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Further along...

Was able to adjust offset and gain of ALC response on 5086-7906 TG per the service manual procedure. Performance is within specification, but there is a big 'hole' in the flatness at the low frequency end that varies with output level. Perhaps this is the nature of the beast, given the absolute accuracy specs that allow for ~3-4 db variation total over the operating range. Wanting to check the spectral purity, but lack a receiver / analyzer in the 100 - 500 MHz range at the moment (VNA in 'Tuned Receiver' mode tried, needs further study). It might be spurious outputs confusing the detector, as has been suggested in these threads, ??#103226???#103124???Or, something else. Have seen some instability and oscillation in the ALC at the low frequency end from cold start. Would very much like to see a schematic of the 5086-7906..! ALC control voltage from the A10 board looks good, see stable ramps in sweep and quiet at zero span. Using 8471A detector and external ALC did not achieve leveled condition, but some response to level adjustment was seen. Have found little information on the external ALC mode for 8593E, other than with some special options it cannot be used..??

Waveforms show open loop output, 0 dBm, and -10 dBm at 0 dB step attenuator.

Thoughts..?