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

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

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.