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Old Oct 31, 2006, 12:14 PM
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Ottawa, Canada
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Quote:
Originally posted by Volante24
I just want to have all the
receivers on a negative shift.
Here is another option. If your receiver that needs to be shift reversed is double conversion, chances are there is a 10.245MHz or a 11.155MHz crystal in there. If there's a 10.245, replace it with a 11.155. Conversely, if there's a 11.155, replace it with a 10.245. These are standard frequencies for second conversion from 10.7MHz to 455KHz. Crystals and filters used in the RC receivers could be found in old cordless phones. There is nothing fancy about them. Hopefully you're going to find the right size. Usually there is no retuning required.

Good luck,
Cirip
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Old Oct 31, 2006, 12:36 PM
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Quote:
Originally Posted by Cirip
Here is another option. If your receiver that needs to be shift reversed is double conversion, chances are there is a 10.245MHz or a 11.155MHz crystal in there. If there's a 10.245, replace it with a 11.155. Conversely, if there's a 11.155, replace it with a 10.245. These are standard frequencies for second conversion from 10.7MHz to 455KHz. Crystals and filters used in the RC receivers could be found in old cordless phones. There is nothing fancy about them. Hopefully you're going to find the right size. Usually there is no retuning required.

Good luck,
Cirip
A cool idea! Thanks, it did not occur to me. The crystal in RCD3500 (negative shift) is on 11.155MHz. Interesting to see which one is in its postive-shift twin. If I recall correctly the same 11.155MHz crystal was in the negative-shift Electron 6.

Nick
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Old Oct 31, 2006, 11:20 PM
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Quote:
Interesting to see which one is in its postive-shift twin
Maybe it's worth mentioning that there's no rule such as 11.155 is positive and 10.7 is negative or so. It all depends on the frequency plan of the receiver whether the transmitter's original shift is reversed or not up to the detector. What I am saying is that whatever the second conversion crystal frequency is, changing it will change the shift at the detector input for sure.

Cirip
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Old Nov 01, 2006, 03:20 AM
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United Kingdom, Wrexham
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Take a look here

I have recently built a few Rx18 rxs from this site:

http://home.nordnet.fr/fthobois/anglais/rx17-b.htm

It explains about the frequency shift for a dual conversion rx. I had to fit 11.155Mhz crystal for them to work here in the UK on my Futaba FF6 Transmitter.

Another alternative is to use a transisitor circuit to invert the ppm signal, as the designer has done on his smaller single conversion rx:

http://home.nordnet.fr/fthobois/rx22.htm

If you do a search around the forum there is lots of information about it. I believe it can be modded on the transmitter, as a lot of commercial units these days, Hitec or Sanwa have switchable PPM "postive or negative"

One last alternative is to use a DSP decoder utilizing a PIC microcontroller. There is an excellent piece of code offered by Bruce Abbott which can be adpoted for either shift. Here is a link to Bruce's site:

http://homepages.paradise.net.nz/bhabbott/decoder.html

He often posts on the forum here. Good luck

Ash
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Old Nov 01, 2006, 05:24 AM
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East Anglia, UK
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Quote:
Originally Posted by Cirip
Maybe it's worth mentioning that there's no rule such as 11.155 is positive and 10.7 is negative or so. It all depends on the frequency plan of the receiver whether the transmitter's original shift is reversed or not up to the detector. What I am saying is that whatever the second conversion crystal frequency is, changing it will change the shift at the detector input for sure.

Cirip
The same trick would work in single cobnversiuon RX's too, but getting the right Xtals is non trivial.

I AM glad we don't have this problem in the UK...
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Old Nov 01, 2006, 08:21 AM
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Quote:
Originally Posted by Cirip
Maybe it's worth mentioning that there's no rule such as 11.155 is positive and 10.7 is negative or so. It all depends on the frequency plan of the receiver whether the transmitter's original shift is reversed or not up to the detector. What I am saying is that whatever the second conversion crystal frequency is, changing it will change the shift at the detector input for sure.

Cirip
It was pretty clear, thanks.

