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Direct Injection in President Mckinley AM-FM-SSB

A

Alessb

New Member
Aug 25, 2016
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Hello everyone.

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. 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 been reading in the forum about direct audio injection in similar CB radios, in particular to the Cobra 148gtl. It seems that in the circuit layout of this Cobra, the direct injection procedure (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. Below, I have uploaded two schematic diagram of PC-893 to discuss the audio chain for the TX 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.


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.


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
 

Thanks 399, I will search a little more about this issue. If things are like they seem to be, I'll give it a try soon.

If it works, I'll upload some pictures and comments of the mod.

Regards,

Alessb
 
Greetings,

I've successfully pulled this off with a Cobra 2000 and 148, going around the high level modulator and using the AN612 as a low level modulator and running the drive and final in linear mode.

Although I only glanced at your schematic, I didn't see where in AM mode you go around the SSB filter. It looks while in AM mode, to me on the block diagram, the carrier oscillator goes through the SSB filter. It's that way on my Cobra 2000/148 as well. I think your keeping the radio in the AM mode so the RX works normally using the mode switch? I found to make this mod worth it the mode switch had to be correct.

If, in fact, the radio is in AM mode but there is no provision to switch out the SSB filter you can say goodbye to all that audio and your carrier. I used a 4066 to route IF signals around, including bypassing the carrier (+modulation) through the ssb filter while in "AM" mode.

When switching to SSB mode, you will only have a different tone, not much in bandwidth because the SSB filter will strip off the carrier and either sideband down to somewhere around 2.1 KHZ.

You can, again, jumper around the SSB filter but you will have DSB (double sideband) output. It will contain all the bandwidth you can give it but the listener probably has a CB style rig and has the same ugly filter in their RX chain. So there goes all the wide band width at the receivers end. I guess you could jumper around the SSB filter on RX but the selectivity would be destroyed. Not really an option.

So, overall this mod really kicks a$$ on AM. It is certainly superior to the MMM or other "modulators" (Whatever that means??). The bandwidth is absolutely incredible (My 2000 is 1 - 20khz + ). Audio is smooth as slik, so articulate and I feel the actual nuance of one's voice is projected more precisely resulting in a natural tone at the listener's end.

Modulation percentage's appear to break the law of physics. Although it looks cool on the scope, many receivers in CB's use simple diode detection and are incapable of clear demodulation when lowering the carrier in an attempt for more audio. This type of carrier lowering is different from other methods which use some sort of active or passive device in the the B+ line of high-level modulated CB transmitters. The bottom line is pulling out to much carrier because it looks good on the scope won't do much except cause the nastiest sounding crispy edges on a diode detector however using a PLL type AM detector will give a significant decrease in distortion with "lop-sided" AM.

I bring that up for a reason. ALC - having control of the ALC and carrier level is really wonderful and it's nice to have. On the stock 2000 (in SSB mode) the RF output is sampled and fed back to the audio preamp. Which sucks because feeding the AN612 directly bypasses that part of the transmitter. There are many an option here to get a working ALC signal. I've investigated a few ALC methods and currently the 2000's ALC action is at the RF level and not the audio level but audio based versions seems to work well too.

Some other findings I came across:
The AN612 seems to like a buffer in between the audio source, which in my case is the line-out from a pc, and it's audio input. I used a J-FET acting as such as it worked great. I highly recommend this.

This may be an issue to the 2000 only. I found the carrier control used to balance out the carrier to have a lack of bias to obtain 4 watts output so a new bias source with more control was cooked up and worked great. I recommend a variable bias here if lots of carrier control is wanted or the current ssb balance adjustment is unable to supply enough bias to generate a suitable carrier.

The collectors of the driver and final were connected to B+ directly as the stock AM collector volts was around 8, or so. This only matters if you have the radio mode switch in AM mode while in rx.

Verify class AB operation - super important.

Audio processing is a must. Currently my setup consists of a variable Q 12 band graphic EQ, side chain d-sser, mutiband compressor and finally a mutiband limiter. It's all software on a pc running linux studio and the right price. Free.

I hope this helps you. I've done both my 2000 and a "newly" acquired 148gtl with this "mod'. I think it's fantastic and always great audio reports. Proof that one doesn't need MMM's, extra power supplies, a new radio, or what ever. I spent my money on a new mic not a board with few op-amps.

Cheers!
 
  • Like
Reactions: Shadetree Mechanic, Alessb, LeapFrog and 2 others
Just curious, if this mod only works on mostly AM, why not just use an AM radio instead of this one? Not trying to be offensive, just more curious. These are classic radios man. No need to mess them up trying to gain some bandwidth is there?? Or am I way off base here? Why not just use one of the newer radios like others have done and get the same or better results? Again, I'm more curious than anything so please don't flame me for asking, or if it seems like a dumb question. Have a great day and thx for any and all responses.
73 and God Bless.
 
  • Like
Reactions: tecnicoloco and rabbiporkchop
Hello 222dbfl!!

The basic reason I , personally, started down this road is simply audio bandwidth vs. cost vs. hack factor. I'm sure Alessb has his or hers own reasons.

1. Audio bandwidth of 1-20khz+ is not something a cb or export radio will do out of the box.

2. It may seem like the radio gets all hacked up but in all reality it doesn't. Well, I guess that also depends on your tech and how meticulous he/she is.

