Doesn't sound like the LO drive. If the drive level is 3-4 dB below the nominal, conversion loss will only increase by less than a dB for most mixers. Sounds more like a pre-amp failure if it has one - or some sort of unwanted attenuation in line with the input.
Alan
One maybe less obvious source for such attenuation might be the connector. I've seen several dB of attenuation at several GHz in the past when people mixed up 75 Ohm and 50 Ohm N-connectors. You can damage the connector that way, because the center pin is thicker but will fit if you try hard enough ;-) After that, there might be a small airgap if you use the proper connector. This airgap (or reduced spring strength of the connector) can cause several dB of attenuation.
Peter
Surely, the Marconi 2383 has a 50 ohm input socket if it goes up to 4.2 GHz? The issue with N-connector centre-pins is when a 50 ohm plug is forced into a 75 ohm N socket, as used on link analysers, attenuators and suchlike, usually not for microwave frequencies.
Many seem to believe the same is true of BNC's but all the 75 ohm ones I've come across have had the same diameter mating part of the centre-pin.
Chris
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