The -5 volts DC that should be present at TP4 is generated by the LM 320 (aka 7905) that is identified as VR2. This is a pretty simple circuit that includes a couple of diodes (CR7 & CR8) and a few capacitors (including C37). If the board was just serviced and confirmed as working, I would trace the problem back to your connectors. Do you have connectivity from J1 pin 7 to the positive side of the capacitor labeled as C37? J1 pin 7 on the S&T sound board comes from the power supply board in the bottom of the cabinet (J4, pin 9) via a solid orange wire. See if you also have connectivity from the power supply board J4 pin 9 to the S&T sound board J1 pin 7. J4 pin 9 on the power supply board also feeds the backbox general illumination. Are all of the general illumination lights in the backbox working?