3. The hack factor is cool. Not alot of High-Level CB radios are modded in this way.

4. More precise control of modulation percentage.

5. The audio quality is better than a MMM or Top Gun modulator but the cost is about 80% less. Even less if one has a 148/2000 around.

6. It's fun to modify the C2000/148 to do something it was never intended to do. Which, in this case, has a dramatic effect on the quality of transmitted audio.

7. Lightens the load on the power supply of the 2000 in AM mode.

8. I agree with you in there are many who consider the 2000/148 a classic radio. And I recon it is. But imagine for a moment. A broken, used up, butchered 148 can be had for $60.00 or so. If one is adept at fixing theses dead/tweakers paradise radios, an afternoon of bench work can take that piece of doodie radio and turn it into a well oiled AM audio machine - a far cry from the rode hard and put away wet 148 that showed up on the door step. The radio gets a new life and the station gets a new addition of something that is kinda cool, definitely different. IMHO.

9. It doesn't require any external power supply, or installation of larger transistors in the series pass, works with all frequency mods, and if setup properly the mode switch works so it switches seamlessly between ssb and am - normally.

I came to know this mod, actually, from reading about a mod for a Kenwood. I just duplicated it on a 2000, then the 148. So fun, it was a great project.

My next project will be to adapt this to a Cobra 29. It will be interesting to say the least.

Have a good one!
 
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Reactions: LeapFrog, tecnicoloco, rabbiporkchop and 2 others
I totally agree. Like I said, it was more curiosity then anything else. I can totally understand now and as long as it not f'ing the radio up if you know what I am saying, then it sounds well worth doing it. I got it now and understand as to why now. Thx for making this clear and easy for someone to understand! Have a good one man.
73 and God Bless.
 
If this will work on the ts440, will it work on a kenwood ts140s?? Just curious?? Thx for any and all replies. Might be one for me to look at as I do have an ol ts140s. It's my backup radio to the Icom 746. After that it's the optima mk3!! Anyways. Again, thx for any and all responses. I'll read the post and link sent about the td440s as well! (y)(y);)
 
Hey Technicoloco!!!

How the heck are you?

I read the page (your link to AM window) in reference to the 440 and the heart of it's AM generation is exactly the same as the Low-Level mod. Using the AN612 as an AM modulator.

The caps, resistors, and the filter designations are not compatible with the C2000/148. But the idea is exactly the same.
 
  • Like
Reactions: tecnicoloco
Wowww, excelent post cbphreaker! Very interesting!

I apologize for my very late reply, I missed all the comments. Regarding your comments...

cbphreaker said:
I've successfully pulled this off with a Cobra 2000 and 148, going around the high level modulator and using the AN612 as a low level modulator and running the drive and final in linear mode.
Click to expand...
Great! Really glad to hear that!

cbphreaker said:
Although I only glanced at your schematic, I didn't see where in AM mode you go around the SSB filter. It looks while in AM mode, to me on the block diagram, the carrier oscillator goes through the SSB filter. It's that way on my Cobra 2000/148 as well. I think your keeping the radio in the AM mode so the RX works normally using the mode switch? I found to make this mod worth it the mode switch had to be correct.

If, in fact, the radio is in AM mode but there is no provision to switch out the SSB filter you can say goodbye to all that audio and your carrier. I used a 4066 to route IF signals around, including bypassing the carrier (+modulation) through the ssb filter while in "AM" mode.
Click to expand...
Do I need to go around the SSB filter? My intention is to bypass the original audio amplifier of the radio, replacing that signal with audio from the external sound chain. The rest of the circuit remains unchanged, so I'm not sure about what to do with the SSB filter.

cbphreaker said:
When switching to SSB mode, you will only have a different tone, not much in bandwidth because the SSB filter will strip off the carrier and either sideband down to somewhere around 2.1 KHZ.
Click to expand...
That is right, and I managed to get a new filter on e-bay for $2. It is a 7 KHz filter (much wider, but it was the only one available for that frequency).

cbphreaker said:
I bring that up for a reason. ALC - having control of the ALC and carrier level is really wonderful and it's nice to have. On the stock 2000 (in SSB mode) the RF output is sampled and fed back to the audio preamp. Which sucks because feeding the AN612 directly bypasses that part of the transmitter. There are many an option here to get a working ALC signal. I've investigated a few ALC methods and currently the 2000's ALC action is at the RF level and not the audio level but audio based versions seems to work well too.
Click to expand...
That is very interesting. Can you tell me how to put the ALC at the RF level of the radio?

cbphreaker said:
Some other findings I came across:
The AN612 seems to like a buffer in between the audio source, which in my case is the line-out from a pc, and it's audio input. I used a J-FET acting as such as it worked great. I highly recommend this.
Click to expand...
Great, I will keep that in mind.

cbphreaker said:
Audio processing is a must. Currently my setup consists of a variable Q 12 band graphic EQ, side chain d-sser, mutiband compressor and finally a mutiband limiter. It's all software on a pc running linux studio and the right price. Free.
Click to expand...
Yes, I am using a complete audio chain for audio processing.

Thank you and kind regards,

Alessb
 

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