Matching crystals, and testing them…

[Chrissy JParticipant @chrissyj94964]

Hello everyone!

I have in my hot little hands, an eBay special: a 40Mhz 3-channel radio set, with four crystals, a gift from my partner. Through dint of close examination, I've found that two are marked 40.975 and the others 40.725.

Am I right in thinking that I need to use matching-numbered crystals, as I recall doing so with 27Mhz a long time (1988) ago?

Google is not being helpful and is talking about how the crystals don't actually resonate at the frequency that the can says, about underrating and overrating, conversions and gods only knows…

Also, is there a way of testing individual crystals to see if they work? For all I know, they could all be dead….

The set is being used on the submarine model featured in this month's Model Boats magazine, BTW, hence the requirements for 40Mhz.

[Chrissy JParticipant @chrissyj94964]

[Charles OatesParticipant @charlesoates31738]

You've got it right, matching numbers together, in addition, they may be marked for transmitter tx, and receiver rx.

There is plenty of techy stuff online about resonant frequencies, but it's not much use for normal modellers.

To test, just plug in and switch on. Without test gear I can't think of another way.

Good luck with the sub, it should be great fun.

Chas

[Charles OatesParticipant @charlesoates31738]

You've got it right, matching numbers together, in addition, they may be marked for transmitter tx, and receiver rx.

There is plenty of techy stuff online about resonant frequencies, but it's not much use for normal modellers.

To test, just plug in and switch on. Without test gear I can't think of another way.

Good luck with the sub, it should be great fun.

Chas

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by Chrissy J on 27/09/2017 17:13:08:

Hello everyone!

I have in my hot little hands, an eBay special: a 40Mhz 3-channel radio set, with four crystals, a gift from my partner. Through dint of close examination, I've found that two are marked 40.975 and the others 40.725.

Am I right in thinking that I need to use matching-numbered crystals, as I recall doing so with 27Mhz a long time (1988) ago?

…..

Um. You are correct that you need 'matching' crystals for the band you are working on. But I would usually expect the Tx and the Rx crystal to have a slight offset, due to something called the 'intermediate frequency'. That used to require a 455Hz difference in the old AM days (except for McGregor, who used 470!). That's why you can't swap a Tx and and Rx crystal, and why they are marked that way.

What is the kit you have got? And what is marked on the crystals? Incidentally, I don't expect there would be any damage caused by trying them out – if they don't work they just won't work. But if we can find out what the frequencies OUGHT to be we will be in a much better position…

This link might help… https://www.rcgroups.com/forums/showthread.php?808548-How-do-you-identify-modulation-mhz-and-frequency

P.S

Having looked at the other messages here, I see we are talking about a Hitec Ranger set. It looks like Hitec crystals are readily available, and, looking at the shops, I see that they seem to print the channel frequency on BOTH the crystals, rather than the actual frequency each works at. See here, for instance –  https://www.modelhelicopters.co.uk/electronics/crystals/hitec.html

That's a bit confusing if you are trying to follow first principles!  However, it seems likely that you have a pair of channel 975 and a pair of channel 725 crystals – now all you have to do is find out which one of each pair is the Tx and the Rx crystal, which you can do by trial and error. Unless there is another marking or some kind of indication…

Edited By Dodgy Geezer on 27/09/2017 21:46:06

[Charles OatesParticipant @charlesoates31738]

I've no idea why my reply appears twice, sorry chaps.

It's true that different makers seemed to use different protocols over the years, and that there is a difference between what is technically correct, and what a modeller expects to see. The upshot of it all is the same, a user just plugs the matching crystals in and sails the boat. I've seen plenty of accasions where mixing RX and tx crystals makes no obvious difference, and the opposite.

Chrissy, plug the obvious pairs in, test for range, if it works, all is fine.

Chas

 

Edited By Charles Oates on 27/09/2017 23:31:36

[Chrissy JParticipant @chrissyj94964]

Posted by Dodgy Geezer on 27/09/2017 21:27:42:

That's a bit confusing if you are trying to follow first principles! However, it seems likely that you have a pair of channel 975 and a pair of channel 725 crystals – now all you have to do is find out which one of each pair is the Tx and the Rx crystal, which you can do by trial and error. Unless there is another marking or some kind of indication…

Upon further examination with a magnifying glass, each crystal is marked 'TX' or 'RX' on their casing, and the coloured tabs on their printed labels support this. (I understand that TX crystals have a black tab and RX crystals have a green tab).

The TX crystals are also in holders for fitting to the transmitter, but the holders have labels with different numbers printed on them – on first glance, it looked like I'd got four different crystals, which is why I wasn't sure whether they needed to match or not. I've had to peel back the labels on each item to be sure.

Anyway. I'm happy that I have two matching pairs, I've ordered a couple of servos and a battery holder for the receiver and when they come I'll have a play with them.

Thanks for your help. Sorry my first post was such a newbie question but I promise I won't ask many more!

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by Chrissy J on 28/09/2017 02:46:02:

Thanks for your help. Sorry my first post was such a newbie question but I promise I won't ask many more!

No question is too 'newbie'. We all learn something from them. Before you asked, I was not aware that it might be a practice not to stamp a crystal with the actual frequency, but to put the transmission frequency on both and put 'Rx' on the one which was 455 less than the marked value! Don't stop asking such useful questions!

Superhetrodyning is a technique used to help process the weak incoming high-frequency signal. It works by mixing that signal with a slightly different frequency, which results in a resonant 'beat' at a much lower frequency – much like two out-of-sync motors produce a characteristic beat. No matter what the initial high frequency is, the lower 'beat' frequency (called the 'intermediate frequency&#39 will stay the same with different pairs of crystals if the 'offset' between them is the same, which makes things easier to handle in the rest of the circuitry. So the frequency difference is small, but critical to the receiver operation….

[Malcolm FraryParticipant @malcolmfrary95515]

Back in the elder days, when buying crystals from such as Maplin, when they did such stuff, the TX crystal indicated what the transmitter was going to transmit. The RX crystal was numbered to give its "real" frequency, i.e. it showed the offset value. You had to provide your own paint to fall in with the colour coding. The more modern practice is to just show the frequency to or from air, with an indicator to show which end it belongs to. That was 27MHz, 40 went straight to using either channel numbers or the air frequency. Channel numbers could create its own problems, since different countries had their own ideas regarding which channel belonged to what frequency.

It was a not unknown, if naughty, practice to "reverse" the crystals to get away from a signal clash. This would leave the transmitter working, but effectively out of the permitted frequency bands. Since the offset was the same, the system still worked, but there was the outside chance that a legal user might have suffered interference.

[Chrissy JParticipant @chrissyj94964]

Posted by Dodgy Geezer on 28/09/2017 09:15:00

No question is too 'newbie'. We all learn something from them. Before you asked, I was not aware that it might be a practice not to stamp a crystal with the actual frequency, but to put the transmission frequency on both and put 'Rx' on the one which was 455 less than the marked value! Don't stop asking such useful questions!

See, this was exactly why I asked my first question. At first I thought I had four individual crystals; I was dimly aware that a TX/RX pair would have a small difference between them in actual frequency and I didn't know if, for one particular crystal, I'd have to pair it with another crystal of a higher or lower frequency.

Google was not helpful, but you good people were. Thanks again.

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