The 8570A spectrum analyzer (similar to the 8569B analyzer) works fine on all six bands except the 3.5 to 8.5 GHz band. This third band behaves erratic. Sometimes the signal level is reduced by about 30 db for no apparent reason, while other times it works fine. I can somewhat compensate for this "gain" reduction by changing the Preselector setting but it is not a stable result, since the gain can change frequently. The instability is easily seen when I use the internal comb generator to view the signal strength across the 1.7 to 22 GHz region, which covers five of the six bands. When everything is working good I see a progressive decrease in signal amplitude with increasing frequency. However, when the 3.5 to 8.5 GHz band becomes unstable the signal level decreases in this band and becomes abruptly very small from 5.5 to 8.5 GHz. I can partly compensate for this change in the spectrum by adjusting the Preselector control but it is not a stable adjustment. The only thing that does not change in time is the stepwise baseline pattern (defining the background noise level), which remains constant for this band as well as all others. I have read through the operator and maintenance manual describing the instrument and adjusted the bias and slopes for each channel to calibrate the instrument but this does not resolve the problem. So far I have not been able to isolate the cause of my problem. Any help would be greatly appreciated.