I have a battery pack from a Uniden scanner that won't hold a charge
any more. The pack has the charger circuit built in, and the
batteries are nicads in a pack of six AA size cells of 600 mAh
capacity. The charger circuit receives an input of 12v at 500 mAh
from a wall-knob. If I replaced the 600 mAh batteries with 1200 mAh
batteries what would be the effect on the charger? Would it just take
longer to charge, or?? If I am going to replace the batteries I would
like to increase the capacity at the same time.

The pack will still take a charge. Charging voltage at the batteries
is 8.5 volts. It just discharges fairly quickly. The radio is
probably 20 years-old, so it's a miracle the batteries lasted this
long.

Dick