27 MHz vs. 75 MHz

[Timmahhh]

27 was worldwide air and ground. USA only got 6, but I thought there were 12 total?
72 started as air and surface (a few upper channels switched to air only when 75 came around)
75 was most popular in the USA
50 and 53mhz was a ham band and if you had a license it was a great way to avoid problems at the track

At some point, 72 became air only in the US, right?

75 was most popular at the club level, but I remember a lot of big time racers using 27mhz... maybe because of its international compatibility?

[GoMachV]

Yeah I want to say by the early 80’s 72 was all aircraft
This is from the manual in an old Cox radio I own.
Interesting that there was a 75mhz aircraft only as well

[Incredible_Serious]

Conspiracy

Everything's a conspiracy with you.....

[matt1ptkn]

Bringing up this topic again, as I have questions. To start, its seems the more I learn, the less I know. Anyway, I recently acquired a large lot of RC crystals from my (now closed ) LHS. As I am going through them all and trying to match up pairs and organize everything in numerical order, I've found that there are more frequencies than I thought for RC. I'm only familiar with 27 and 75 MHz AM and was aware of 72 MHz for air use. I have no experience with other frequencies, FM or PCM. After 75 MHz AM, I went straight to 2.4 GHz. I've also got some odd ball frequencies that I can't find anywhere, mainly in the 30-40 MHz range. I'll get some photos to back up my questions.

1. AM / FM: I understand (enough) about the difference, but what (if any) is the compatibility? I've read that 27/72/75 MHz transmitters work on both AM and FM, but that receivers are only one or the other. True or False? Will there be any connection between AM and FM crystals that are the same frequencies?
2. Different Brands: Some crystals state that they are for specific brand(s). Will these work at all in other brands? Is the branding specific to the transmitter, the receiver or both?
3. Conversions: I have no clue what the single conversion versus dual conversion is. Does each type only work in specific transmitter/receivers? Is there any compatibility between single and dual conversion crystals?
4. Single Crystals: I have a handful of NIP single crystals. I thought you always needed a matched pair, ie: same frequency, same band, same type, same brand. Will these single crystals work with any others on that frequency, or are they specific to only that brand, etc.?
5. Identification: Is there a way to ID a blank crystal? Or if it is only labeled with the frequency, is there any way (other than trial and error) to determine what it (should) work with?

If you've read this far, thank you for your time. If you have answers, please

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why-dont-you-explain-this-to-me-like-im-five.jpg (38.69 KiB) Viewed 1337 times

. THANK YOU!!!

[jwscab]

you should in general as much as possible, use matched crystals that are specified by the manufacturer, ie, futaba matched pairs, airtronics matched pairs, etc.

Now, in theory, the crystals should work in any system, am or fm. In practice, crystals can be manufactured that operate slightly differently, and so you might have a crystal that would work in an am system, but not an fm system. it has to do with how 'tuned' the radio is, without getting too complicated, some crystals are much more finicky with their tune or actually don't naturally vibrate at the frequency printed on the casing. these types of crystals are what's called an overtone crystal, so for instance, the crystal actually would be 25.xxx Mhz, but can be tuned to operate on it's third overtone, or 75.xxx Mhz.

there are also different circuit designs for modulators and demoduators, and some are more critical of crystal parameters than others. am vs fm are totally different topologies and functions, so they are not compatible, although the crystals may work in either.

single conversion vs dual/multiple conversions add complexity in the radio circuit, but improvements can be made in adjacent channel rejection and noise improvements. essentially having multiple conversions allows you to add additional selective filtering to make the radio link more robust. The down side is complexity, cost, and additional tuning, since the frequency gets converted to yet another frequency. as far as the crystal is concerned, it's not relevant.

additionally, due to the age of the technology and hardware, these circuits may have drifted so far out of tune that they wouldn't even work with the right crystals today. So there is that giant hurdle as well.

[Pepito]

Hello everyone!
I have 5 very old crystals which I cannot figure out what they go to.
They have frequencies I have never seen before.
They are printed with:
1. 76.165 KDS 6A
2. RA 74.978 KDS 7G
3. R75.258 KDS
4. R75.018 KDS
5. R75.218 KDS
These are frequencies that are 75 Mhz or "close enough" but they do not natch any of the old frequencies 60-90 as I believe the all only had only a two digit suffix (i.e. 90 - 75.99Mhz).

Please help me figure out this mystery.
It is bugging the crap out of me.
Thanks!!