[Milsurplus] ARC-5 Pot Taper

[Mike Morrow]

John wrote:

> Does anyone know if the 50k Ohm pot in the standard ARC-5 Control Boxes
> is Linear or Log taper?

The ARA RF gain pot for CBY-23155, -23251, -23261 is specified as:
"Resistor, variable, 0-50,000 ohms, tapered per assembly 6488"
It is A.R.C. part number 6488.

The SCR-274-N RF gain pot for BC-450-A, -473-A, -496-A, is specified as:
"Variable, 0-50,000 ohms"
It is A.R.C. part number 6488.

The AN/ARC-5 RF gain pot for C-26/ARC-5 is specified as:
"Resistor, variable, composition, 0 to 50,000 ohms +/- 20%"
It is A.R.C. part number 6488.

Note that the parts description for the ARA implies a taper of some sort.
Since all three systems use A.R.C. part 6488, there is then a taper of
some sort in all three systems.  It is a long-standing ham *myth* that the
command set receiver RF gain potentiometer is linear taper.  There *is* a
pronounced taper.  The potentiometers that I have tested (from all three
systems) typically have a maximum resistance of about 60,000 ohms, which
just meets the 20% tolerance in the AN/ARC-5 specification.  The resistance
to ground from minimum RF gain to maximum is typically in the vicinity
of: (CW = Clockwise)

  0% CW     60,000 ohms   [Mimimum RF gain, Full CCW]
 25% CW     18,000 ohms
 50% CW      5,000 ohms
 75% CW      1,000 ohms
100% CW         40 ohms   [Maximum RF gain, Full CW]

With ageing, some of these pots will have significant increases in the
maximum resistance...75,000 to 95,000 ohms maximum is common.  But
the taper is preserved.

All that having been said, I've always found 50,000 ohm linear taper
pots easy to find, and audio taper versions difficult to find for
that resistance.  Linear taper seems to work acceptably.   

Mike / KK5F