TOPIC:

Hi guys,

I got a nice Robbe receiver not too long ago and I was happy to see that it worked with my Modelcraft (Conrad) transmitter.
So the 40MHz FM crystal from the original gear was a good match.
I decided to get an extra pair of crystals from eBay so I could switch receivers more easily.
But to my surprise the setup isn't functional.

I reported this to them and got an instant refund (cool) but I'm curious why this is happening nonetheless.
Or could it be they didn't survive the trip from Down Under?

with certain brands you have to purchase the brands own cristals to get everything working, think this includes Robbe(=Futaba) or just be lucky when a set is a match

have a look at my wanted add think I posted some links which explain about cristals, dont quite get it myself.

Yeah, I also googled a bit and read that there can be problems among brands...
Picking the exact bandwidth apparently isn't enough.
Guess I got lucky with the generic Modelcraft crystal.
Although from that I expected the ones from eBay to work as well.

Maybe I'll have a go at some other Conrad crystals (or Futaba).
And get you some AMs while I'm at it.
Hope they work though.

Grrr... now C says their crystal sets are for Acoms - Sanwa.
So this'll be the next try :

If it were me, I would've tried the crystals in a different Tx/Rx setup to prove it's definately the crystals that were the problem.
I've seen mismatched Tx/Rx setups not work together with matching crystals, which is more to do with the way the commands are sent/received (PPM, PWM, coded etc), but this usually only happens when trying to run really obscure stuff together. Never had any problems mismatching Futaba, Acoms, Sanwa, Nikko, HSP, HBX, Hitec etc, but then 27AM is a bit more forgiving anyway (Another reason why it's so prone to interference).
I've also seen very old gear require mis-matched crystals to work (Sanwa, possibly to do with age/inaccuracy of other components on the board) & also only require the crystals to be "somewhere near" eachother's frequencies (ie instances where half-bands talk to whole bands).
For 27AM, I've used XMods, Nikko, white package, etc crystals in all of my gear & never had a problem.

Writing & reading back, I think there's the clue - "variations in amplitude" (Lower frequency AM) probably doesn't need to be as accurately regulated as "variations in high frequency" (Higher frequency & frequency modulated FM). It's like reinventing the wheel - 40FM was preferred over 27AM cos there was less interference, cos it had to be more accurate to work.

As always, I'll go one step further - could it be that FM crystals are marked FM because they're manufactured to closer tolerances than those for use with AM? - did we ever get a result from trying FM crystals in an AM radio? (Or was it the other way around?)

I did vary the setup as much as I could with the gear I have...
There's one generic transmitter with it's original Rx and crystals - plus the recently acquired Robbe R123FA Rx.
That all works fine together with the original xtals and doesn't do anything with the new ones.
I suspect they may work with gear from another brand. Should have asked before I ordered maybe...
The receivers that are mentioned in the second add make me more hopeful for a succesful combination.
Especially after seeing this page :

Futaba receivers

But I'll not cheer too soon. I think it's a tricky subject.
If I find wb some AMs, I could try 'em out on the FM gear...

Okay, I've done a bit more research...
First I came across an ad for a TX and Rxes :

That wrote:

TX.

* 1X Futaba Megatech 3PM-FM HRS.

RX.

* 1X Micro 3CH Futaba R203HF HR.
* 1X Micro 3CH Futaba R133F.
* 1X Micro 3CH Futaba R123FA.

Crystals.

* 1X TX Futaba 40.835 FM Type 40-6.
* 3X RX Futaba 40.835 FM Type 40-6.

It's still a bit beyond me but I am suspecting it has to do with single or dual conversion...
He seems to be using the same crystals and receiver at least.
The clue may lie in the (single conversion) Rx I have with a dual conversion crystal.
Looking into the types - 40-6 and 40-12... not much to find on the net.
And nothing like that on the original crystals I have.
Furthermore - the second pair I ordered seems to say 40-10.

Yep, that's it. I have a dual conversion crystal. Type 40-12.
I've done some research before I ordered them but had not gotten to the bottom of this.
All I knew was that they should match Futaba with such type references.
4-6 and 40-10 are single conversion so I should have more chance of succeeding with the next pair.



Btw - the description of the ad above does not match the crystals in the picture.
Those 40-12s should not be useful. Maybe he made the same error...

That's something that's been bugging me for a while, what's the single/dual conversion thing about, what's it referring to?

From what I read, the dual conversion receiver filters the signal in two steps instead of one.
Don't ask me how exactly...
But it'll have more reach and less chance of interference that way.
Only applies to Rxes btw.

Oddly enough most of the info on the subject that's on line seems to be full of switcheroos.
From my own crystals, some info on eBay and a Dutch RC plane forum I concluded 40-12 must be the dual version.
We'll hopefully soon see if it gets confirmed that 40-10 are single conversion crystals.
Only difference with 40-6 should be that they're a bit longer...