SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Nov 06, 2006, 07:05 AM
Registered User
Joined Oct 2006
63 Posts
Quote:
Originally Posted by pilotpete2
Hi John,
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

Pete
Can you share with us where one can get such crystal for under a buck? I would like to buy them either!

Nick
Volante24 is offline Find More Posts by Volante24
Reply With Quote
Sign up now
to remove ads between posts
Old Nov 06, 2006, 07:15 AM
Registered User
Joined Oct 2006
63 Posts
Quote:
Originally Posted by plane_spotter
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.
Where I can find more information, description and specs, if possible, for these receivers?

Nick
Volante24 is offline Find More Posts by Volante24
Reply With Quote
Old Nov 06, 2006, 07:34 AM
Registered User
Joined Oct 2006
63 Posts
Quote:
Originally Posted by bravokilo
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.

BravoKilo
Which band do you speak about for this conversion?

Based on my experience with Hitec Laser 4, if you don't care about complying with the FCC requirements, this should be pretty straightforward. Depending on the precise design and the components, it may be not easy to pull the original "AM" crystal by 2.8 to 3 kHz (the PDF). You may check this experimentally, or look if the FM versions use the same or a different crystal. Or just put there the TX crystal from another brand FM TX and tune it properly.

The FM transmitter typically uses partially smeared falling (?) edge of the modulating pulse to narrow the emitted bandwidth, which seems a bit superfluous to me. Even this is easy to implement, but if you do not care about an extra kHz at a -20dB level, you can skip this. I guess then the receiever will work even better.

Nick
Volante24 is offline Find More Posts by Volante24
Reply With Quote
Old Nov 06, 2006, 08:42 AM
Registered User
plane_spotter's Avatar
United Kingdom, Wrexham
Joined Jun 2004
622 Posts
RX-18 info link

Hi Nick, here you go. All the info is avaliable here. There is a massive range of homedesign equipment. You will be blown away.

http://home.nordnet.fr/fthobois/anglais/engl-index.htm

enjoy,

Ashley
plane_spotter is offline Find More Posts by plane_spotter
Reply With Quote
Old Nov 06, 2006, 07:40 PM
Registered User
Joined Sep 2006
74 Posts
THANKS NICK! I have always wondered about the difference between am and fm xtals.
The Graupner I have is already in FM and will use it as reference. With the proliferation of the FM NB RXs and its simplicity, might as well convert thoese AMs to FMs(TX).

Thanks Again.

BravoKilo
bravokilo is offline Find More Posts by bravokilo
Reply With Quote
Old Nov 07, 2006, 05:24 AM
Registered User
Joined Oct 2006
63 Posts
Quote:
Originally Posted by plane_spotter
Hi Nick, here you go. All the info is avaliable here. There is a massive range of homedesign equipment. You will be blown away.

http://home.nordnet.fr/fthobois/anglais/engl-index.htm

enjoy,

Ashley
Thanks, a great site. It remains to learn French to fully benefit from it.

By the way, how can I distinguish between the quartz "crystal" and the quartz filter at around 10.7MHz? There is one on the first IF in my receivers, but I am not sure which type is it...

Nick
Volante24 is offline Find More Posts by Volante24
Reply With Quote
Old Nov 07, 2006, 06:40 AM
now that's a wattmeter...
simingx's Avatar
Singapore
Joined May 2002
1,136 Posts
On the JR R700, shift change is accomplished by changing a single solder bridge... I've tried and verified it myself.
simingx is offline Find More Posts by simingx
Reply With Quote
Old Nov 07, 2006, 08:39 AM
Registered User
Joined Oct 2006
63 Posts
Quote:
Originally Posted by simingx
On the JR R700, shift change is accomplished by changing a single solder bridge... I've tried and verified it myself.
Great, this would make all sense to me. Can you describe where that bridge was? Maybe it was grounding or pulling up a certain pin in some chip?

Nick
Volante24 is offline Find More Posts by Volante24
Reply With Quote
Old Nov 07, 2006, 09:00 AM
now that's a wattmeter...
simingx's Avatar
Singapore
Joined May 2002
1,136 Posts
On this page (which is a GREAT page BTW!!) you can see a solder bridge just beside the 22uF capacitor. That's it.
simingx is offline Find More Posts by simingx
Reply With Quote
Old Nov 07, 2006, 12:18 PM
Registered User
Joined Oct 2006
63 Posts
Quote:
Originally Posted by simingx
On this page (which is a GREAT page BTW!!) you can see a solder bridge just beside the 22uF capacitor. That's it.
Do you mean the first or the third pictuer? The same 22muF capacitor is on the both...
Volante24 is offline Find More Posts by Volante24
Reply With Quote
Old May 02, 2007, 05:16 PM
Registered User
Joined Oct 2006
63 Posts
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 question to the experts: in DC receivers the first IF is 10.7 MHz. Which filter is used there -- quartz or ceramic? What is its normal bandwidth? I saw ceramic filters at 10.7MHz with the bandwidth like 250 or 180 kHz (apparently for usual WFM) which looks too much for RC, but I wonder if it makes a difference, for the main selectivity is in the second IF path.

