Fault
Calculator displays E666666 when first powering on. Possible cause dry capacitors.
Power
AC operation
4.5V current 200mA
Batteries
1.2V 450mAh rechargeable nickel-cadium
Capacitors
10uf 16V
4.7uf 50v
10uf 16v
47uf 16v
Fixing E666666 initial output
I started swapping each capacitor until I found the one which was causing the fault. The culprit was once a 4.7uf close to the power input jack.