The controlled lamps are dimmer than they should be and test point TP1 on the rectifier board reads around 2.8 V DC instead of about 5.4 V.

Controlled lamps dim and TP1 reads about 2.8 V instead of 5.4 V: half the bridge open

Cause: One of the four diodes inside the lamp supply bridge rectifier has gone open, so the bridge only rectifies half the waveform and the output drops to about half.

Checks

• Check for about 7.8 V AC from the transformer at the bridge input.
• With power off, test each of the bridge's four internal diodes in diode mode, in both directions: each should read about 0.4 to 0.6 V one way and open the other. One that is open both ways is bad.

Fix

• Replace the bridge rectifier (shim a thicker modern part so all the bridges touch the heat sink plate) and reflow the board's header pins while it is out.

Systems

• Bally MPU AS-2518-35

Games

• Star Trek

Related tips

• Backbox GI out and fuse F6 blowing: burnt rectifier J3 plug and a shorted bridge
• Rectifier test point reads below its range
• Rectifier board test point voltages
• Rectifier test point ranges with fuse and rectifier
• No +5V on any board and no MPU LED: bad return pin in the solenoid driver board J3 plug
• No +12 V at rectifier board TP3
• MPU won't boot and driver board test points read low: replace the solenoid driver board
• Solenoid board 5 V collapses under load: open 12 V bridge on the rectifier board

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