How to make a JR Radio to work with Futaba RX?

[Isaac F]

Guys, I have a JR XP783 radio (72mhz) and a lots of 72mhz Futaba receivers.
I know JR is + Shift and Futaba is - Shift but is there any mod that I can make to my TX so I can have a switch to select - and + shift in order to use JR and Futaba receivers.

Can somebody give a schematic for this mod?

THX,

Isaac

[macr0t0r]

The search engine is your friend! Search for "JR Shift." Here we go...
http://www.rcgroups.com/forums/showthread.php?p=4014422

Schematic and all. Can't remember if your JR has the freq module capability or not. If it does, then you put the inverter circuit where the module connects. In other words, either right before(in TX) or right after(in module) the Module connector plug.

Warning! This will be in violation of the FCC code. Adding any circuitry between the logic and RF boards is considered taboo.

I have not done the mod myself for this purpose, but I've seen it utilized.

- Jim

[vintage1]

Theoretically you COULD do it by changing the RX crystal so the local oscillator is on the other side of the incoming signal..if you can find one with 950Khz difference to the one that 'ought' to fit..

[Miami Mike]

Warning! This will be in violation of the FCC code.That really shouldn't be too much of a concern for someone in Panama.

Isaac, are you sure your Futaba receiver is a negative shift unit? Outside of the USA and Canada, most of the r/c world uses positive shift.

Vintage1, that's a new one on me. I'll have to give it some thought.

[macr0t0r]

(ahem)

He stated clearly he's using 72mhz systems, not 75. Those are US bands, which means they have the lovely shift fiasco that usually only us Americans enjoy. ;)

- Jim

[Miami Mike]

Ahem?

Ahem?

Aren't you the guy who just told someone in Panama to be worried about the United States Federal Communications Commission? :rolleyes:

I'll grant that the probability is probably pretty good that Isaac has a negative shift receiver that was meant for North America, but we don't know that yet.

Here's an interesting website (http://www.rc-network.de/magazin/artikel_05/art_05-027/art_027-01eng.html) that features a listing of radio control frequencies used around the world. Of the few countries I checked, I found that Japan, Israel, Hong Kong, and of course Panama, all use 72 Mhz for radio control, and I'm sure there are many, many, more. However, the general consensus here on RCGroups is that negative shift is unique to the USA and Canada.

They are not necessarily "US bands".

[vintage1]

Ahem?

Ahem?

Aren't you the guy who just told someone in Panama to be worried about the United States Federal Communications Commission? :rolleyes:

I'll grant that the probability is probably pretty good that Isaac has a negative shift receiver that was meant for North America, but we don't know that yet.

Here's an interesting website (http://www.rc-network.de/magazin/artikel_05/art_05-027/art_027-01eng.html) that features a listing of radio control frequencies used around the world. Of the few countries I checked, I found that Japan, Israel, Hong Kong, and of course Panama, all use 72 Mhz for radio control, and I'm sure there are many, many, more. However, the general consensus here on RCGroups is that negative shift is unique to the USA and Canada.

They are not necessarily "US bands".


I think the truer statement is that negative shift is unique to some brands of radio designed to operate on 72Mhz.

[Isaac F]

Schematic and all. Can't remember if your JR has the freq module capability or not. If it does, then you put the inverter circuit where the module connects. In other words, either right before(in TX) or right after(in module) the Module connector plug.


Hello Guys,

Here in Panama Central America we get all of our RC things from USA so we suffer the same shift fiasco as you guys :eek:
Also, here no body cares about FFC code or what ever :rolleyes:

My JR XP783 use a rear removable module. Its look like my radio was available for a short time in USA until JR release the 8103. The JR XP-783 looks exactly the same as a JR-388 just that mine have one less chanel.

So, Is there is like a DYI or a schematic of how to wire the inverter. I will like to install it inside the radio and put some switch to select - of + shift depending of what RX brand I will fly.

Here is a pic of my Radio:http://www.scaleaero.com/tnXP783.jpg

Also, I have detailed pics of the PC board of my radio so if someone need to see the back of my radio with the cover removed to point me what I need to mod, send me a PM and I will e-mail them.

Thx for your help,

Isaac

[Isaac F]

Ok guys, check this Schematic. I found it at http://fritzthecat.250free.com/inverter.html I e-mail Fritz and he point me to the updated one with a Zener Diode and a 270 ohm Resistor.