Another question: is it Ok to use the 455kHz filter with the 6 kHz bandwidth (+-3kHz) in the second IF circuit, or it is too narrow?

Finally, maybe somebody can recall -- half a year ago there was a post with an url to a guy selling surface-mount Murata narrow-band 455 kHz filters. I now use a different computer and could not find it...

Nick
Volante24 is offline Find More Posts by Volante24
Reply With Quote
Old Dec 18, 2007, 08:02 AM
Registered User
Joined Oct 2006
63 Posts
Quote:
Originally Posted by vintage1
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...
I now know how to flip shift on some Hitec DC receivers, not difficult at all, but depends on the model.
And the trick with 11.155 <--> 10.245 also works, although I faced the problem -- while 10.245 are readily available, I could not find 11.155 crystals. My friend blew his Supreme in the crash, I diagnosed the 2d het crystal, yet could not find a replacement. So, had to put a 10.245MHz one, and then flipped the shift.
Volante24 is offline Find More Posts by Volante24
Last edited by Volante24; Dec 19, 2007 at 03:16 PM.
Reply With Quote
Old Dec 19, 2007, 01:10 PM
Registered User
Ottawa, Canada
Joined Jul 2004
203 Posts
Hi Volante,

Sorry, I missed your post long time ago. Here are some answers:

Quote:
Originally Posted by Volante24
in DC receivers the first IF is 10.7 MHz. Which filter is used there -- quartz or ceramic?
Doesn't really mater. Usually the quartz filters have less attenuation, but they are more expensive.


Quote:
Originally Posted by Volante24
What is its normal bandwidth? I saw ceramic filters at 10.7MHz with the bandwidth like 250 or 180 kHz (apparently for usual WFM) which looks too much for RC, but I wonder if it makes a difference, for the main selectivity is in the second IF path.
Exactly. The adjacent channel selectivity is being achieved mainly in the second IF, i.e. 455KHz. The main reason for the higher first IF is the better image rejection. Bandwith of the first IF is not that important in the overall picture, although some selectivity is beneficial in order to prevent second mixer overloading/swamping.


Quote:
Originally Posted by Volante24
is it Ok to use the 455kHz filter with the 6 kHz bandwidth (+-3kHz) in the second IF circuit, or it is too narrow?
Too narrow, I guess. Check Carson's rule for FM bandwidth estimates. Keep in mind that the max frequency in the PPM signal is around 3KHz and the frequency deviation can be 3-5KHz peak. The PPM/FM modulation follows the mobile voice communication standard to some extent.
http://en.wikipedia.org/wiki/Carson_bandwidth_rule

Cirip
Cirip is offline Find More Posts by Cirip
Reply With Quote
Old Oct 02, 2012, 02:29 PM
Registered User
Joined Jun 2011
12 Posts
hii i'm try to build rx18,bt
http://home.nordnet.fr/fthobois/anglais/rx17-b.htm
in avobe site ...i dnt understand what rf coils they are used in rx=18 or rx-17 fm receiver,
plz help me in thos rf coils.
thank u.
Sm692048 is offline Find More Posts by Sm692048
Reply With Quote
Old Oct 02, 2012, 08:16 PM
Registered User
EloyM's Avatar
SantaAna CA
Joined Aug 2003
318 Posts
Years ago, there was a magazine named Radio Control Modeler (RCM). Writing for RCM on matters electronic was Jim Oddino, easily the most knowledgeable RC electronics man of
the times. Though actually writing about NiCds at the time, Jim once wrote: .....everybody is an expert.....
You guys just proved it re RC receiver shift. Lots of words, theories, etc - mostly not even close. Nothing to do with decoding, etc., it is all frequency related.
Think about it - Double Conversion. Simply stated, that means that the incoming frequency is converted twice before going to the IF (455 KHz) stages. In the Airtronics & JT positive shift receivers, it is mixed (converted) with a 11.155 MHz crystals. For negative shift,
a 10.245 MHz crystal is required, From then on, things work as they did on Single Conversion units - all 455.
I've done it numerous times - it works. Do the math!
Wonder if I qualify for Oddino's "Expert" title??

PilotPete - I have some spares - if you want to make a test I can send one along.
Direct: eloymz@att.net

Eloy Marez - Orbit---Futaba ---Airtronics!
EloyM is offline Find More Posts by EloyM
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Multiplex EVO and negative shift RX owners twohsieh Radios 84 Mar 27, 2004 10:27 AM
FS: Hitec 555 RX - Negative Shift cadconversions Aircraft - Electric - Airplanes (FS/W) 5 Feb 12, 2004 07:57 PM
FS: Hitec 555 rx - Negative shift - Save $17.50 + S&H now! cadconversions Aircraft - Electric - Airplanes (FS/W) 7 Feb 10, 2004 12:12 PM
WTB- negative shift RX butaford Aircraft - Electric - Airplanes (FS/W) 1 Nov 13, 2003 08:36 AM
WANTED: GWP-4P Rx, Negative (Hitec/Fut) Shift DaJudge Aircraft - Electric - Airplanes (FS/W) 2 Dec 17, 2001 01:05 PM