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 Post subject: Schematic Interpretation
PostPosted: Feb Sat 14, 2004 8:38 am 
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Location: Merrick,NY,USA
Hi Guys,<P>I'm working on a Packard Bell 45M and I've run across a schematic symbol that I don't quite understand. It is for the volume control. It looks like a 7500 Ohm control except a straight line runs thru the symbol on one end and an extra 175 ohm section is added. There is a notation under the symbol that says: "volume control has special curve" The original control on this radio is gone and was changed to a 10K. But it only adjusts volume at the last qurter or so of the pot revolution. I'd like to try and approximate the operation of the original control but can't quite decode the symbol. Any ideas?<P>Mike<P>------------------<BR>


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 Post subject: Schematic Interpretation
PostPosted: Feb Sat 14, 2004 9:13 am 
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Hi Mike,<P>This is a common volume control configuration. The schematic symbol denotes a control which has a mechanical stop such that you can't turn it all the way to the end in the clockwise direction.<P>The pot is actually connected to perform two different functions with a single control. The left-hand terminal provides a resistive attenuator at the antenna. As you turn the volume up, the resistance to ground increases. The right-hand connection provides the cathode bias resistor for the RF amplifier stage. As you turn the volume up, this resistane to ground decreases.<P>The idea of this control is that when you turn the volume all the way up, you still have 300 ohms in the cathode circuit of the RF amp, to prevent excessive plate current.<P>You can duplicate the function with a standard 10,000-ohm control by adding a 300-ohm fixed resistor between the right-hand terminal and the cathode. The schematic I checked (Rider's 6-4) shows the original value as 10,000 ohms with a 300-ohm cathode bias leg.<P>The "curve" is called a logarithmic taper, or simply a log taper. Commonly used for volume controls. The control currently installed in the radio is probably linear taper, which would explain the unequal control over the range of rotation.<P>VERY IMPORTANT: This control must be wire-wound. You can't use a carbon composition control in a circuit which carries current, i.e. in the cathode circuit.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


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 Post subject: Schematic Interpretation
PostPosted: Feb Sun 15, 2004 9:07 am 
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Location: Merrick,NY,USA
Hi Leigh,<P>Thanks for the thorough, detailed reply. This helps a lot. Can you elaborate a little on why you can't you use a carbon comp pot ? Are we talking a lot of current here that only a high wattage wirewound can handle? <P>Mike<P>------------------<BR>


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 Post subject: Schematic Interpretation
PostPosted: Feb Sun 15, 2004 9:52 am 
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Hi Mike,<P>Yes, it's a question of current carrying capacity within the control. In the cathode portion of the winding, the current increases as the resistance decreases, because you're decreasing the bias voltage on the tube. Carbon comp elements are only designed to handle a few mils. So, at the high-volume end of the rotation, you might draw enough current to damage the element. If the element opens, the radio is completely dead, because you've opened the cathode circuit in the first RF stage.<P>You might get by with a carbon control, but I wouldn't vouch for its reliabilty. The type of radios which used this volume control circuit usually didn't have much volume, so you would expect to run the control near the upper end.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


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 Post subject: Schematic Interpretation
PostPosted: Feb Sun 15, 2004 6:21 pm 
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There's a table on page 99 of the Mallory / Yaxley Radio Service Encyclopedia volume 1 which gives maximum current capacity for carbon controls of both linear and log tapers. A 10,000 ohm control is ratted as 15 ma for linear and 10 ma for log @ 100%, i.e. full resistance. A 100k control is rated 4 ma linear and 2 ma for logarithmic @ 100%.<P>These are just examples, but it points out that carbon controls are not designed to carry significant current.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR> <A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A>


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