My experience is typically that the U6 or U7 (U50 on system 11's) is damaged and this damage is usually caused by a bad special solenoid coil/transistor. When a special solenoid locks on I find it usually takes out the TIP102, the fuse, the coil and about half the time the TTL chip that turns on the transistor. On system 11's where I usually see this One TTL chip supplies the signal to 4 of the special solenoids and the other supplies the signal to 2 of the special solenoids, then one of the extra gates on that second chip is used for the enable signal for the flipper relay and special solenoids System 9 looks just a touch different with U6 handling 3 special coils and an enable and U7 also handling 3 specials and the enable.
7406 Is an inverter gate (inverts low to high and vice versa)
7402 is a NOR gate (low and low are inverted to high, all other combinations are low)
The Enable signal (En with a line over it) should be HIGH in attract mode and should go low whenever a game is started or when the game enters the diagnostic mode. This is at U6 pin 11.
The Blanking is low for a few seconds at start up and is then high after a second ( this gets inverted to low at U59 pin 8 ) and hits U6 at pin 12 and is low at all times
So at attract mode the inputs at U6 11 and 12 are high and low respectively and the output at 13 is low. Then you start a game and 11 and 12 are both low so the output at 13 goes high. U58 then flips the signal so the output on U58 goes from high to low which then activates the other NOR gate at U7 so that the flipper relay is active. The low output on U58 is also now used on U6 and U7 at all the special solenoids and when combined with the low from each direct solenoid switch will activate the solenoid coils.
If U6 was broken before it might have been STUCK with a high output regardless of the input. If the new chip now has all the special solenoids disabled during game play then double check the inputs into U6. Pin12 is the inverted blanking and should ALWAYS BE LOW Pin 11 should be high during attract mode and go low during game play. If Pin 12 is high instead of low there might be a blanking issue with the MPU board.
My Forte is with System 11 and 9 is 90% similar but uses different numbers for the chip locations and some small differences. Like the blanking only uses 1 other gate on system 9 (U41 a 7404 ) and this chip appears to ONLY be used for the one gate. ON system 11 they used some gates from the special solenoid chips on the blanking so I see alot of damaged blanking circuits on system 11C games at the special solenoid chips