[R-390] RF GAIN pot ?
Craig
hamfish at comcast.net
Fri Mar 18 11:14:11 EDT 2016
Jacques,
No, not crazy............but I'll add more confusion. Those numbers on the
back of CTS pots might only have meaning to CTS. Haven't been able to find
squat. The RF gain pot in my junk box has on the back......SM-C-283204-2
5K CTS8524............go figure?
But according to the web and how to measure a pot; it is 5K ohms reverse log
taper.
Still learning,
Craig
-----Original Message-----
From: Jacques Fortin [mailto:jacques.f at videotron.ca]
Sent: Friday, March 18, 2016 7:55 AM
To: 'Craig'; r-390 at mailman.qth.net
Subject: RE: [R-390] RF GAIN pot ?
Hi Craig,
So I am not crazy: the original RF GAIN pot on the R-390A is a REVERSE LOG
one !
Meaning that it have less resistance variation at the end of the shaft
rotation than at the beginning.
So the reverse that the AF GAIN / LINE GAIN controls does.
I never figured this before...
CTS SW1376 also leads to NATO P/N 5905-01-148-3835, but even searching for
this one does not provide more clues about the track taper.
And the R-390 part is the same, btw, in the October 1953 Collins manual
(first manual printed for the R-390, order 14214-P-51) on page 199.
Wondering why it is that... to facilitate the squelch adjustment ?
You know, the Squelch option (relay operated) that was never fitted in
production for the 390A but being there in the 390 ?
If someone can otherwise explain why a reverse-log pot was fitted there,
please do !
73 to all,
Jacques, VE2JFE
-----Message d'origine-----
De : R-390 [mailto:r-390-bounces at mailman.qth.net] De la part de Craig
Envoyé : 18 mars 2016 01:27 À : 'Jacques Fortin' <jacques.f at videotron.ca>;
r-390 at mailman.qth.net Objet : Re: [R-390] RF GAIN pot ?
Jacques,
Yes, I have audio pot on the brain! My mistake. So digging into the junk
box..........found a CTS 5K pot from an R-390/A. It still has the jumper
between lugs 2 & 3. Must be a RF gain pot. Removing the jumper and measuring
shows this pot is a 5K ohm audio (log) reverse taper, I think?
Putting us on the same page. With the shaft facing me and turning the shaft
fully counterclockwise lug #3 per the schematic is the counterclockwise lug
(least resistance from wiper). End to end the pot measures about 4.63K ohms.
Counterclockwise lug to the wiper with the shaft half rotation, is about
3.48K ohms.
I Googled the part number ((71450) type SW1376)) for the RF gain pot and got
nowhere as to its specs.
Hope this helps and makes sense.
Regards,
Craig
-----Original Message-----
From: Jacques Fortin [mailto:jacques.f at videotron.ca]
Sent: Thursday, March 17, 2016 4:58 PM
To: 'Craig'; r-390 at mailman.qth.net
Subject: RE: [R-390] RF GAIN pot ?
Craig, RF GAIN I'm speaking about now...
For your question: no, not really. A reverse log pot wired in reverse (as
you suggest) will have the minimum volume at 10 and the maximum at 0.
But at least the progression will be "normal" for a volume control.
Good for the other side ??
73 ! VE2JFE
-----Message d'origine-----
De : Craig [mailto:hamfish at comcast.net] Envoyé : 17 mars 2016 17:46 À :
'Jacques Fortin' <jacques.f at videotron.ca>; r-390 at mailman.qth.net Objet : RE:
[R-390] RF GAIN pot ?
AND THE PLOT THICKENS
To Jacques & All,
The 14-PH-56 rings a bell, got one of those also! It appears to be
normal.......has CTS line & local pots 2.5K audio (log) taper. The Motorola
supplied AF, IF decks, etc., while trying to make the "Too Loud Amelco"
normal.
The parts list, Y2K R3, calls for a RV4ATSA252D pot which is a normal log
taper 2.5K 20% tolerance.
Question: Ok, if it is reverse taper......reverse the wires connecting to
the ends of the pot and zero would be less (no) audio & ten would be loud
audio?????
Craig,
-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Jacques
Fortin
Sent: Thursday, March 17, 2016 12:51 PM
To: r-390 at mailman.qth.net
Subject: [R-390] RF GAIN pot ?
Hi to all,
Is it possible that the RF GAIN pot (listed as a 5K) is a reverse log one ?
The one I have in my parts bin (came from a Moto 14-PH-56) measures 10k,
despite it is identified as a 5K on the cover: 318K147 5K CTS650 and when
the shaft is dead center, I got 8K between CCW contact and center, 2K
between CW contact and center.
Looks like a reverse log, right ?
73, VE2JFE
-----Message d'origine-----
De : R-390 [mailto:r-390-bounces at mailman.qth.net] De la part de Jacques
Fortin Envoyé : 17 mars 2016 13:53 À : fengjs at juno.com;
r-390 at mailman.qth.net Objet : Re: [R-390] LINE /AUDIO POTS
I'm sorry but.... not sure of that !
73, VE2JFE
-----Message d'origine-----
De : R-390 [mailto:r-390-bounces at mailman.qth.net] De la part de
fengjs at juno.com Envoyé : 17 mars 2016 11:02 À : r-390 at mailman.qth.net
Objet : Re: [R-390] LINE /AUDIO POTS
It seems to me that a well-placed capacitor could give the pseudo-log taper
without upsetting the bias point.
---------- Original Message ----------
Date: Thu, 17 Mar 2016 10:01:00 -0400
From: "Jacques Fortin" <jacques.f at videotron.ca>
To: "'Norman Ryan'" <nnryann at yahoo.com>, "'Steve Toth'"
<stoth47 at yahoo.com>, <jupete at internode.on.net>
Cc: <r-390 at mailman.qth.net>
Subject: Re: [R-390] LINE /AUDIO POTS
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"
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 Envoy? : 17 mars 2016 03:44 ? : 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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