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VX7r - the truth


alan_k
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This should be of interest to you techies out there.

I have been wondering just what the circuitry of this radio is comprised of, particularly following all the discusion/supposition about AM transmitting.

I now have the circuit diagram and can see how it all works.

The radio transmits on 3 frequency bands, refered to on the circuit as LB, MB and HB. (Low, medium and high band).

The transmit strip uses the same active devices (BJTs and FETs) for all 3 bands, with different tuned circuits switched in for the respective band.

The control of the power level on FM transmission is done via a feedback circuit to control the supply level to the driver stages, this is standard design for this purpose. The response time of this feedback circuit is fast so that the FM power level stays constant.

Now comes the AM transmit. The response time of the feedback circuit is slowed down considerably so the instantaneous power level can follow the modulating signal (i.e. voice) so giving an AM transmit function, albeit at a low level.

EDIT The modulation depth is given as 60% in the setup instructions.

The control signals coming from the control board select LB, MB or HB on 3 control lines and AM on another control line. I guess the controller programming will only select AM when LB is selected.

It should be a reasonably easy modification to select AM when MB is selected although it wouldn't be frequency dependant, you would get AM transmit on air band and 2m frequencies.

I don't have a VX7r to try this on so I can't comment on the quality or modulation depth. (Prior to EDIT above)

The standard VX7r does NOT transmit AM on air band, just in case you were in any doubt.

Cheers,

Alan

Edited by Guest
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Hi Alan,

I am techie, but not in the radio arena unfortunately so all that makes no sense to me. But I am impressed it does to you :)

My VX7R has been modified (available from a simple googly woogly) to TX on AM. I have successfully conversed with other airband radios with no problem (also marine, 2m and PMR [ish - having a stepping issue]). However, when I read other forums (radio specific) they mention that it is not transmitting on a true AM frequency? Does that agree with your fandangled stuff above?

Cheers

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..... that it is not transmitting on a true AM frequency?

There is no 'true AM frequency' as you say, AM is not related to any particular frequency as far as technicalities go, you could have amplitude modulation on any frequency if you wished. The air band is only AM because it was set up a long time ago when AM was the prevalent modulation, before FM was thought usable as a modulating scheme;

Ahhh now, as I said I don't have a VX7r to try out (any donations :?: ).

The specification says AM only on the 50MHz amateur band, that's the LB as mentioned above.

As far as I am aware the solder bridges in the back of the radio only affect the frequency coverage, not the type of modulation for each of the 3 frequency bands.

Have you had any physical mods done to the printed circuit boards in your radio, or just changes to the solder bridges :?:

Alan

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OK Huffhuff I understand where you're coming from.

What information did you have for changing the solder bridges :?:

I'm still searching for a definitive list of what all the bridges do. They connect directly to the processor with no markings as to function on the circuit diagram :cry:

Cheers,

Alan

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Some of the reading I have done on many radio fora, suggests that the solder bridge modifications that allows the VX 7R to transmit on airband simply allow the transmission of an under-modulated FM signal. This it would seem from the info I have available, will probably work over a very limited range, and possibly only to certain other radios.

Whilst there may be modifications that could be done to allow the VX7R to work on airband as a true AM transmitter, they seem to be pretty involved, and to date I have found no evidence that anyone has actually succeeded in doing it. Personally, I don't feel it's worth it, and would rather use a dedicated set and sort out a switching system with priority given to airband (as in the FlightCell devices), or just a manual switch over system.

The other option I've not tried yet is to wear a pair of ear buds for pmr/vhf inside my headset which is connected to the airband.

I did have a panel mount aircraft radio that would monitor the vhf frequencies, but was blocked from transmitting on them. By using dual watch, you could at least hear what your mates were saying and still be monitoring airband.

Phil

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Whilst there may be modifications that could be done to allow the VX7R to work on airband as a true AM transmitter, they seem to be pretty involved,

Depends what you call pretty involved Phil, a couple of diodes and resistors to steer the switching the way you want it should work.

