HI-FI on Yaesu FT-840 and President Mckinley AM-FM-SSB

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Alessb
Audio Head
Posts: 2
Joined: Tue Aug 23, 2016 1:52 pm

HI-FI on Yaesu FT-840 and President Mckinley AM-FM-SSB

Post by Alessb » Tue Aug 23, 2016 2:58 pm

Hello to all users of VooDoo forum, this is my first post, although I have read a lot of useful and interesting information.

I currently have two radios, a Yaesu FT-840 ham radio, and a President Mckinley (export) CB radio. I plan to make the Hi-FI modification (by direct injection) on both on them, and I would like to ask a few questions before I get to work.

1. I managed to buy a couple of AM 6KHZ filters for the Yaesu FT-840, one for AM, and the other to replace the narrow original SSB filter (perhaps I could make a relay switch), so the filter part is solved. I have read that driving audio from a mixer or console directly to the balanced modulator (direct injection to the balanced modulator) will give the radio a nice broadcast audio quality.

1) My first question is: if I drive audio directly to the balanced modulator of the Yaesu FT-840, will this improve audio to both SSB and AM modes, or will it only work in SSB?

With regard to the President Mckinley export CB radio, it has the Uniden PC-893 board (identical to the Teaberry Stalker 9 Fdx) and has AM-FM-SSB modes. For what I have read, it seems that in most CB radios the balanced modulator is only used for SSB, and that for the AM mode they use a transistor that series-modulates the final. This radio has the AN612 balanced modulator, and a transistor labeled TR27 (2SC2945AQ) for the unbalance switch.

2) Hence, my second question is, where should I drive the audio for the AM mode of this CB radio, and where for the SSB mode?

Any help would be appreciated. Thank you in advance,

Alessb

Alessb
Audio Head
Posts: 2
Joined: Tue Aug 23, 2016 1:52 pm

Post by Alessb » Thu Aug 25, 2016 2:48 pm

Hi everyone.

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. Below, I have uploaded two schematic diagram of PC-893 to discuss the audio route for the AM mode and FM mode.

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.
Image

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.
Image

I did not upload a diagram for SSB mode, but I figured that the 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,

Alessb

kd2nfc
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Joined: Sun Jan 28, 2018 4:50 pm

Post by kd2nfc » Sun Jan 28, 2018 5:17 pm

1. I managed to buy a couple of AM 6KHZ filters for the Yaesu FT-840, one for AM, and the other to replace the narrow original SSB filter (perhaps I could make a relay switch), so the filter part is solved. I have read that driving audio from a mixer or console directly to the balanced modulator (direct injection to the balanced modulator) will give the radio a nice broadcast audio quality.

how did you make out with this for the FT-840? I'd like to do this mod, do you have any diagrams on the changes I gotta make.

show me the way :shock:

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