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Properly interfacing the VIC-1 or VIC-4 to a CB radio

OPCOM

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Here is something I did that might be of interest. This gives transmit and receive function of a CB radio over to the VIC-1 intercom system, just as if the CB were an RT524. To operate the CB normally, just unplug the adapter from the CB's mike jack and plug in the CB's mike. As with anything as grossly non-uniform as a CB radio, some changes might be necessary and your mileage may vary. The CB I use is an old-style pre-ST Cobra 148GTL.

Notes:
The headset of the VIC intercom system is driven directly from the military radio's headset amplifier. The impedance is 150 ohms. The 8:150 ohm transformer matches the CB audio output to the VIC-1. The typical CB however has alot more audio power (4 to 8 watts) than the RT524, so a series resistor on the 8 ohm side and a 'volume' control on the 150 ohm side help the level to be correct. Adjust to make the level with the CB receiving and producing a strong and undistorted audio signal to be the same loudness as the unsquelched RT524 noise.

The headset microphone drives the RT524 directly through the VIC-1. The level may be more than the CB will handle without distortion. Therefore the level control is used. On a good CB, the radio will have a microphone gain control and audio processing circuits, and this is less critical. Some cheap CBs may require more audio into the microphone input than the headset provides, this requires experimentation. I will be finalizing this and mounting it in a metal box this weekend and using an oscilloscope to tweek it. Pictures will be forthcoming unless something happens..

The VIC-1 keys the RT524 by grounding a key line. This is used here to control a relay that performs the same function as pushing the 'talk' switch on the CB mike. Note that many CBs use the switch to also disconnect the speaker during transmit. This is done because the speaker is otherwise connected to the CB's modulator and would feedback to the mike in the normal mode. The disconnect function is provided more for propriety than otherwise, and the VIC-x provides it again internally via its own relays.

The squelch on the CB should be set how you like. There is no provision for 'monitoring' (the "all' position on the VIC-1) but it could be done by adding a small additional amplifier with a 0-5V adjustable 150 ohm output inside the CB and feeding that to pin K of the VIC J501. This is made necessary since the CB has only one audio output, while the VRC-12 radio sets like the RT524 have two.

Transformers are used to isolate the CB from the VIC. The reasons are that the CB and VIC operate from different voltages and also it's very important to eliminate ground loops which would cause all kinds of noise and poor performance.
:wink:

schematic revised to version 0.9 due to error in drawing.
schematic revised to version 1.0 to accommodate fixed and variable audio input.
 

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cranetruck

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Good work Patrick! I'll be looking closer at it for sure, my problem with CB in the the deuce has been the high background noise. By using the current headset, H-161, the experiments can continue...
 

OPCOM

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Thanks, this is all a labor of love! The adapter is debugged and working properly after some fiddling. The VIC-4 seems to require the receive audio to be put to the "K" terminal (The VIC-4 amplifier unit I have is found to be questionable so I will replace it tomorrow). I had only the VIC-1 schematic so I had no way to know this before. VIC-1 owners may try K if H does not work (but H should be correct if the VIC-1 schematic is accurate). Microphone cable shielding from the adapter to the CB is of higest importance and caused me some issues so a quality cable must be used or RF feedback from the CB may cause a squeal. I have not put in any RF bypass capacitors or filters, it would probably be a good idea. The "new" relay I used is intermittent so I will install a new one. As it is, the levels are good when the CB's mike gain and audio level are set to half, and the "TX AUDIO LEVEL" pot is set to 50% and the "RX AUDIO LEVEL" is set to 20% (Likely because I used an 8:600 ohm transformer instead of an 8:150 and therefore twice as much voltage is developed across the secondary [remember the voltage ratio is the square root of the impedance ratio]). The 39 ohm resistor is kept, removing (shorting) it increases the RX audio in the headphones too much. Both pots are linear taper pots. Also, I used a 24V lamp for the indicator, so it will never burn out. It's not very bright but it does not have to be. While I was at it, I built the adapter with two channels. Only one is wired but the parts are in place for the other in case I want to hook up a ham radio or a toaster oven or something else. Tomorrow I will take some pictures of the adapter's layout and also some signal level readings and post them so as to help those who want to try this themselves. This is really alot of fun.
 

cranetruck

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So, you'll be testing this on your trip East, I presume. I don't have a radio in my deuce yet, only the CB (plus the VIC-1) at this time, but if you are able to stop by we can compare notes.
Did you cannibalize a cx-4723 cable for the interface?
 

