[Milsurplus] ARC-5 Pot Taper

J. Forster jfor at quikus.com
Mon Jul 29 23:52:13 EDT 2013


Thanks Mike.

I'm thinking of trying a 100K audio taper, or possibly a small SP rotary
switch and a half-dizen or so resistors in a log ratio.

Best,

-John

===============



> 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
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