going from the battery holder contacts to the main pcb, and the power switch.
Remove the batteries, open the machine case, use a multimeter to check for good continuity, meaning resistance values below 0.5 ohm on each power cable.
Sometimes a bad contact is found at the battery terminals as these use metal rivets to maintain electrical contact between the batery terminal and the flat cable. Dry electrolysis may cause oxidation here.
Also check for previous battery leakage. This for sure would cause corrosion and would need total disasembly to clean the components without the risk of destroy the keyboard faceplate.