Hi Alan Bill and Dennis. Thank you for your helpful replies.Unfortunately, the attenuator switch is not similar to the HP652a, it's a double 16 pole 10 way (balanced with 1 dB and 10 dB steps). Today I've tried various cleaners and I think the problem was old switch lubricant that had hardened. After soaking it in Electrolube, and spraying it with a strong detergent (Power Force kitchen cleaner) it was rinsed with boiling water. Now, after drying, it looks good. The only problem is that? the waveform is distorted when the frequency is below 50 Hz.
Chris Hayes