[sdiy] de-tuned by transport?

[Tony Clark]

>     Also, you should lock the pot down with glypt (I think I spelt 
> that right).  If you can't find any, I have used finger nail polish to 
> do this job...in fact, one place I worked, we purchased AVON polish.  
> The woman who serviced our account must have thought we were pretty 
> strange...a bunch of guys buying all that finger nail polish....

   Funny!  We use nail polish here too!  Usually whenever we need any, we 
either ask the girls here if they have any that they don't like OR we go 
to any dollar store where the "gaudy" colors are dirt cheap!  We've even 
used nail polish with glitter!  We shoulda charged more for those units!

   Anyhoo, it's usually not necessary to lock the turning mechanism down 
since most quality trimmers are fairly stiff to start with.  The only 
benefit that glopping the trimmers has is if you want to know if anyone 
else has ever mucked around with your settings.  Great if you ever have a 
"warranty" repair yet find the trimmers have been moved...

   The question that should be asked is what _kind_ of trim pots were 
used.  There are basically two types of multi-turn trimmers.  The square 
kind and the elongated rectangular kind.
   The best type to use are the square kind because as far as I know, the 
wiper action is rotary.  In the elongated rectangular type, the action is 
linear, which has a disadvantage in that there tends to be some measure 
of "sticky"-ness to the wiper motion.  I have noticed, when calibrating 
units with these types of pots, the wiper can make sudden "jumps" when 
turning the pot, causing overshoot if you are calibrating something quite 
delicate.
   I got into the habit of actually "thumping" pots to make sure that 
there wasn't any latent pressure being applied to the wiper that might 
cause it to be unseated later (like during transportation).  Usually 
giving the pot a good whack on the top is enough to remove any pressure 
from the pot.
   Now while I haven't really seen this kind of behaviour from square 
trimmers, it is possible that the same conditions may apply.  One way to 
avoid this would be to reduce the amount of effect that the particular 
trim pot has on the circuitry, that way any slight change in the pots 
position won't have a great effect on the behaviour of the circuit.
   Good luck figuring it out!

   Tony

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