[R-390] Allen Bradley Potentiometer Numbering Schemes anyone?

[2002tii]

Barry wrote:

>Does anyone know where I can find out the value/type of an AB pot I 
>have?  It's part of a "new"
>(to me) Krohn Hite function generator and I have no schematic for 
>it.  It's the main frequency
>control that's getting a bit erratic and would like to look for a 
>possible replacement.
>
>All it has are some numbers and letters stamped on the back and I 
>assume some of those indicate
>the resistance, type, taper, etc.,

Not necessarily a good assumption.  The Allen Bradley Type J was a 
very common part for decades.  They are carbon composition pots, 
available from stock in both linear and "log" taper (actually two 
different linear tapers approximating a log taper).  However, many, 
many custom types were made (reverse log, three-decade log, 
four-decade log, multiple taps, shaft options, etc., etc).  The 
number could well be a Krohn Hite number.

You should be able to determine resistance and curve with an ohmmeter 
(center the wiper, measure the resistance from top to bottom and from 
one of the ends to the tap.  The resistance to the center will tell 
you if it is linear or log, and if log, how many decades and whether 
it is normal (CW) taper or reverse (CCW) taper.  You should be able 
to find resistance curves for the various log tapers easily on the 
web, and they will make evident what you are measuring.  Don't expect 
precision -- it's a composition pot -- pick the closest curve to what 
you measure, and look for one of those.  (If it turns out to be a 
reverse log part, be prepared to look a long, long time.)

Have you contacted Krohn Hite?  They are usually pretty good about 
having parts for older units.  http://www.krohn-hite.com/

Best regards,

Don