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This sequence begins with TR33. The base terminal is held at a constant 1 Volt, more or less. The emitter gets a divided-down copy of the modulated B+ that powers the RF driver and final courtesy of R125 and R125. TR33 remains turned off until the difference between the base and emitter terminals reaches about 0.6 Volts. The modulated audio waveform will drive the emitter of TR33 down to around 0.4 Volts when the negative side of the AM modulation gets near to 100 percent. 100 percent negative modulation is zero amplitude. For this half of the audio waveform, the lower voltage is the higher modulation percentage.

TR33 will only turn on during those brief downward peaks, and when it does it provides a current path to ground from the base of TR31. When this happens we get current from 8 Volts out the collector of TR31 and into the base of TR32. C90 smooths out the current pulses from those negative audio peaks. This smooths out the action of TR32, serving to reduce the mike audio level by shunting it to ground before it can reach the mike amplifier. TR32 is, in effect a variable resistor, controlled by its base current.

Whew!

TR33 is a comparator, triggered when modulation amplitude falls below its sensing threshold voltage. TR31 is a charge pump, feeding the attenuator transistor TR32.

This circuit has been the industry standard for sideband CB radios for decades. Uniden, Cybernet and RCI all use this circuit, albeit with minor changes.

73

<blockquote data-quote="nomadradio" data-source="post: 825674" data-attributes="member: 22">This sequence begins with TR33. The base terminal is held at a constant 1 Volt, more or less. The emitter gets a divided-down copy of the modulated B+ that powers the RF driver and final courtesy of R125 and R125. TR33 remains turned off until the difference between the base and emitter terminals reaches about 0.6 Volts. The modulated audio waveform will drive the emitter of TR33 down to around 0.4 Volts when the negative side of the AM modulation gets near to 100 percent. 100 percent negative modulation is zero amplitude. For this half of the audio waveform, the lower voltage is the higher modulation percentage. TR33 will only turn on during those brief downward peaks, and when it does it provides a current path to ground from the base of TR31. When this happens we get current from 8 Volts out the collector of TR31 and into the base of TR32. C90 smooths out the current pulses from those negative audio peaks. This smooths out the action of TR32, serving to reduce the mike audio level by shunting it to ground before it can reach the mike amplifier. TR32 is, in effect a variable resistor, controlled by its base current.Whew!TR33 is a comparator, triggered when modulation amplitude falls below its sensing threshold voltage. TR31 is a charge pump, feeding the attenuator transistor TR32. This circuit has been the industry standard for sideband CB radios for decades. Uniden, Cybernet and RCI all use this circuit, albeit with minor changes. 73</blockquote>

[QUOTE="nomadradio, post: 825674, member: 22"] This sequence begins with TR33. The base terminal is held at a constant 1 Volt, more or less. The emitter gets a divided-down copy of the modulated B+ that powers the RF driver and final courtesy of R125 and R125. TR33 remains turned off until the difference between the base and emitter terminals reaches about 0.6 Volts. The modulated audio waveform will drive the emitter of TR33 down to around 0.4 Volts when the negative side of the AM modulation gets near to 100 percent. 100 percent negative modulation is zero amplitude. For this half of the audio waveform, the lower voltage is the higher modulation percentage. TR33 will only turn on during those brief downward peaks, and when it does it provides a current path to ground from the base of TR31. When this happens we get current from 8 Volts out the collector of TR31 and into the base of TR32. C90 smooths out the current pulses from those negative audio peaks. This smooths out the action of TR32, serving to reduce the mike audio level by shunting it to ground before it can reach the mike amplifier. TR32 is, in effect a variable resistor, controlled by its base current. Whew! TR33 is a comparator, triggered when modulation amplitude falls below its sensing threshold voltage. TR31 is a charge pump, feeding the attenuator transistor TR32. This circuit has been the industry standard for sideband CB radios for decades. Uniden, Cybernet and RCI all use this circuit, albeit with minor changes. 73 [/QUOTE]

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