There was a known issue with some TDS scopes caused by poor solder on the metal shield on the ceramic attenuator boards. The legs on that shield provide the ground return for the relays. So a bad solder joint can cause all sorts of odd front-end issues.
Be aware that re-soldering is tricky because of the ceramic. If you can't pre-heat the board, be careful and don't apply too much heat from the soldering iron or it will crack. I have soldered a bunch of these, so it's not a super-high risky thing, but just be careful.
