I just looked again at the OP, noticing the 7 dB down figure. I'm not sure if these are full-or half-wave detectors, but if full, then there are two diodes. If one burns out, then I think it will be all out of balance, and lose at least half the power signal. If it turns out to be equivalent to half the voltage, then there's 6 dB right there, pretty close to the observation. If one diode remains, it should still get some measurement signal, so might zero out OK but still not measure right.
If there's only supposed to be one diode, then I think it could be a partial damage scenario as others have said.
Ed
