[Collins] Source for 75S-1 S-meter adjustment pot?

[Dr. Gerald N. Johnson]

On Tue, 2008-12-02 at 13:22 -0800, Ian wrote:
> If it’s a 3 terminal linear pot reverse the leads on the outside
> terminals and you may find that the wiper then rests on an unworn area
> and will work better.  It’s a quick and dirty attempt at a fix.

Or wire a fixed resistor in parallel from wiper to ground. Say 470 ohms.
That also will move the zero location on the pot, but that resistor may
have to be selected to keep in the range of the pot. Also that shunt
resistor will divert some of the current and the heat from the pot. A
small (24 or 33 ohms) resistor in series with the pot will also move the
operating point.

Last year, I wrote this to an individual asking about the pot: 

> I don't see anything perfect. The closest, is 31VA205-F, 500 ohms
> instead of 250 ohms. It might adjust easier when paralleled in the
> 75S-1
> with a 470 ohm fixed resistor. In fact you could move the operating
> point of the old pot again by hooking a resistor, my guess is about
> 330
> ohms in parallel with it. 
> 
> While it makes adjustment touchier, the Bourns cermet pot ought to
> last
> a lot better than the Alpha pot ($1.40) above. But its $9.16.
> 652-3852A-162-102A. Its 1K so I'd put a 330 ohm resistor in parallel
> with it.
> 
> I'm presuming the S-meter pot is about 7/8" diameter, 1/4" shaft, 3/8"
> mounting bushing. All I have for the 75S-1 is the Yellow book
> schematic, no
> pictures or parts list.
> 
> There is some chance that Mouser has values in stock for these pots
> that
> they don't list in the catalog. Searching for those catalog numbers
> should get you a list of similar parts that you can look through for a
> 250 ohm pot.
> 
> A couple years ago, Mouser had Allen Bradley type AB pots which were
> the
> standard to which all other makers couldn't meet for high quality. And
> then they had military rated pots that were as good as the AB pots.
> Not
> in their catalog now.
> > 
> 

I've seen type RV-4 pots in other catalogs like maybe Digi-Key or
Jameco. Molded pots of military quality. The price on the Bourns pot is
now $10.91.

Mouser shows a RV-4 part, but shows no dimensions. At 500 ohms its a
882-RV4NAYSD501A, $11.04 in stock. They show a 250 ohm but for $8 each
you have to order 85 minimum.

Better yet, Jameco (www.jameco.com) shows a 250 ohm Clarostat RV4 pot in
stock. Jameco P/N 241584 Mfg CLAROSTAT/HONEYWELL Mfg # RV4NAYSD251A
$9.29  each in stock. Its 1-1/8" diameter, 3/8" mounting, shaft long
enough for a knob. You might need to saw a screwdriver slot in the
shaft. Or mount a small knob.

But digging deeper in my receiver manuals, the pot is the size of a 9
pin socket. Not the RV4 size over an inch in diameter and from the
pictures it looks like a 1/4" mounting with a 1/8" shaft.

Like maybe a Mouser 594-149-1102 1K for $7.82 in this catalog, probably
more this year.

Or a Digi-Key number SPRU251C-ND in 250 ohms mil type RV-8 for $8.86.

Or closer to the original, DK number 381N251-ND for $9.79.

With rear terminals DK RV6N251C-ND for $10.73.

With rear terminals, screwdriver adjust, and shaft lock, DK RV6L251C-ND
for $11.83. In stock. Probably the best of the lot. A better quality pot
than the original, still fits.

> Ian, K6SDE
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer