Another FT-290Rmk1

Many thanks to my old friend and colleague Steve G7TAO, who has donated to me his Mk1 Yaesu FT-290.

Not only does this radio have a few of the ancillary items mine doesnt, it also doesnt have the USB carrier balance fault mine has! allowing me to get back on 2m SSB whilst mine is 'in the shop'. But it does have one odd issue

The radio works perfectly on external power, but not on batteries. On batteries, switching the set on shorts the pack, but also the pack never reads above 10v! This is despite the batteries being perfectly fine used in my set! Whats more odd, is that after briefly switching on and then off, I can watch, on my meter, the voltage work its way back up slowly! The power input socket seems to be good. Im starting to suspect something strange like one of the protection diodes is at fault.