As taken from the bible
Problem: Star Trek Next Generation diverter coil stuck on!
Star Trek Next Generation (STNG) uses more coils than there are transistors on the Power Driver board. Williams solution was to add a small auxiliary driver board, mounted above and to the right of the main driver board in the backbox. This small auxiliary driver board holds more TIP102 driver transistors for the additional coils needed in STNG. This board needs +50 volts for a "tieback diode" voltage for the circuit. The power is connected by a thin violet/green wire which connects to the playfield's single drop target coil (at the back of the playfield). If this wire breaks, or if some other power wire in this coil power daisy chain breaks, it can cause the two diverter coils to lock on (after they are first activated in game play!) If the problem is not found quickly, the diverter coils and their driving TIP102 transistors (usually Q15 and/or Q7) can fail. Transistors on the auxiliary driver board will fail in one or even a couple of activations if the tieback voltage is not present on the board.
Answer: If the two diverter coils lock on after a game is started, check the violet/green tie-back wire which connects to the playfield's single drop target coil. This wire than daisy chains to the other coils controlled by the auxiliary driver board. It's not a bad idea to add a second back-up wire from the single drop target coil (or another adjacent coil) to the circuit board, just in case one wire breaks. Additionally, add two 1N4004 diodes to each of the under-the-playfield diverter coils (banded side of the diode to the power lug with the thick wire).
Also check D7 and D15 on the aux board with a DMM's diode test (and while you're at it test TIP102 trans Q7 and Q15). With the coil power fuse removed, you can also test U1 pin 13 and pin 8 with a logic probe or DMM - if high (and the diverter is not supposed to be energized), then that U1 is bad (74LS576). That is, the U1 pins that connect to the driver transistors are normally high (when an output U1 pin goes low, the driver transistor completes the ground path for its associated coil). So a logic probe or DMM is useful to look at the U1 output pins (anything low and the associated coil will be energized). Another test of this is to use an aligator jumper wire connected to ground, and touch each U1 output pin - the associated coil should energize. The Aux8 U1 chip is driven through the ribbon cable from the CPU board's U7 chip. A damaged cpu-to-aux8 ribbon cable can also cause some wacky behavior.
Also make sure the diverter coils are the correct type and resistance. The correct coil type is very important (AE-25-1000, but always confirm with the manual). Remove one wire going to each coil, and measure the resistance with a DMM. It should be around 12 ohms and no less. Another common problem is when moving the game and the backbox is laid down, the ribbon cables get pulled, and it wasn't plugged in fully on the board. So if a wire in the ribbon cable is faulty, a diverter coil can lock on and burn and ruin its associated driver transistor on the auxiliary board in the process.
Finally, these 8-driver Auxiliary boards are not necessarily exchangable between game titles. The boards are the same, but there are a set of four jumpers on the Aux board, and the jumpers vary depending on the game title. So if a Aux8 board is transplanted from say Demo Man to STNG, make sure the Aux8 board jumpers are changed accordingly.
The STNG tie back wire on the drop target coil. Picture by Jelle Nelemans.
