I've noticed excessive through attenuation with my HP attenuator set (8497K / 8494H).
The repeatability was out of spec. After switching there would be a difference of 0.1 to 0.2dB.
I tried IPA on the RF relay switch contacts with no effect. I was hesitant to try anything more aggressive but with little to loose I applied Deoxit Gold ... and it fixed the problem completely.
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The spec for through attenuation is 0.4dB + 0.09dB/GHz & 0.6dB + 0.09 dB/GHz. The repeatability is now much better than spec.
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(I'd already replaced the o-rings)
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vk2bea