[Boatanchors] HP8554B Mixer failure?

[Arthur Shulman]

Does anyone on the list have experience diagnosing possible mixer
failure in this device?

Symptoms are very low sensitivity (-20 db down) to known levels of input
signal. LO seems at normal level of feedthrough.

I cannot account for the failure. The input of the instrument was never
knowingly exposed to excessive levels. The measured losses through the
input circuitry & attenuator seem to agree with the values in the
troubleshooting section of the manual.

If this is a mixer diode failure, what is the recommended replacement
part?