[Racal] RA6830-516 Spin Knob Noise Elimination

[Dallas Lankford]

As mentioned previously, microprocessor noise became audible when I =
touched the main tuning knob as I tuned through the 19 meter band while =
checking out my new spin knob.  Apparently this is why Racal used =
plastic tuning knobs on 6830's.

When working on 6830's in the past I had noticed that the main tuning =
shaft (and encoder disc) was completely insulated from ground.  I took =
the knob off and ran a wire from the shaft to the headphone jack.  The =
noise was gone.  I tinned some thin (0.003") copper sheet, used a hand =
paper punch to punch a 0.25" hole in it, cut it with scissors to a =
washer shape with solder tab, soldered a #24 insulated stranded wire to =
it, mounted it on the inside end of the shaft between the metal washer =
and plastic bushing, and soldered the stranded to a nearby ground point. =
 After putting everything back together I turned on the 6830 expecting =
the noise to be gone when I touched the metal spin knob.  But it was =
still there!  Evidently not all grounds are the same.  So it all came =
apart again, another thin tinned washer was made, but this time a small =
hole, to clear 2-56, was punched in the tab using a leather punch, the =
washer was fitted on the outside end of the shaft, between the metal =
washer and plastic bushing, the shaft was inserted and brought down =
flush, the tab was bent around the edge of the bushing and flat against =
the front panel, and a General automatic center punch used to punch the =
drill start point.  It was drilled completely through with a 50 gauge =
drill and tapped out with a 2-56 tap.  A 4-40 would probably have done.  =
When all was reassembled, the microprocessor noise due to touching the =
knob was gone.

It is curious that grounding the shaft to the PC board was ineffective, =
while grounding it to the front panel worked.  This reminds me of a =
similar arrangement used by Collins on the 51J-4.  There a collar with =
set screws (which clamped the collar to the tuning shaft electrically) =
was positioned to keep tension on a tension (bowed) washer so that the =
washer also made positive electrical contact with the metal shaft =
bushing (that in turn made positive electrical contact with the front =
panel).  This grounding arrangement was made to eliminate static-like =
noise which would have otherwise been made as the tuning shaft was =
rotated.  Similar static-like noise was produced by the tuning encoder =
shaft of some R8's, and could be eliminated with an arrangement similar =
to the 51J-4 arrangement.  This noise elimination mod to the 6830 is =
made in the same spirit.  There is a tension (bowed) washer at the rear =
of the shaft.  I do not know whether it originally went between the flat =
washer and the plastic bushing, or between the metal washer and the =
encoder disc.  And I do not know whether the hump of the washer pointed =
in or out.  Since the grounding washer is not mounted at the inside end =
of the shaft, it probably does not matter.  The outside end of the =
tuning shaft contains a retaining ring, a bronze (copper? or?) washer, =
and a steel washer, in that order (from outer to inner).  The home made =
washer with tab goes between the steel washer and the plastic bushing.  =
Positive electrical contact with the front panel is made by the steel =
washer and home made washer interface (under tension due to the tension =
washer at the inner end of the shaft) and by the head (and lock washer =
and flat washer and) the threads of the 2-56 mounting screw.

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