[R-390] re: Call for measurements - 100dB carrier reading

[Jim M.]

Also, a 100-ohm 10-turn
is no better than a 10-ohm 1-turn.
------------------------------------
???
A 10 turn pot (such as the one I identified in previous email) will allow 
much finer adjustment of zero, and I believe would be more stable.  I have 
used that replacement in several 390a's now and the meter zero is always 
dead on zero regardless, once I set it.  The 10-turn pot provides good 
vernier control as compared to a single turn.  It's wattage is more than 
adequate.  And it mounts easily.  And it is sealed.  And the shunt resistor 
stays across it.  AND THE METER NEVER DRIFTS.  Me thinks you guys are making 
the problem much harder than it needs to be in terms of the meter zero 
adjustment.  Not to criticize, but I think this subject has turned into a 
"mental candy bar" as they say in engineering, sometimes called "analysis 
paralysis."


----- Original Message ----- 
From: "David Wise" <David_Wise at Phoenix.com>
To: <r-390 at mailman.qth.net>
Sent: Thursday, August 30, 2007 1:23 PM
Subject: RE: [R-390] re: Call for measurements - 100dB carrier reading


Bob Camp already asserted that wattage was not
an issue.  I'll fill in the detail.  13mA * 67.5mV
is less than 1mW.  Going beyond worst-case,
13mA**2 * 100 ohms is still only 17mW.  Unless
you were being deliberately perverse or have
access to nanotechnology, you couldn't make a pot
that can't take this.  They picked a big one so
techs wouldn't need a magnifying glass, that's all.

A 10-turn cermet would be fine electrically, but
aren't they pcb-mount trimmers or something?
There would be mechanical challenges and
it would look odd.  Also, a 100-ohm 10-turn
is no better than a 10-ohm 1-turn.

I haven't read Chuck's ER article.  If he was
just changing R537, he was perpetuating the
same old foolishness.  The lower the shunt,
the more likely that tube variations will
tip you past the point where zero is not
achievable.  Lame!  That's what drove me
to look for something better.

Regards,
Dave Wise

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Perry Sandeen
> Sent: Wednesday, August 29, 2007 10:14 PM
> To: r-390 at mailman.qth.net
> Subject: [R-390] re: Call for measurements - 100dB carrier reading
>
>
> Gents,
>
> over the years the 100 ohm pot was replaced 10 turn pots but
> still using a 2 watt unit.
>
> IIRC, Chuck Felton in his ER article on R390 upgrades shunted
> the post with a low value 1/2 watt
> resistor.
>
> The question becomes: Could one use one of the multi-turn
> cermet type pots instead? They are cheap
> and plentiful.  Multi-turn 2W pots are in the $20 range now.
> I do concede the point that is won't
> look "original".
>
> Had anyone tried this?
>
> Regards,
>
> Perrier
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