[R-390] Potent I/O Meters...

[Drew Papanek]

On potentiometers, John KA1XC wrote:

>I don't know what to recommend as a good cleaner or lubricant now, it 
>almost
>seems like voodoo. Some audio guys swear by WD-40, and others swear at it.

I've used WD-40 and it seems to work reasonably well, although it gums up 
after  a while.
I have also had success with 100% isopropyl alcohol (applied generously with 
an eyedropper).
It also works well to remove solder flux and to clean tape heads.

Let it evaporate before operating the equipment; alky is extremely 
flammable.

The most readily available small quantity source I have found is isopropyl 
formula gasoline dryer/antifreeze.

Rubbing alcohol is usually 70% iso alky and the rest water; hence does not 
work  well.

>Often the 390 pots are not just a little dirty but actually worn out. I've
>had a couple of RF Gain pots that were mechanically worn out at the 10
>o'clock position that they normally sit at; there was simply no more
>resistive element at that one spot.

Sometimes the wiper can be bent to sweep a different radius and ride on a 
previously unused portion
of the element.  That worked well for a Mazda volume airflow sensor in a 
friend's car (the sensor is a pot whose shaft is coupled to a spring-loaded 
vane in the air intake).

>I'd definitely like to find a good replacement source for the Audio/Line
>pots, but 2.5K panel pots with .25" shaft and audio taper are near
>impossible to find;  if they were 5K it wouldn't be so bad.

One could use a 5k unit with 4.7k or 5.1k fixed resistance parallelled 
across the element.  That would keep the load impedance presented to the 
previous stage at the design level.  The source impedance feeding the 
following stage would change but  would have no effect  as the input 
impedance of the following stage is many times higher than that from the 
pot's wiper in any case.

>  I've actually thought of trying a dual 5K pot with all the connections 
>paralleled.

How about a "stereo" pot?  Concentric line and local level controls would be 
a neat feature if suitable knobs could be found  :)

Drew

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