72 crystals
 

Well I guess I made a mistake and bought the wrong crystal. I needed a channel 29 crystal and ordered one on eBay. It arrived and would not work in my receiver. The package says it is dual conversion. My radio is a JR SX600. I am guessing I need a non dual conversion channel 29. Anybody need a channel 29 dual conversion crystal lol .

I don't understand why it would matter single or dual conversion. ?

It matters - a single conversion receiver just like your SX600 will require a single conversion crystal - end of story you cannot use a dual conversion crystal even if it is a JR brand crystal, it must be a single conversion crystal.

John

+1

Receivers use a crystal to generate a signal thats mixed with the incoming signal and the difference is taken, thats called the IF because lower frequencies are easier to work with.

Dual conversion receivers do that twice, so the frequency the crystals generate are different.

http://en.wikipedia.org/wiki/Direct-conversion_receiver if you want to know how it works

Why didn't you check before ordering if you need a single or dual conversion?

RC systems have gone through many levels of technology over the past 75 years. Unless you are well versed in communication theory, it's difficult to understand what is really going on in any RF design.

Give the guy a break.. Remember what forum this is posted in, we've all made and make unknowing mistakes.

Also another brand like Futaba, of Hitec may be opposite shift. Positive, or negative will not work unless it is correct. Just so it will confuse you more. There is a hobby shop in Ohio Radical RC, that has a $5 crystal swap that is sometimes handy. This forum may have something too that could be free, depending on the poster.

The negative/positive shift turned out to be a marketing ploy and really made no difference. I didn't find this out until just a few years ago when I was talking to Tony Stillman at Radio South. Seems the only difference in the receivers was the red dot Futaba put in them.

Hmm, my Airtronics radios don't work with Futaba and Hitec crystals, and am pretty sure vice versa. Don't have any JR transmitters to try but think they are the same as Airtronics AFAIK. My Airtronics has a thing to program pos. and neg shift on the transmitter. I had to use it once. Some transmitters actually could change frequencies, just before 2.4 came in. Now I just mostly use the cheap $6 Orange 2.4 receivers in my little planes. Cheaper and smaller than most crystal type receivers. But if that is all you got....

Thats not positive/negative shift. You're thinking of the high band/low band nonsense. Also there was more to it than just a dot, it was how they were tuned, but yes, it was more marketing hype than technical advantage

Yes shift is important. If your receiver has positive shift, the crystal has to be higher by the IF (intermediate frequencey) of the actual transmitted frequency, if Negative shift, it has to be lower than the transmitted signal. For single conversion, the IF frequency is 455 Killo cycles, for dual conversion, it is usually 11.7 megacycles. Plus, some recievers use 3rd order harmonics while some use 5th order so again, diferent brands of receivers may require different cuts of crystals to work properly. It is not simply an act of matching frequencies.

Yep, a brain fart

Let me see if I can clear this up.

In a superheterodyne receiver the signal from the antenna is slightly filtered then mixed with a signal that is offset from the signal of interest. If the receiver is single conversion, the frequency is offset by 455 kHz as set by the crystal. This frequency can either be above or below the transmitter signal, it doesn't matter, as the mixing stage produces both the sum and the difference of the two frequencies as well as many other combinations of these two frequencies in harmonics. The reason for 455 kHz was that the IF parts ( tuned intermediate frequencies) were cheap common parts used to build AM radios. Unfortunately, the closeness of other strong signals getting through the lightly filtered front end at the antenna meant that you often go shot down in areas with a lot of other signals.

So the next improvement in RC RF was the addition of a second mixing stage that was offset by 10.7 MHz, which happen to be used in cheap FM radios. So what this really did was make the slight filter at the antenna work quite well at rejection of image signals, that it could not do very well when they were only 910 kHz away. Signals that are 21.4 MHz away are attenuated fairly well. Again, it doesn't matter if the frequency of this oscillator is above or below.

Now radios produced after Frank Hover developed the narrow off spike that made AM RC sets work without the AGC circuit going nuts allowing any stray signal to mess up the pulse train, RC development was off and running from the mid-60's and all was good until the mid to late 70's with AM radio sets. But thanks to cheap FM chipsets, it got a lot cheaper and easier to build FM RC sets. They didn't work any better or worse, but just cost a lot less to build. However, at this point the manufacturers had to decide if the narrow off pulse was a high side frequency shift or a low side frequency shift.

Interesting- I flew with thoes sets back in the old days. Started out with escapements in the early 40s.

What's a crystal? Just kidding. I will check my stash of old radio gear too see what I have that you might be able to use. If I don't have a ch29 ,I'll check too se if I have a matched set in another channel

Here's an idea , call or contact Tony Stillman at Radio South , tell him what you have and what you want .
Sit back for a few days and it'll be delivered to you .
Its that simple .

Or you can search ebay for a high end 72mhz transmitter that has selectable frequency box. That is what I did and after turning two dials it was tuned to my receiver. I found it easier than trying to score crystals. BTW I paid only $30 for the TX and it basically has all the features the modern 2.4ghz systems have for a small fraction of the price a new system would cost you. Just an idea.

Bad info since the post is about receiver crystals.

Because I am a beginner and I thought that channel 29 was the only thing I needed to check for when I ordered. Thanks for your input.

Thanks for everybodys help to clear up the confusion about crystals. I had no idea that there was a difference. Its no big deal really. I only paid 5 bucks for it. I ordered another crystal "single conversion" and it works fine. I could easily switch to 2.4 tomorrow if I wanted to . I like my JR 72 radio because a RCUniverse member was kind enough to gift it to me to help me get started in the hobby. Plus my red flag on my 3 foot antenna looks cooler than y'alls short stubby 2.4 antenna lol.

That is true, I have a nice piece of yarn on my antenna to tell wind direction. When I'm on 2.4 it's still there but I can't really see it unless I look down. Cool factor counts for a lot too!!!:cool:

[QUOTE=L8cruiser;11951447]Thanks for everybodys help to clear up the confusion about crystals. I had no idea that there was a difference. Its no big deal really. I only paid 5 bucks for it. I ordered another crystal "single conversion" and it works fine.


You are welcome L8. There is nothing wrong with using the 72 stuff and even the much older 27 stuff if one takes the time to learn the pitfalls. As for the two negative posters on this thread please just ignore and there is no need to explain yourself at all Just Enjoy.

John:cool:

I still use mostly 72 stuff because I have to use it up. The smaller ones I have on 2.4 mostly because the aerial is too long on the 72 mhz, but also because the Orange receivers are only $6. It is hard to find crystals for that much, much less receivers. There is also a Lemon receiver that is supposedly better yet and smaller.