Nick
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Old Nov 01, 2006, 05:57 PM
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I think that change the decoder part using PIC IC like Bruce's is more cheaper and faster instead of X'tal.
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Old Nov 03, 2006, 09:11 AM
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The Northeast Kingdom, Vermont
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Cirip,
My Positive shift Airtronics Rx do use the 11.155MHz crystal, will try replacing with 10.245MHz on one of the 6 Ch models (piece of cake) the 4 Ch models will be more of a pain as they are "doppeldeckers".
Thanks for all the suggestions.
Pete
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Old Nov 04, 2006, 10:01 AM
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Hello Gentlemen;

Am sorry to disappoint you but it cannot be done at least easily. The "shift effect" is not on the audio section of the RX. The section of the RX that discriminates the shift is the detection. Here is the explanation: Positive shift means that the TX's frequency ,say 72.510 Mhz is shifted upwards by say 5khz or to 72.515 to modulate it(JR radios). Negative shift means, it will be downward to 72.505 Mhz. So, the RX's detection circuit can be made to look for a positive or negative shift. After it has detected the information, everything will be the same onwards to the decoder.

For the sake of learning (which I assume is why we are here in this forum), after detection, what is sent to the decoder is a uniform set of short pulses with fixed widths, usually .2 to .25 milliseconds. It is the delay between these pulses that creates another pulse which is the controlling pulse recognized by the servo. Almost all servos today requires a positive pulse. Negative pulse is a thing of the past. Therefore, it is not a matter of inverting the pulse signal (audio).

BravoKilo
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Old Nov 04, 2006, 03:19 PM
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Changing shift:

[QUOTE: Changing RX shift CAN be done by changing the oscillator Xtal from 10.245 to 11.155...or the other way round. This is nothing to do with servos. Plane spotter and myself did successful shift changes just as he described earlier in this thread.

Despite other comments this IS THE EASIEST WAY TO DO IT this does however assume that it's a Dual Conversion RX.
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Old Nov 04, 2006, 03:58 PM
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Hi John,
Thanks for your input, yes they are all dual conversion Airtronics and have the 11.155MHz crystal. Yesterday I talked to an old buddy that is an EE and 40+ year Ham and he agreed with this approach, He's not an R/C guy, but understood the issue.
Swapping out the crystal seems the best way to me, as it requires no mods,(I'm lazy, like things easy) but only a component swap, and it looks like the price of the crystals will be less than a buck each so will order enough for all my receivers, will try out one first, luckily a flying buddy has a Polks TrackerII so we can do a range test with his Tx as it has shift select and freq. synth. if it works, well I'm set
Thanks,
Pete
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Old Nov 04, 2006, 08:38 PM
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Thanks John, that confirms it and is straigth to the point on how to do it. I didn't know how to do it cuz this issue came up just the last couple of days for me. I have a JR PCM10S and I saw the TA7761 chip and the 11.155 crystal. like Pete, I like to do it the easiest way without disturbing the neatly couched components in tight RX. I downloaded the JR FAQ which helped. Of course its all clear now. The 11.155 xtal is above the incoming 10.7 while the 10.245 is below (11.155-10.7=.455), (10.7-10.245=.455). The shift is in the 10.7. A negative shift like in Futabas, will shift the .455 downward. While in JR, a negative shift of the 10.7 will shift the .455 upnward. I don't see much bandpass problem, definitely not on the .455 since the bandwidth there is +/-3Mhz upwards(mostly). The 10.7 Mhz path is the same. The difference is just in the second mixer. Obviouosly, when I saw the RX22, the MC3362 actually detects both shifts but will output the audio signal in an inverse directions. Neat ckt in there to just insert an inverter and use a selection sw. My previous statement about positive/negative pulse to the servo was an attempt to expalin that we cannot put an outside module to reverse the pulse in each channel as suggested in one of the posts. What is inversed is the signal before the decoder.

BravoKilo
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Old Nov 04, 2006, 10:58 PM
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Gentlemen;

I was wrong in saying that it cannot done easily. Changing the LO xtal in the second mixer is in fact EZ.Finding the xtal is something else. I still have to find a cordless phone with a 11.155Mhz. Most of them are 10.245 Mhz. I was also inaccurate in saying that after detection, its the same all the way. Well, after looking at RX22, not really. It is more accurate to say that either shift, the audio pulse is there except it is inversed. Was enjoying reading and learning from all your posts at 2 am our time here (Manila). Sorry, I stand corrected!!!

By the way, there is something wong with this JRpcm10s. By chance , would anyone have the schematics? I hate looking at those multi pin square IC but then again, the trouble may be in some minor external parts.


Thanks Again to everyone!


BravoKilo
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Old Nov 05, 2006, 10:44 AM
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The Proof is in the Pudding

Hi Bravokilo, glad to see you got there in the end. As John mentioned earlier we did succeed in shift changes by swapping the 2nd oscillator frequency.
The hard part is probably actually finding the xtals. 10.245 seem easy to get hold of but 11.155 were a lot harder. Here is a picture of an rx-18 built and tested by John and myself. I owe the credit to John, for the setting up of what is a very high quality design. The Rx-18 has fantastic performance.


Sorry for the clarity of the second photo but you can just make out the frequency of the xtal: 11.155Mhz. This allows me to use this receiver on my Futaba FF6 Transmitter.

Also like Vintage1 said it is possible to do on a single conversion with xtal frequencies. I am sure I have seen half size crystals either 455khz above or below the channel number. Must have a look at that. I am sure I have on somewhere.
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Old Nov 05, 2006, 12:09 PM
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Hi PLANE_SPOTTER! Beautiful pictures and beautiful layout. Yesterday,I was actually reviewing the Rx17/18 and indeed it looks like a very good if not excellent receiver. After seeing the schematics, I think I like to build this. To be practical about it, I will go for 50 Mhz (6mters) since I could use those coils and inductors from cordless phones. Cordless phones for the US works on 46/49.9 Mhz and lowering the capacitor a little bit might just do it. I could go 40 Mhz but I will have to wind my coils. I had some experience in using FET front end designed by Mike Dorffler in the Royal Electronics kits. I will check though if the original bipolar RFand mixer transistors will perform adequately. The cordless phones use bipolars and are already there. Coincidentally, a friend has probably 50 units cordless phones in his inventory which use the MC3362. My problem is they do not have the HT ceramic filter and best I got arD's and E's (+/-4.5Mhz). I see the AGC, very nice. I actually could get 2 MC3362 from each set complete with the CF, the quad coil,and the common R's and C's.

On my post yesterday, I should have waited for daylight. I was sleepy at 2 am in the morning but the topic was quite interesting cuz I was actually researching on how to convert a JR PCM10S. Worse case, I will remove the second board where all the PCM stuff is and replace it with a board with the 4015 of the RX18 or the 74HC164. I could have one more DC RX. This one is at 72Mhz. I have a Futaba TX module for it. i caught the discussion on an external converter "where each channel....". Oops, I thought there seem to be confusion of the pos/neg shift to the pos/neg signal to the servo. Thus my comment that it has something to do with modulation which as John told me is not really true FM. It's FSK, am I correct? I should not have said that it cannot be done easily though much less venture into implying that the detection is set to positive or negative shift detection. In fact I realize as in the RX22, the same detector will detect either one without any mod. Well anyway, I have learned so much with FM RX in such a short time with great people here in this forum. Building our own radio is a bif trill. I am grateful, especially to John who took time out to communicate with me.

Pls advice me on how to convert the AM transmiter to FM. I have a Graupner FM on 27Mhz and I see that the encoder is the same as the AM. It is using the 4015 to commutate a set of control pots. The difference is in the RF where a varactor is taking the signal from the encoder and shifting the frequency in the oscillator. Can I fix the AM modulator portion and put a varactor in an AM RF deck have have FM? Is the AM crystal usable in FM? If not, why? Am not familiar with xtal properties. If this can be done, then I will convert all my AMs TX's and build several RX18's! I still have those AMs of the 70's Kraft, Heathkit, Futabas,Sanwas using discrete components.


PLANE_SPOTTER, thank you so much. The name reminds me of the film Battle of Britain with those plane spotters counting the Junkers, Heinkels, Messers, etc...coming in across the channel!


BravoKilo
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