The touch panels use +5 and +12 VDC, and the control panel uses those voltages as well as the -12 VDC. The easiest place to measure those voltages is on the Power Regulator board. There are also LEDs and voltage test points on the rear panel of the DC power supply.
The problem might be due to one or more open fuses on the Keyprocessor I/O board. There are eight 3A pico fuses on the board. F8 is a spare.
If you need support or parts, Advanced Ultrasound Electronics can help you. We have most parts in stock for the HP/ Agilent/ Philips Sonos 5500 and 7500 systems.