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Question about radio and fader
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Chrome58
Posted 2017-08-04 12:50 PM (#545567)
Subject: Question about radio and fader



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Posts: 1316
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Location: Belgium, 40 miles south of Brussels
Hi everyone,

A question for the electronicians around here.
I'm trying to replicate a fader between the front and rear speakers.

Ma Mopar sold one (see pic), and it was simply a single potentiometer, regulating the voltage between the 2 ouputs.
The problem is that every potentiometer that you can buy cannot manage more than a fraction of a Watt of power.

So how do they did it ?

I know the power of the radios at the time was not high, but it was a few watts nonetheless ...





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miquelonbrad
Posted 2017-08-05 12:52 PM (#545635 - in reply to #545567)
Subject: Re: Question about radio and fader



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Posts: 1737
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Location: Hay Lakes, Alberta, Canada
Hi Vincent,
How they did it is, they used a wire wound potentiometer instead of the carbon potentiometers that became common in the early 30's. Prior to that time, radio circuitry was a lot "cruder", so voltages going through the controls was usually much higher. Wire wound potentiometers were the norm, as they can be built with larger gauges of wire, to suit the voltage/current passing through.

I can look through the parts bins in my radio shop, to see if I might have one...
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miquelonbrad
Posted 2017-08-05 3:19 PM (#545638 - in reply to #545567)
Subject: Re: Question about radio and fader



Expert

Posts: 1737
100050010010025
Location: Hay Lakes, Alberta, Canada
I looked at your radio schematics, and they used a 15 ohm pot for that fader control. I don't have anything with that small of a value...
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Chrome58
Posted 2017-08-06 3:22 AM (#545663 - in reply to #545638)
Subject: Re: Question about radio and fader



Expert

Posts: 1316
1000100100100
Location: Belgium, 40 miles south of Brussels
Hi Brad,

Thank you for the answers.

By "wire wound potentiometer", you mean a rheostat, right ?
Something like that : http://www.ebay.com/itm/25W-100-OHM-High-Power-Wirewound-Potentiome... ?

By the way, 15 ohms seems like a really low resistance value.
From what I gathered here and there, I had my mind on a 10K ohms potentiometer for that fader job.
Or at least that's the value they suggest for pre-amp use.
But I suppose that as I'm using it after the amplifier, the resistance value might be different.
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miquelonbrad
Posted 2017-08-06 3:37 AM (#545664 - in reply to #545567)
Subject: Re: Question about radio and fader



Expert

Posts: 1737
100050010010025
Location: Hay Lakes, Alberta, Canada
Yes, rheostat is another term. Term is used more today, than back when the radios were produced...

If you get too high of a resistance, it will load down the output circuit of the radio beyond what it is designed to handle. Another problem that occurs when you put in a large rheostat is that the volume you hear from the speakers will be very low, due to the rheostat using up a lot of the voltage.
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mstrug
Posted 2017-08-06 5:17 AM (#545666 - in reply to #545567)
Subject: Re: Question about radio and fader



Expert 5K+

Posts: 6487
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Location: Newark, Texas (Fort Worth)
15 ohm rheo:

http://www.mouser.com/Passive-Components/Potentiometers-Trimmers-Rh...

18 ohm pot:

http://www.mouser.com/ProductDetail/Bourns/51CAA-E28-D15L/?qs=fqXUT...

more:

http://www.globalspec.com/search/products?page=gs#sqid=21918799&que...

https://www.digikey.com/products/en/potentiometers-variable-resistor...



Edited by mstrug 2017-08-06 5:23 AM
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