Roomba 4200 Series: Anyone Have A Charging System Circuit Diagram?

Hi! I picked up several Roomba Discovery 4200 series robots and have refurbed a few and got them running. They're remarkable little machines! I'm having trouble with two of them... they run fine on a full charge but the on-board charging system won't work. I tried connecting the charger directly to the unit (bypassing the dock/charging pad) without success. I do not get the slowly blinking red power light; nothing is lit. When docked, the two green lights are lit on the dock showing a connection but I get nothing on the Roomba itself. I did replace both the U2/U4 transistors but there was no change. There's obviously another problem with the on-board charging circuit. I'm trying to find a main board and charge port circuit diagram so I can attempt to troubleshoot it. Can anybody help? Thanks! confused