The only thing that I need will be to find the signal input pin to the RF module in order to insert the circuit there. So how do I find this imput pin on my JR 783 :confused:

Please check the schematic and tell me what you think

http://www.honesty.com/imagedata/5/9/4/13586594.jpg

YHX,

Isaac

[macr0t0r]

Ah bugger, you got me on the FCC thing. Allow me to take back my "ahem". :p Solder away!

Hmmm...there must be more involved than I thought, for this is far more complex than what was posted in the other thread. Anyhow, here's a link to a picture of what pin is what, so you can see which is the signal pin:
http://www.rcgroups.com/forums/showthread.php?t=540007
In that thread, it is referred to as the "Mod" pin.

Tell us how it goes, for there is yet to be a post of someone who has successfully done it.

- Jim

[electrotor]

Theoretically you COULD do it by changing the RX crystal so the local oscillator is on the other side of the incoming signal..if you can find one with 950Khz difference to the one that 'ought' to fit..

An interesting theory vintage 1 - please explain.

Are you sure you are not confusing conversion (RF + local oscillator = IF) with frequency shift due to modulation (FSK)?

[Bruce Abbott]

If the receiver's Xtal frequency is lower than the channel frequency, shift is preserved (eg. 72.000MHz~72.001MHz - 71.545MHz = 455KHz~456KHz). If the Xtal frequency is higher, shift is inverted (eg. 72.000MHz~72.001MHz - 72.455MHz = 455KHz~454KHz). The final effect depends on the polarity of the signal after demodulation and amplification, so it's quite possible for different designs to respond to the same 'shift' using Xtals on opposite sides of the channel.

Things get even more complicated with a Dual Conversion RX, as the 2nd mixer Xtal can also be lower (10.245MHz) preserving shift, or higher (11.155MHz) inverting shift again! Also, the 1st oscillator may not work properly on the opposite side, due to the large difference (21.4MHz) between upper and lower frequencies.

Modifying the TX should be easier and much cheaper (no extra Xtals to buy!).

Isaac, the Schematic looks OK, but I would discard the 270 Ohm resistor and Zener diode, instead feeding Vdd from the on-board +5V regulator. The RF module probably has its modulation input on pin 2. To maintain the correct bandwidth you might need to insert the invertor at an earlier point (eg. where I marked a red 'X' in this partial schematic of an X378).

[Miami Mike]

I wouldn't ground pins 4, 10, or 11. Sure, they ought to go low by themselves if the corresponding input pins are reliably grounded, but what's the point of forcing them low?

[Isaac F]

Isaac, the Schematic looks OK, but I would discard the 270 Ohm resistor and Zener diode, instead feeding Vdd from the on-board +5V regulator. The RF module probably has its modulation input on pin 2. To maintain the correct bandwidth you might need to insert the invertor at an earlier point (eg. where I marked a red 'X' in this partial schematic of an X378).

Hey Bruce, thx for you advice. Just cuirious, The PRE RF schematic pic you post belong to a JR X378, Do this is the same for my JR XP783? I am pretty sure they are the same BUT better ask :rolleyes:

Also, I have a JR XF622 that I want to use it for testing this before I do it to the JR 783. Do the 622 is the same?

http://www.rotory.com/LAUNCH/image2/xf622.jpeg

THX,

Isaac

[electrotor]

If the receiver's Xtal frequency is lower than the channel frequency, shift is preserved (eg. 72.000MHz~72.001MHz - 71.545MHz = 455KHz~456KHz). If the Xtal frequency is higher, shift is inverted (eg. 72.000MHz~72.001MHz - 72.455MHz = 455KHz~454KHz). The final effect depends on the polarity of the signal after demodulation and amplification, so it's quite possible for different designs to respond to the same 'shift' using Xtals on opposite sides of the channel.

Surely that's conversion, not shift. I think you are confused about terminology.
RF + local oscillator produces sum and difference signals, the difference being what the IF stage processes, the sum being filtered out. Shift is the amount the carrier is pulled due to modulation by the encoder according to positions of joysticks, etc. That's why it is correctly known as Frequency Shift Keying (FSK), a form of FM.
Conversion and shift are entirely separate and different processes.

[Bruce Abbott]

Shift (http://www.its.bldrdoc.gov/fs-1037/dir-016/_2387.htm) has both magnitude and sign. For our purposes only the sign is relevant, as the magnitude is effectively fixed by bandwidth and compatibility requirements.

