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Old Sep 09, 2004, 09:17 AM
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Manila, Philippines
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Procedure to re-tune a Dual Conversion Receiver

Hi Guys, did anyone here know how to re-tune a dual conversion receiver, I have a Hi-Band 72Mhz dual conversion receiver, it work perfectly on a Low-Band crystal 72.350 Mhz.... I am just wondering if anyone here know how to re-tune a receiver to exactly match my current frequency at 72.350, I have access to signal generator, spectrum analyzer and frequency counter..., but I do not know the actual procedure of doing it....I like to learn how to do it by myself.....this is just my idea, but never actually done it, I was thinking of sweeping a signal generator from 72.000 to 72.990 in a increment of 5 Khz, then monitor the output of the receiver and adjust some IF transformer to get the maximum signal at 72.350.....Is this correct? I was thinking its just a band pass filter and adjust some IF transformer to get maximum signal output at center of working frequency (72.350)?

Some people will just said send it to Futaba Factory for Re-tune, But I already have access to all equipment, and just want to learn the procedure.
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Old Sep 09, 2004, 03:44 PM
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Andy W's Avatar
Marietta, GA
Joined Jun 1999
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The crystal itself tunes a Rx. You don't have to tune them to work on different frequencies.

Transmittters, however, are different, and do need tuning, hence the need to send them to an authorized service center.
..a
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Old Sep 09, 2004, 04:43 PM
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While in general Andy is correct Futaba Rx's need to be tuned to either High or low band. I am not sure of the procedure to change it, but I am sure some one on here does.

Hmmmm. Not that often that a mod is wrong.
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Old Sep 09, 2004, 05:36 PM
RIP Ric
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Quote:
Originally Posted by W'rkncacnter
Hmmmm. Not that often that a mod is wrong.
Poor misguided fools..



Wow. I never realized this. I've never heard of "low" and "high" band receivers, but when looking at a site that sells Futaba equipment, I found that this was indeed the case - Futaba uses different parts for these. No Hitec, Berg, FMA or GWS Rx I've ever bought (and I've bought several of each, especially Hitec) requires this.

Personally, I would not buy a Futaba Rx based on this (although I've never bought another Futaba, as they are large and heavy compared to all the others)..

Sorry for the bad advice.
..a
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Old Sep 09, 2004, 05:42 PM
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Tuning a R/C Rx to a specific frequency will optimize it for best performance. We used to do it all the time in the old 27Mhz days. But on a modern Rx, the improvement might not be noticed unless the Rx was goofy before the tuneup.

I often wonder what the REAL performance hit is when a Futaba Rx gets an xtal from the opposite Hi/Lo band. Has anyone done any controlled tests with a Futaba Rx as several different xtal/freq range are measured (via range testing or lab equipment)? If so, what were the results?

RC-CAM
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Old Sep 09, 2004, 06:34 PM
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Normally the only thng you have to retune is the input satge before the first mixer.

IF you can identify which coil or coils it is and IF you have a non metallic tuning tool., its dead easy.

Juts do an antenna down range check, and tune for minium servo chatter and the edge of the range.

Not sure whats sort of design you have there, but usually its the single coil right near where the antenna goes into the board.

If there is an RF stage as well there miht e another more imporatnt one to tweak, - post up a pikkie or ten and it may be possible to ID the right coil or coils.

If there is a trap tho, it may be harder.
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Old Sep 10, 2004, 08:02 AM
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I have access to this equipment, I wish I have this kind of Instrument at home.

Mr.RC-CAM, your right...Its so happen, that somebody just sold me a FUTABA Dual Conversion Receiver (Hi-Band) and I use 72.350 MHZ (low-band), I know it will work but its just not optimize, but before I put in in my Helicopter, I compare it to my FMA Dual convertion with 72.350, Both receiver give the same range.
I guess they have the same SENSITIVITY......I have not test its SELECTIVITY, I have to look for another Transmitter with very close to my frequency say 72.330 or 72.370 any put it near the 2 receiver under test, and perform a range check......may be next time If I was able to borrow a transmitter and test its selectivity, will let you know.

I believe the Futaba Dual Conversion Receiver is more Superior that FMA Dual Conversion Receiver, here is the proof: On my Scale Helicopter I use an On-Board Igniter made by FUTABA GH-1, it use the Receiver battery to Ignite the Glow Plug, whenever I use a FMA Receiver, and activate the On-board Igniter, all the servo is shaking, but It work perfectly whenever the On-Board igniter Deactivate, I guess its the electrical noise coming the Igniter circuit get thru the FMA receiver....But when I change the Receiver to FUTABA Dual Converter, Everything Works perfectly.

