Faulty proving unit?

[LCD]

My Megger MPU690 Proving unit didn't appear to be working, so I replaced the batteries. When I installed the final new battery, the unit made a fizzing sound, all of the voltage lights lit up and then it just died. After this, I tried testing my voltage indicator using the proving unit and nothing lit up on the proving unit.
I've checked the following on the proving unit:
Continuity of wires, pcb for condensation, dust or anything that might be causing shorting.
Any ideas before I throw it away and buy another one?

 

[telectrix]

i'd guess at a dry joint on the pcb. inspect with a magnifying glass. when found, solder.