OPCOM

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I will be doing that. I plan to try to make it to VA on Saturday or so, there should be no rush driving. Mr. Haney had a couple of ends off one of those cables so he threw them into the pot when I bought the system. I also installed a Lowrance 3500C GPS today using a RAM mount to hold it off the dash in the area where the rifle storage mounts usually go (seeing as i don't generally carry rifles). Might help me from taking too many wrong turns. Also updated the schematic and took some pics. for now the're offline, but i will try to replace the above schematic, with version 1v1.
 

popacom

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I have a Question for you OPCOM I have seen you refer to VIC-4 in your article, What amp is used in that set-up ?is that the same as the F.I.S.T amp and com boxes?

Thanks popacom / Bill:mrgreen:
 

OPCOM

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Thank you for the notice. I do not get to read all the threads as I would wish. There are so many threads daily, that they often are scrolled a couple pages down by the time I can get to the BBS.

The VIC-4 components are listed here:
Installing Aux. 24V and 12V Power and an AN/VIC-4 Intercom in an M35 Military Truck
The amp and crew boxes are incompatible with VIC-1. The same headsets are usable.

I found that the CB radio interface to the VIC-4 is subject to RF getting back into the VIC-4 through the cabling on transmit. This is coming from the bumper-mounted whip antenna on the M35. The correct antenna parts are in a topic. It uses the same antenna as the GRC-106 uses. The HF whip. The way to get that to work with a CB (SWR wise) is to use two long sections and one short one. I do not recall the exact numbers, pls. search for the thread I posted.

The VIC-1 and -4 (and probably -3) are designed to be used with FM sets, therefore AM and SSB sets are likely to cause issues if the antenna is within sight of the headset or there is an SWR issue causing RF currents to radiate from any coaxial cables. I tried ferrite beads, in-line RF filters, and bypassing the mike inputs at the headset connectors. It was -almost- good enough. Forget high powered AM and SSB radio sets unless the antenna is well away and shileded from the VIC cables.

At this point, I recommend finding a way to use the CB's original mike, and use the VIC for reception, unless you have plenty of time to experiment. I do not believe the adapter box i built contributes to the problem. I had success when reducing the CB radio's carrier power to about 1 watt and a 4 watt PEP level (instead of three watts and twelve PEP) Furthermore the VIC-4 has a lot more electronics inside than the VIC-1. If you need only two transmitters, and not four, use the VIC-1.

Be aware the signal levels in the VIC-4 seem to be very low, so the voltage from the CB speaker must be heavily attenuated. It is also important to electrically isolate the CB audio output and input from the VIC by using transformers or another method (optical?).

I am starting a new truck project at this time and will likely put an all band ham radio in the cab. I will try and interface to a VIC-1 and report whatever I find here.

If anyone has better success with interfacing a CB to the VIC system, please advise.
 

Boatcarpenter

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I have just begun my search for a way to be able to talk on a CB while having a H161U/E headset on for use with the VIC-1 intercom and overcome the background noise problem in a Deuce.
So far I found this headset that can be used with a CB.
http://www.wearecb.com/store/twinpointworkman-mad1-superstar-hands-free-cb-p-2911.html
Perhaps someone clever could figure out a way to disassemble the headset and mount the boom mic along side the VIC-1 headset mike. The next challenge would be to get the little ear speaker inside one of the VIC-1 headsets ear pieces. Then, to talk on the CB, one would only have to turn off the intercom with the chest switch so you were only listening and talking on the CB.
It doesn't say if the CB headset mic is noise cancelling or not so that might be a problem in a Deuce.
Perhaps the CB headset could be made to fit comfortably under the VIC-1 headset but it might get uncomfortable after a while.
Always looking for input on this matter.
Any suggestions :idea:?
BC
 
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OPCOM

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I'd like to add that the VIC-4 is very sensitive to AM signals such as CB. the VIC 1 and 4 are designed for FM sets. No data on the VIC-1 but I am glad you did get it working. AM or SSB sets?
 
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