Concentric Control Knob

There are a very limited number of options to fit more controls on the front panels of rebuilt Heathkit SB-Line units. It is possible to reduce the spacing between controls a little so that three controls can be placed to the left and right of the main tuning control but this may not be sufficient so an alternative strategy was required. I chose to convert some single controls to a concentric type where the appropriate potentiometers or switches were available.

The photo below shows the original Heathkit control knob on the left and the new control on the right.

The skirt was removed by carefully drilling into the retaining metal inside the knob with a large sharp drill until the skirt came loose and could be removed.

All mechanical work except the hole for the grub screw was carried out on a Seig micro-lathe. The new control is made from 1.125inch diameter stock solid aluminium bar. The central 0.25inch diameter hole is drilled first.

Then the front cavity created but do make sure that the cavity is not so large that the retaining screw for the inner knob is obscured. The corners are deburred with a fine file and finished with a fine rubbing down paper. The outer surface was knurled just enough to provide an easy gripping surface.

The hole for the grub screw was created using a pillar drill and the correct sized tapping drill and the thread tapped using a tapered tap first and then a second or plug tap. Remove any burrs.

The final finish can be a clear or coloured Anodise or an Alocrom depending on the colour required.