Sears Volume/Power Control - Printable Version

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+--- Thread: Sears Volume/Power Control (/showthread.php?tid=1551)

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Sears Volume/Power Control - NV3G - 12-13-2008

I am presently working on a Sears Model 1708 radio. It has a dual potentiometer of which 500K is the volume control and 1K is the variable cathode resistor for the RF ( 78 ) tube and the Oscillator/Detector tube ( 6A7 ). I do not understand this type of design. What did the engineer have in mind? How does it work when your increasing the volume control for audio and decreasing the cathode resistance in the cathode circuits of the RF and Oscillator/Detector Stages?

Any help would be greatly appreciated and where do I find a replacement control? Perhaps a modification is in order.

I am confused.

Thank you,
nv3g

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Re: Sears Volume/Power Control - Texasrocker - 12-13-2008

Is the Silvertone 1708 a trf design? Does the volume control the amt of antenna into the circuitry? If so, I may be able to help explain, depending on your radios-chassis design.Is the pot a "linear taper" rheostat type control? Randal

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Sears Volume/Power Control - NV3G - 12-13-2008

The Sears 1708 radio is a superheterodyne receiver with a 175 KC intermediate frequency. The 500K potentiometer is between the ( 75 ) tube detector and audio grid. The schematic can be found at

http://www.nostalgiaair.org

I do not know what the taper of the potentiometer or rheostat is. The rheostat is the variable cathode resistor in the RF amplifier ( 78 ) cathode and the first Oscillator/Detector tube ( 6A7 ). I have not seen this design before where your increasing the audio volume and decreasing the cathode resistor at the same time.

Thanks for your interest.

nv3g
Oscar -

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Re: Sears Volume/Power Control - Texasrocker - 12-14-2008

Im not sure if this theory applies to your set,most probably not if your set has avc circuit, but I have run across similar circumstances in TRFs before using a dual-type pot resistance setup. Whereas, when the wiper is turned up from the ground end of the pot, the resistance (the signal strength) increases at a slow rate for about the first 1/3 of rotation, then, much more rapid increase in volume as pot is turned all the way up to max volume.
The overall effect on the trf sets ,is that this gives a variable amt of control, slower increase at lower volume ( higher signal strength)end of pot, and a faster increase after about 1/3 of full rotation. In the trf sets,this is a early attempt to acheive "audio taper" from a linear taper design. I have the JF Riders books, I will take a look at the schemat when I get back to my workshop. I have trouble seeing the nostalgiaair pages on my small laptop. Randal

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