Why doesn't CD spin motor power up in JVC audio system?

I have a CD/casstte/radio JVC model no. CA-MXS2BK. When the CD tray closes, the CD lens motor powers up from the circuit board via ribbon cable, but the spin motor doesn't. The open and close switches communicate correctly to the circuit board; the ribbon wires/junction plugs are sound: there is no voltage at the circuit board junction plug for the spin motor (SM+, SM- pins). Power connections to the CD circuit board are OK. There are no components/joints visibly amiss. The spin motor itself does work on test. The display shows "no disc" because the CD doesn't spin.