[R-390] LINE /AUDIO POTS

Craig hamfish at comcast.net
Thu Mar 17 10:37:27 EDT 2016 

Hello Jacques,

BTW: I'm guilty (not complaining) of noticing the difference between stated values vs measured values of the Line & Local gain pots. Yes, my "Too Loud Amelco" had that issue, sort of? 5K log for the Local gain pot & a 2.5K log pot for the Line gain. (going by memory, didn't write down notes)

Looking at the wiring & solder joins (condition & visual) it had to be from the factory. Or so it seems. The Amelco didn't have a scratch on the green head screws, original tubes, BBOD's, etc. 

I've posted many replies here on the R-390 e-mail reflector. 

#1: The belief here, there is a reason for those pots. 
#2: How many others have measured what is in their R-390/A?

After several years of fooling around with it, (it ain't in the AF deck, ain't in the IF deck) I think the difference is somewhere in the wiring harness. Damned if I can find it. With 2.5K audio (log) pots in both the Line & Local locations, the volume is just fine 20 feet outside of the radio shack with the local gain at zero. 

When the telephonium rings, I'd like to turn down the volume, not the RF gain. At this point in my life, lifting 75lbs of radio is getting old and 5K log pots work.

Regards,
Craig

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Jacques Fortin
Sent: Thursday, March 17, 2016 7:01 AM
To: 'Norman Ryan'; 'Steve Toth'; jupete at internode.on.net
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] LINE /AUDIO POTS

Hi to all,

Despite the trick presented by Steve works practically to "imitate" a log potentiometer from a linear one, it cannot be used on R-390/390A because the end to end value of both local and line potentiometers are a part of the follower tube bias system.
So using a linear pot with the ~500 ohms resistor will change the bias of the tube with the sliders positions, except when both potentiometers are at zero, which is not very practical, listening wise...
Bottom line, log law potentiometers are needed there.
The only other "trick" that will work is to use a higher value LOG potentiometer replacement with a fixed resistor in parallel to make the 2,5K value end to end.
Ex: you find a 5K LOG pot so you put a 5k (4K99) resistor in parallel to obtain 2.5K end-to-end and all will work OK.

BTW: who on this thread was complaining about potentiometers found in a R-390A that measured 5K LOG but were stamped 2500 on the cover ???
I just found two of those "anomalies" in my R-390 parts bin....

73, VE2JFE


-----Message d'origine-----
De : R-390 [mailto:r-390-bounces at mailman.qth.net] De la part de Norman Ryan via R-390 Envoyé : 17 mars 2016 03:44 À : Steve Toth <stoth47 at yahoo.com>; jupete at internode.on.net; r-390 at mailman.qth.net Objet : Re: [R-390] LINE /AUDIO POTS

Great suggestion, Steve. So, with a 2.5k linear pot I just connect a 500 ohm (20% of 2.5k) resistor as you describe -- or should the resistances of pot and resistor be proportionally higher so they add up to 2.5k?
73Norman

      From: Steve Toth via R-390 <r-390 at mailman.qth.net>
 To: "jupete at internode.on.net" <jupete at internode.on.net>; "r-390 at mailman.qth.net" <r-390 at mailman.qth.net>
 Sent: Thursday, March 17, 2016 1:29 AM
 Subject: Re: [R-390] LINE /AUDIO POTS
   
If you need  log pot response from a linear pot one method that works is connecting a resistor that's 20% of the value of the linear pot between the ground end solder lug  and the center variable contact solder lug.
- Steve  K7PZN"Always look for a positive solution then Improvise, Adapt, and Overcome"
"Sometimes it is the people who no one imagines doing anything, who do the things that no one can imagine".....
 

      From: Pete Williams <jupete at internode.on.net>
 To: r-390 at mailman.qth.net
 Sent: Wednesday, March 16, 2016 8:53 PM
 Subject: [R-390] LINE /AUDIO POTS
  
​    G'day  All.....
  Gary Schneider  from  'Playthings Of the  Past... have just  told me that 2.5k pots  only  available in  linear taper.
Cheers
Pete
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