The MPU board has a signal specific for enabling the flippers. It comes from MPU U11 pin 16 and out on MPU J4 pin 7 to the SDB at J4 pin 8. It's a Blue-White wire according to the Meteor schematics.
When that signal is High, the flippers are disabled (relay is OFF).
In this state, the high signal coming out of MPU U11 pin 16, downstream results in pin 3 of U4 on the SDB to come up to around 0.9 volts. U4 is just a transistor array (it contains 7 internal transistors).
When U4 base pin 3 is 0.9 volts, it results in the collector pin at U4 pin 2 to drop to around 0.3 volts. U4 pin 2 connects to the non-banded leg of diode CR15 and when U4 pin 2 is around 0.3 volts, no voltage is passing through diode CR15.
This results in the base leg of driver transistor Q15 to be at 0 volts from the pull-down resistor R39 which switches the Q15 transistor OFF at the collector leg (metal tab) and so the flipper relay coil doesn't engage.
When MPU U11 pin 16 goes low to enable the flippers, downstream it results in SDB U4 pin 3 to drop to around 0.3 volts which releases the U4 pin 3 collector leg that's connected to the non-banded side of diode CR15. The voltage here rises to around 2.1 volts. The non-banded side of diode CR15 going to the base leg of driver transistor Q15 rises to around 1.4 volts which switches ON transistor Q15 so the collector leg (metal tab) connected to the relay gets switched to ground placing 43 volts across the flipper relay coil that engages it.
See the voltages in the chart below. Confirm your game is roughly in these ranges when it's working. When it's not and the flipper relay is buzzing, look for any abnormal voltage change.
SDB_Coil_Drive_Voltages.gif
SDB_Flipper_Schematic.gif