The secondary power supply (SPS) is a board that supplies regulated +5VDC and -8VDC. It also has the circuitry to control the standard lamp (SL), mercury lamp (HG) temperature monitoring, stepper motor power and high voltage power (HV).
Note - Part of the SL, HG, and HV circuitry lives on the heatsink.
Note2 - The SPS board also includes the rate meter circuitry, however this is not used and usually shorted out to by pass it.

To identify if the SPS board is using the new or old temperature circuit, you will need to locate the following