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

[bipi at comcast.net]

Jim,

That is my exact experience as well...the 10-turn precision pot was easy to adjust to zero and it has never moved since I made the modification.  It was so good that I put one in my 51J4 with identical results.  I also have one (of a different value) to put in a 51S1 when I get around to it.

All that being said, I have enjoyed the technical discussions.

Thanks

Mike, K7PI
Mercer Island, WA

-------------- Original message -------------- 
From: "Jim M." <jmiller1706 at cfl.rr.com> 

> 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" 
> To: 
> 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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