After all the bulbs were changed to LEDs, the ring of lamps in the circle doesn't light, although each LED works when tested on its own.

A group of lamps won't light after fitting LEDs: their diodes were clipped off

Cause: The diodes across those lamp sockets had been cut off at some point (the clipped leads were still soldered to the sockets), probably taken by a previous owner for coil repairs.

Checks

• Look under the playfield at the lamp sockets: compare with neighbouring sockets and look for stubs of clipped diode leads.
• Use the lamp test to see whether the lamps light.

Fix

• Fit new diodes (1N4004 type) where the old ones were cut off, with the banded end the same way round as on the working sockets.

Systems

• Williams System 4

Games

• Laser Ball

Related tips

• A single lamp does not light
• How the lamp matrix runs 6.3 V bulbs on 18 V
• Tester LED check of the lamp columns
• Tester LED check of the lamp rows
• No lamp matrix voltage
• Diode-test the lamp column and row transistors
• Light a lamp row by grounding its transistor
• Diode-test the lamp 7408 and 7406 chips

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