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 Post subject: Airline 62-187 dual volume pot and different volumes by band
PostPosted: Feb Sat 10, 2018 3:37 am 
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Location: Amherst, WI
I’m working on an Airline 62-187 (chassis shared with 62-185, 190 & 196) and found that the volume on the SW band reaches a full, comfortable level with the control less than half open, while the BC/Police bands require opening the control almost all the way to achieve a similar volume.

A closer review of the schematic (http://www.nostalgiaair.org/pagesbymode ... 011330.pdf ) revealed a DUAL VOLUME CONTROL (R2 @ 2500 ohms and R10 @ 500k ohms).

Attachment:
Airline 62-187 dual volume pot.JPG
Airline 62-187 dual volume pot.JPG [ 70.89 KiB | Viewed 285 times ]


It appears that R2 is engaged only when the set is switched to the SW band. R2 is shown just above T1 in this segment of the schematic:

Attachment:
Airline 62-187.jpg
Airline 62-187.jpg [ 110.69 KiB | Viewed 285 times ]


And here is R10:
Attachment:
Airline 62-187 #2.jpg
Airline 62-187 #2.jpg [ 52.27 KiB | Viewed 285 times ]


I don’t understand why a dual volume pot is used or what the function of R2 is supposed to be. Further, I don’t understand why there is such a difference in volume levels depending on the band selected. Is R2 responsible for that or are there other possibilities? How can I bring the volume level for the BC and Police bands up to the level I’m getting on the SW band?

The volume control was very stiff and felt gritty when the shaft was turned so I’ve been dribbling DeOxit into the seams separating the segments of the control and it seems to be helping. The pot may be dirty or damaged as the volume on the BC and Police bands becomes intermittent in the final few degrees of clockwise travel. I noted the strips of metal that hold the sections together but have been reluctant to bend the tabs to access the segments of the control for fear that the tabs would break at the bending point.

I'm assuming that this control is uncommon and I need to make do with what I have. The set has been re-capped, all out of tolerance resistors have been replaced and an alignment has been performed as well. I would define my knowledge and skill level as somewhere between beginner and intermediate.

This might be one of those situations where it would be best to live with it as it is but my curiosity still wants to understand the role that R2 is playing in the operation of this radio. Any insights or suggestions would be appreciated.

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Mike, K9UW
Amherst, WI


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 Post subject: Re: Airline 62-187 dual volume pot and different volumes by
PostPosted: Feb Sat 10, 2018 3:49 am 
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Location: Livermore, CA
Mike

I see where we have a problem. Your link ends up leading to a wrong schematic. Problem on Nostalgiaair.

http://www.nostalgiaair.org/pagesbymode ... 011329.pdf

Most radios only use R10 to control volume. R2 adjust bias on RF tubes. Try jumping out this section of volume control.

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Norm


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 Post subject: Re: Airline 62-187 dual volume pot and different volumes by
PostPosted: Feb Sat 10, 2018 4:07 am 
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Joined: Jan Fri 06, 2012 11:49 pm
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Location: Amherst, WI
Thanks for the information and suggestion, Norm. The schematic link I originally posted does get you to the right schematic but, for some reason, the first two pages are unrelated.

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Mike, K9UW
Amherst, WI


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 Post subject: Re: Airline 62-187 dual volume pot and different volumes by
PostPosted: Feb Sat 10, 2018 5:54 am 
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Norm Leal wrote:
Mike

I see where we have a problem. Your link ends up leading to a wrong schematic. Problem on Nostalgiaair.

http://www.nostalgiaair.org/pagesbymode ... 011329.pdf

Most radios only use R10 to control volume. R2 adjust bias on RF tubes. Try jumping out this section of volume control.


Just tried your suggestion to jump R2, Norm. Works like a champ. Thanks!

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Mike, K9UW
Amherst, WI


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 Post subject: Re: Airline 62-187 dual volume pot and different volumes by
PostPosted: Feb Sat 10, 2018 9:19 am 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
R2 adjusts the resistance between the RF amp tube cathode and ground, and therefore controls the tube's gain. It is switched out (bypassed to ground) in shortwave mode giving maximum stage gain; it is switched in when the set is in the other two bands, and increases RF amp stage gain as the audio stage gain is increased. I suppose that the radio designer's thinking was that shortwave stations were more likely to be weak and needing maximum amplification as opposed to high-powered local standard-broadcast stations.

Many contemporary radios had an RF amp stage ahead of the converter; but the gain was usually held constant (a fixed resistor to ground from the cathode was used). Some sets had a separate pot (not ganged to the volume control) to control the RF amp stage gain but this proved to be problematic (when the control got "accidentally" set to minimum gain, either by operator error or by a prankster, the radio was extremely deaf and this precipitated many unnecessary service calls).


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 Post subject: Re: Airline 62-187 dual volume pot and different volumes by
PostPosted: Feb Tue 13, 2018 4:40 am 
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Joined: Jan Fri 06, 2012 11:49 pm
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Location: Amherst, WI
Thanks to both Norm and lorenz200w for explaining why this set was designed with dual volume controls and for suggesting a solution to the performance problem I encountered.

Your willingness to help is genuinely appreciated!

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Mike, K9UW
Amherst, WI


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 Post subject: Re: Airline 62-187 dual volume pot and different volumes by
PostPosted: Feb Wed 14, 2018 3:07 am 
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Location: Vancouver Island, Canada
On a side note, the right formulation of DeOxit can be the right stuff, but contact cleaner is not always the best for volume controls.
Since a lot of people here use only WD40 on volume pots, I am not spraying hit or miss anymore. I ruined a restorable volume pot by washing the carbon out of it.

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Burl Ives, RIP, oldtimer.
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