After changing the playfield SPACE and SHUTTLE flashers (and backbox flashers) to LEDs, they stay lit all the time instead of flashing.

LED flashers glow all the time: disconnect the flasher warm-up resistors

Cause: The game has small resistors that pass a trickle of current through the flasher bulbs to keep them warm. That trickle is enough to light an LED.

Checks

• Find the larger warm-up resistors on the small boards near the flashers and in the backbox.

Fix

• Unsolder one end of each warm-up resistor for LED flashers; it can be resoldered if incandescent bulbs go back in.

Systems

• Williams System 9

Games

• Space Shuttle

Related tips

• System 9: Sorcerer uses two game EPROMs, Comet and Space Shuttle one
• Flasher resistor reads far below 330 ohms with bulbs fitted: that's normal

Source: Pinball Repair help group!! (Facebook group)