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I have a President Mckinley export CB radio with the Uniden PC-893 board (identical to the Teaberry Stalker 9 Fdx) with AM-FM-SSB modes. It has the AN612 balanced modulator, and a transistor labeled TR27 (2SC2945AQ) for the unbalance switch.
I would like to make the direct injection modification to improve audio quality. I have a Behringer mixer with mic compressor, a Samson mic, and a passive box (impedance match).
I have read about this procedure in the forum for Cobra and other President radios. For what I have read, it seems that in most CB radios the balanced modulator is only used for SSB, and that for AM mode they use a transistor that series-modulates the final.
My question is, where should I drive the audio that comes from the mixer into the CB radio for the AM mode? And for the SSB mode?
Any help is appreciated, thanks in advance
-- Thursday, 25 August 2016, 13:44 PM --
I have been doing some further reading. Apparently, given the circuit layout, the standard direct injection procedure for the Cobra 148gtl (driving audio to C174 and so on) is only effective for AM only, not SSB. To feed the balanced modulator for SSB, a switch is needed to change the audio feed from the series pass modulator in AM to the balanced modulator on SSB.
However, for the President Mckinley with PC-893, direct injection seems much simpler, and should work on all modes. I could not upload the diagrams I made, but below is the discussion of the audio chain.
In AM mode, the signal from the MIKE GAIN is fed to TR31 and TR29, the mike amplifier circuit. The UNBALANCE SWITCH TR27 shorts to ground pin 1 of the balanced modulator AN612. Hence, AM TX AUDIO is fed to TR46, TR47 and TR48, which are the modulator transistors. The carrier power level is controlled by VR11. The SSB switch TR49 is open. The FM switch TR50 and FM limiter TR30 are open. Finally, the AM TX AUDIO is fed to the DRIVER and FINAL transistors.
In FM mode, the UNBALANCE SWITCH TR27 shorts to ground pin 1 of the balanced modulator AN612. The SSB switch TR49 is open. The FM switch TR50 turns on to short the audio output of TR31 and TR29 (audio amplifiers) before it reaches TR46, TR47 and TR48 (modulators). Instead, the output of TR31 and TR29 is fed to the FM limiter TR30, and from there to the VCO through VR127 and VR8.
In SSB switch TR49 shorts to ground TR46, TR47 and TR48 (modulators). The FM limiter TR30 is open. The UNBALANCE SWITCH TR27 is open (pin 1 of the balanced modulator AN612 is no longer shorted to ground), and hence the output of TR31 and TR29 (audio amplifiers) is fed to pin 1 of AN612.
From all this, it seems that a common node of the audio chain is the output of TR31 and TR29 (audio amplifiers). If I drive the audio output of the mixer to the output of TR31 and TR29, the HiFI audio should work in all modes. Is my analysis correct?
Any advice will be appreciated. Thanks in advance,
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