A bias circuit for biasing a field effect transistor to provide a transconductance that is substantially unaffected by power supply voltage variations. The magnetic brush rubs the image carrier to thereby develop a latent image formed on the image carrier. The service processor executes a routine to poll a memory location in each processor in the system to determine if the processor has entered an error loop with interrupts disabled. The reactor further includes apparatus that can apply different levels of electromagnetic radiation power to different ones of the plural concentric coaxial waveguides.