Crystals?

[Graham54]

Hi all

Just bought myself a Radian PNP version for use with an old Futaba T4VF Tx that came with my first heli. To save ripping the MX6000 receiver off the heli I bought a used Futaba FP-R115F 5ch receiver for it. However, I'm pretty sure I'm going to need a crystal to match as the one in the MX looks really small compared to the one in the Tx. Anyone know if they are compatible? If not, anyone got a spare receiver crystal ch78? Or, any other matching pair that I could buy?

Thanks in advance...

[electrotor]

The physical size of the crystal is NOT a measure of which channel it is for. Crystals for the same channel come in cases of different sizes. In addition tx crystals and rx crystals for the same channel resonate on different frequencies so you cannot mix tx and rx. I can explain more if you want.
What you need to find out is the channel number of the tx crystal, then buy a matching rx crystal. You should also be aware that crystals from different r/c manufacturers are not necessarily compatible so for Futaba gear buy Futaba crystals, for JR buy JR, etc.

[Graham54]

Yes, I realise the physical size is no representation of the channel and I guessed that the Tx & Rx crystals may be different in some way. I was asking if the MX6000 crystal would be compatible (size & frequency) with the Futaba receiver I have just ordered. Not having physically seen one before I wouldn't know. All I know is they are both on Ch78.
I just want to have a suitable crystal here when it arrives or another matching pair on another channel.

[electrotor]

OK but your first posting was confusing :
" I'm pretty sure I'm going to need a crystal to match as the one in the MX looks really small compared to the one in the Tx"

Unless you know the intermediate frequencies of both Futaba and Mx rxs (highly unlikely as this figure is seldom available) and the crystal type for both, there is no way of knowing if the MX rx crystal will be compatible with the Futaba rx. Even if they appear to be physically identical and work at close range, the Futaba rx may not work correctly at longer range. It is not worth the risk for the sake of a few pounds.
Buy a Futaba crystal to match. AFAIK the FP-R115F is single conversion.

[Graham54]

Sorry, I can see how it could be misinterpreted.

Having had only one model on 35mhz I'm not familiar with changing crystals. As far as I knew, all crystals were the same - apart from the different channels. I had heard that receiver crystals may be on a different frequency but thought that may have been the older system. My main concern (when I first posted) was the fact that the MX6000 crystal was so small that it may not physically fit in the futaba Rx. As you pointed out, it's irrelevant anyway, as it's a different make and the electronics will most likely be different.

I think I'm sorted now anyway thanks. A model shop in the next town says he can sort me out with something.

[electrotor]

Crystals are quite delicate and are usually housed in tiny metal cans or packages to give them the correct name. The package has a code, eg HC-49U, HC-6U, etc which merely indicates the physical size and pin spacing. Internally of course you could have any frequency possible.
Going back a few years most, if not all 35MHz gear would have used the same package for both tx and rx, although some manufacturers would stick the package inside a plastic housing for ease of handling. One size predominated (call it standard) but as the rxs got smaller it was necessary to introduce a micro size package, so you then had both standard and micro. Initially all receivers were what is called single conversion - radio theory stuff to do with the internal workings. To complicate matters dual conversion receivers then came onto the market. They required different crystals to single conversion. This is all despite the fact that the channel they were operating on was the same.
If you look closely as some r/c crystal packages you can see stamped on them the actual crystal frequency which is never the same as the actual transmitted frequency. The black magic to sort out the intended frequencies takes place inside the tx and rx. Don't worry about it.
Then we got synthesised txs and rxs which did away with crystals, or so you might think. Actually, although you can now select the frequency and don't have to change crystals anymore, there is still a crystal at the heart of the electronics to give a source or reference frequency. Your PC, laptop, mobile, etc, etc, still use crystals because they are such good frequency reference sources.