Alan

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Well the reason I went for this radio is I figured anybody flying will either have PMR or 2m (and rarely AM but maybe on occassion). The VX7R lets me listen top both PMR and 2m at the same time. I just saw it as one less bit of kit to carry around. As we don't have a standard, some people use PMr, some 2M. I couldn't be arsed carrying both around and swapping depending on the day. Then you always have the possibilty of people using both types on the same day. Plus the VX7R is submersible, fairly small form factor and I live literally on the end of Luton runway so like to hear the chatter :)

Alan, the problem with PMR is something to do with ctcss codes i think. I've tried everything but can only recieve, not transmit :(

Ben

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So what is the real verdict guys? Can the VXR7 be modified to communicate adequately on airband or not?

Sorry but is anything but clear to me.

:cry::cry::cry:

Lets put it this way;

I have not heard of a VX7R that has had the surgery to get it to transmit AM on the airband frequencies. This is not to say it is impossible, but if you go for it, it sounds like you will be a guinea pig. If it was as simple as Alan suggests, I am truly surprised that to date, no geek has published this as a hack on the net (that I have found). Along with computers, radio comms has more than it's fair share of uber geeks who I reckon would be up to the job if do-able.

If you are content with the possibility of short range airband comms to your mates (which may or may not work), together with a full ability to hear what is being said by ATC, the simple solder bridge mods will do the job for you.

Incidentally, the VX2R can be modded to MARS/CAP by Windows software. I also found that I was unable to receive modded VX2R airband transmissions on my ICOM A24, even in the same room. It might well be a similar situation with the '7'.

Hope this put a little clarity on the subject.

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Guys,

Please don't mess with modifying radios to work from one band/freq group to another. If you wanna' use air band, get a radio licence and use an airband transceiver. You don't know what is happening to the signal you are slinging out or where the possible harmonics might be going.

If you start blaring across dedicated ATC or Emergency Service frequencies the authorities will look to the the source with some urgency, then you don't know what the next game will cost you in legal fees alone.

I am not a comms expert but I do suffer as do my colleagues in some countries where this kind of interference happens. Not so much of a problem with low power hand-held devices but who wants to take the risk. Your ability to argue your case will be severely hampered when they discover what it is you have been doing... Try convincing them you have been a responsible citizen?

Just another opinion. :shock:

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So what is the real verdict guys? Can the VXR7 be modified to communicate adequately on airband or not?

Sorry but is anything but clear to me.

:cry::cry::cry:

Lets put it this way;

I have not heard of a VX7R that has had the surgery to get it to transmit AM on the airband frequencies. This is not to say it is impossible, but if you go for it, it sounds like you will be a guinea pig. If it was as simple as Alan suggests, .

Yes but.... the problem as I see it is where and how do you fit an additional switch to an already small (and waterproof) radio.

The alignment procedure for the radio is also pre-programmed so a decent amount of testing would be required to ensure correct performance.

Which leads nicely onto Normans post, I do agree that the best way onto airband is a dedicated radio. I'm looking at the VX7r question in detail because it's something that interests me on the technical side, what with being a radio amateur. And I do like playing with my test equipment :D

Cheers,

Alan

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And I'd love to see it done if it can be Alan.

Presumably you'd need something like a spectrum analyser to see how 'clean' the RF was?

Must confess to liking to fiddle too. Having just got a new phone from Orange, I felt obliged to flash a hacked ROM to it to get rid of the Orange branding and get the Wifi working properly. Then a bit of registry tweaking to do some other stuff, great fun. :-)

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  • 1 year later...

This is and old thread, nevertheless I had been looking for years for a dua/tri-band radio that could tx in all bands, and read all sort of posts about AM Xmit not possible, etc.

I am an airliner pilot and licensed HAM operator. I will buy a VX-7R for the sake of science, mod it and post results.

