Twin Famicom Battery Error

[Telenaphy]

Finally getting around to (trying to) fix up a 500 model Twin Famicom. Recapped the FC PCB, power PCB, and disk drive PCB, plus replaced the drive belt.
Famicom side works fine.

Unfortunately, I'm experiencing an Err. 02/Battery Error when trying to run a disk in the drive. The head doesn't move, the motor doesn't make a sound, the disk read LED doesn't illuminate, just an immediate battery error. Double checked to make sure all the connectors are secured, that caps are oriented correctly, and that solder joints look good... but still no luck.
Using a 7.5v 1.5A power adapter.

Any thoughts on what I should check next?

[Skawo]

Is the interconnector cable under the flap on the back of the unit plugged in?

See if pins 9 and 10 on the drive connector have 5V.
If not, follow the wires to the board and check for any breaks there.