If a transmitted PPM signal is described as 'Positive Shift', that means the pulses are encoded as a higher frequency than the gaps between pulses. When the signal is passed through a frequency changer (eg. the mixer in a receiver) the sign of the shift may change, ie. the pulses may now be encoded as a lower frequency than the gaps. Of course this does not mean the receiver is 'Negative Shift', as it should be designed to account for the inversion (when the correct crystal is installed!).


Isaac I don't have a schematic for the 783 or 622, but all the JR transmitter schematics I do have show the modulation going through a passive filter to pin 2 of the RF board. Here's another example (XP662):-

[ZAGNUT]

Here's an interesting website (http://www.rc-network.de/magazin/artikel_05/art_05-027/art_027-01eng.html) that features a listing of radio control frequencies used around the world. Of the few countries I checked, I found that Japan, Israel, Hong Kong, and of course Panama, all use 72 Mhz for radio control, and I'm sure there are many, many, more. However, the general consensus here on RCGroups is that negative shift is unique to the USA and Canada.

They are not necessarily "US bands".

but most are using 72mhz because of easy availability due to the very large american market. if israel had said only 47mhz was legal do you think any manufacturer would waste their time on us and our relatively small market? all futabas and hitecs sold here on 72mhz are negative shift.


and i'm really not sure that futaba deserves to be blamed for the negative shift "fiasco". since the older american made radios are also negative shift i'm guessing that futaba was just going along with what was popular. but i guess it really depends on who was the first to start making FM gear on 72mhz...

[coloradoz]

Bruce, the signal is already on the board - from your schematic (for the 378 - which should be the same as a 783) the inverted signal is at the base of Q4, why invert it again after it is inverted by Q4?

[Javier Garcia]

I see two other ways to invert the PPM signal:

a) http://users.belgacom.net/TX2TX/tx2tx/english/tx2txgb2.htm.
You can find here schematics for the conectors too.

b) Look at the DSC/trainer socket. The PPM OUT could be different that the one at the RF module socket. This happens, for example, to Eclipse 7. All you have to do is to install a two way conmutator, one "In" from the RF module Socket; the other "In" from the PPM OUT; the "signal exit" to the RF Module PIN.

Un saludo, Javier

[Bruce Abbott]

Bruce, the signal is already on the board - from your schematic (for the 378 - which should be the same as a 783) the inverted signal is at the base of Q4, why invert it again after it is inverted by Q4?
If Q4 is a 'digital' transistor (with a built-in Base resistor) then you probably could use the signal at its Base, and an additional invertor would not be required.

My schematic does not have part numbers, but some of the other transistors almost certainly are digital, so there's a good chance that Q4 is also. In any case it would be a good idea to check the waveforms with a 'scope, to make sure that sufficient amplitude is present (should swing from 0V to +5V, or close to it).

[Isaac F]

If Q4 is a 'digital' transistor (with a built-in Base resistor) then you probably could use the signal at its Base, and an additional invertor would not be required.

My schematic does not have part numbers, but some of the other transistors almost certainly are digital, so there's a good chance that Q4 is also. In any case it would be a good idea to check the waveforms with a 'scope, to make sure that sufficient amplitude is present (should swing from 0V to +5V, or close to it).

HMMMM... I do not have a scope :( I know MAC_MAN is a Engineer for UK's JR Distributor, I going to send him a PM to see if he can find out the part number of the Transistor Q4 and what he think about this. In the worse case I will open my TX and find out :rolleyes:

Isaac

[Isaac F]

HMMMM... I do not have a scope :( I know MAC_MAN is a Engineer for UK's JR Distributor, I going to send him a PM to see if he can find out the part number of the Transistor Q4 and what he think about this. In the worse case I will open my TX and find out :rolleyes:
Isaac

I contacted MAC_MAN and here is his reply:

Q4 on that 378 pcb is a standard transistor, it doesnt have an integrated
>base resistor.
>
>Which radio do you want to do this on ? I have added a single transistor
>inverter to a 347 / 388 / 387 and it "seemed" to work. But as we do not
>have this problem here inthe UK/Europe, it is impossible for me to test it
>out on a Futaba receiver.>
>Tony


I tell Tony to visit this post and share his expertice with us.

THX,

Isaac

[mac_man]

Hi, the transistor used on the 347 / 388 / 783 is a DTC114YF and in this case it DOES have the integral base/emitter resistor. I will try very hard to check the voltage levels and if I can will try and get pics.

[Isaac F]

Hey Tony, You da Man, thank you for post your info. Please keep us updated.

THX,

Isaac