Will try to make different tests using the air-to-air band with other A/Cs in different altitudes, from 40,000' to whichever altitude I can and hopefully unmask the truth about this HT.

I know PAPER manuals/diagrams/specs can hold as much ink you can throw on them, but the truth finally relies on field tests ;)

Cheers.

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Hi Mandrake,

When you've got the radio make up a very simple diode detector that can demodulate AM. Connect this to a scope and see if the waveform is AM.

You should be able to see the AM on the 6m band before any modification of the radio, then see what the waveform looks like on VHF after the mods.

Be interesting to hear what you find,

Cheers,

Alan

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I've got 2 of these radios (great, but I only use 5% of the numerous functions) so would also be interested in any developments.

Does anyone know if they require a specific external microphone ? I can't tell from the circuit diagrams. When used with a full face helmet system some people find my speech distorted as if the mic is too close or over modulating, with massive amounts of wind noise. I've taped extra foam over the mic, and set the "front end attenuation" to 'on' in the menu (but don't know if this only works on the internal mic). Speech is perfectly clear using the external hand held speaker mic.

Any thoughts, or do I need a headset with a different microphone impedance ? Maybe I could just change the external microphone if anyone knows of a better one ? I only use it with PTT, not VOX.

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The 'front end attenuation' is only on the receiver for incoming RF signals.

Most handhelds have too much microphone gain for use with headsets IMHO. The electret microphones do have a spread of output levels, they are not all the same.

You have probably got quite a high output level from your particular mic, if you have any alternative mic inserts it might be worth trying a swap out.

Cheers,

Alan

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  • 7 months later...
Hi Mandrake,

When you've got the radio make up a very simple diode detector that can demodulate AM. Connect this to a scope and see if the waveform is AM.

You should be able to see the AM on the 6m band before any modification of the radio, then see what the waveform looks like on VHF after the mods.

Be interesting to hear what you find,

Cheers,

Alan

Geez, was a while since I logged here. I was busy finishing my training a new company and lost track of this thread :oops:

Well, I finally ordered my VX-7R and Im waiting to get my hands on it. As soon as I get it I'll perform a SW mod (virtual Jumper #3) to enable freeband. Now, Alan is suggesting me to build a Diode Detector (envelope AM detector), or w/e its called, so I can verify if the waveform is an actual AM.

I guess I need an oscilloscope or similar to check the waveform in a graph. Is there another way to perform this? As I currently don't own this equipment. I can try using any SW-based solution if any.

Ive been also looking at the schematics of this radio Vs. its evil twin aviation brother, the VXA-710. Case and motherboards are almost identical. The Processors and transmitters vary as the VXA uses less options than the VX, but has some additional features like a "VOR".

So I guess unless I perform a real field test, all of this chitchat will end in the same, ASSUMPTIONS. :mrgreen:

Cheers.

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  • 2 months later...

So - here's what happens:-

Key in 136.975 - and you'll get AM transmission without hassles.

Click up one stop, to 137.000 - and the software in the radio changes to modulation to NFM (which, as someone mentioned earlier, causes a different wire to be energized in the output stage, which causes the modulation to not be AM anymore).

The way I see it - there's 2 different ways to transmit on AM air-band:

A) add a switch to "cut" the NFM stuff, and "re join" the AM stuff, eg - a hardware hack.

B) modify the firmware software so that it doesn't block AM in the first place.

If someone can get me a firmware dump of this baby, I reckon I could probably handle the (B) option (would also be helpful if someone who's opened one of this up can see what kind of microcontroller is being used, so I can skip figguring out the instruction set for it, and jump direct to the disassembly/hack stage :-)

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I've had another look at the circuit diagram.

The microprocessor is HD6472237TF10.

The control signals that switch between frequency bands come directly from the processor and the modulation control signal come from a shift register.

Looks like the only way to modify this radio is by modifying the firmware.

Cheers,

Alan

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