Late reply to this as haven't been around for a while, but Pete Higgs who serviced our desk said those filters can be prone to cutting the signal & creating issues, but frantically twisting them a few times usually clears the issue. We've had it a bit on our desk. It's something to do with the way they're activated in by turning slightly. We didn't have to recap anything to get ours working nicely, just give the pots a bit of repeated twisting.
