The quality of the electron beam in electron defraction tube S (1013889) is affected by the resistance between sockets C5 (cathode) and F4 (heating filament) on the tube. For optimum results, the resistance needs to be 390 kΩ. Tube holder S (1014525) is accordingly designed such that this resistance is present.
Older designs feature a much smaller resistance and need to be modified in order to work with the new S-series electron deflection tube 1013889).
Tube holders affected: U18500, U185001, 1000610