Siemens V276 mic preamp - loud pops

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I have several Siemens V276 mic preamps. They all work fine,
sonically. But some produce very loud pops when engaging the dB
selection switch. As some of those preamps come from different
sources, I assume that there is a common problem with this.

So far, I changed the selector in one unit (which is very expensive
and time-consuming to install) - with no improvement. Also, I changed
the big electrolytics - no improvement either. An audio engineer spend
quite some hours in trying to find the fault - but he did not succeed.

Does anybody have experienced the same fault, and did find a solution?

Thanks
Ulrich