Careful, sys9 doesn't like MOSFETS. Be sure to do it the 'better new way' on PinWiki

Once MOSFETs are substituted for the TIPs in the lamp driver circuit, it is possible to replace the 27 ohm resistors with simple jumpers or even leave the 27 ohm resistors in place if they are in good shape as the low resistance will have a negligible effect on the operation of the MOSFET. However, the problem with either of these is that the MOSFETs are driven by 18 VDC Source to Gate voltage which is dangerously close to the absolute maximum rating of the IRF9530 and IRF9Z34 of 20 VDC (this also holds true for Data East MPUs). The FQP17P06 has a higher Vgs rating (25VDC) than either the IRF9530 or IRF9Z34N - but you are still driving the part at over 70% of its absolute maximum. Typical uses for these MOSFETs drive them by about 10 VDC Source to Gate voltage (or -10V for P-Channel MOSFET's used here).
http://www.pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#MOSFETs_Installed_in_a_System_9_MPU_Board

OR ELSE!

https://pinside.com/pinball/forum/topic/controlled-lamps-too-bright-lamp-matrix-problem