I Hope there somebody here who are expert on the Receiver, could teach us how to re-tune, or Just give as a Link to the website....Thanks
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Old Sep 10, 2004, 08:41 AM
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Quote:
Originally Posted by lazy-b
I have not test its SELECTIVITY, I have to look for another Transmitter with very close to my frequency say 72.330 or 72.370 any put it near the 2 receiver under test, and perform a range check......may be next time If I was able to borrow a transmitter and test its selectivity, will let you know.
The tuning which MAY be required is as described by Vintage1.

This tuning will optimize the sensitivity, but will have no discernible effect on the selectivity. Selectivity nowadays is obtained via non-tuneable crystal or ceramic (1st IF - 10.7 MHz) and ceramic (2nd IF - 455 kHz) filters.

On the other hand, you say that your tests show that the receiver works real good compared to another of good reputation. What about leaving alone if it works so well already ?
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Old Sep 11, 2004, 04:18 AM
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Yeo. The only specifidc to frequency tuning is the primary crystal oscillator, with one or two specific-to-band coils on the input side. Before the first mixer.

They are primarily there to prevent image frequency and out of band harmonic images getting through to the first mixer.

At 72Mhz with realtively low 'Q' they won't be much out of tune between high and low band anyway.

I n general teh antenna coupling tuned circuity will be very low Q - has to be that way because antennas are not fixed things, and too high a Q will get its tuningknocked off by different antenna placements.

If there is an RF stage before the mixer (good design) that will have a tuned circuit to couple into the mixer. That one can be quite high Q.

How many coils in this beast anyway? Not sure of futaba design philospohy, but i'd expect an input coil, RF coil and somewhere else a discriminator coil with all the rest being ceramic quartz filtering.
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Old Sep 11, 2004, 05:14 AM
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Vintage1: Thanks for the input....I have not tried opening my FUTABA receiver, as JMP Blackfoot said Don't touch it if it work so well....I will tried to get the Picture of Opened Futaba receiver...and post it here.

I just want to learn how the Futaba Service Center Do it, cause I have know a RC flyer here in Philippnes, willing to send their Futaba Receiver to the Futaba Service Center in the U.S.A. just to have it re-tune to its specific frequency, just to have a peace of mind that there receiver is at optimum performance.

When I was a kid, I visit a television factory, and just saw the techician, tuning an IF stage of the Television circuit, in the oscilloscope you can see a Mountain like traces, as the technician adjust the IF transformer you can see Mountain like traces change shape. like become two or three mountain....I guess its the shape of a BAND PASS FILTER, where there is a 4.5MHZ vertical like marker......I wish I could do their JOB.

I guess I have to build some Dual Conversion Receiver Kits, in order to learn the actual of tuning it, you can learn a lot of thing thru Kit Building.....I build a lots of KITS from HEATHKIT like 2 1/2 Digital Multimeter, Power Inverter 12 to 120V, Motorola 6800 Microprocessor trainer.....this is actual building and calibrating, its a lot better that going to SCHOOL....Wow, its a component level kit building, you learn how to trouble shoot, if you understand how circuit work....this is were I learn my electronics......I even Built a Heathkit personal computer, but its not component level, its already a board level, just like a current computer now a day almost anybody here with a common sense can build a personal computer.
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Old Sep 11, 2004, 01:19 PM
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Kits are sold by :

http://www.micronradiocontrol.fsnet.co.uk/

They have a dual conversion receiver kit at 25, which is a good price, plus their receivers are quite good quality.

Otherwise, you could have a look at :

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

and especially :

http://home.nordnet.fr/~fthobois/anglais/regl-rx.htm

where much is explained about building and tuning receivers. Explore this site for much useful information.

Hope it helps.
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Old Sep 11, 2004, 02:43 PM
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Be aware the Micron is I think for 35Mhz only.

No one tunes IF strips anymore: Thats why God made ceramic filters. It was the most appalingly hard thing to do with a wobbulator and RF meter attached to a scope!

Fortunately all uou have to do here is tune for peak output.

I used to tune AM and FM cheap broadcast receivers by ear. Always got them better than the factory did
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Old Sep 13, 2004, 12:16 AM
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Jmp Blackfoot, you just give me the exactly link of what I am looking for.......Thanks.
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