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Old Jun 15, 2004, 03:11 PM
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Can you mount an Xtal outside an Rx, and link the two with wires?

Title says it all really. I'm pretty pushed for space on my homebuilt plank and I'm trying to get all the electronics as flat as possible (so I can put a super shallow canopy over it to make everything more streamlined). Here're some snaps.


Larger image


Larger image

As you can see from the second picture, the crystal is sticking out more than everything else and it's rubbing on the servo. At the moment the crystal's legs are directly soldered to the sockets they usually fit into. Not the prettiest idea but it works and it's made it flatter than normal. My question is could I mount the Xtal in the only space left (on the wood where the black lines are) and run two thick (solid) wires back to the posts on the Rx where the Xtal would normally fit? I've done this on a GWS Rx and I haven't noticed any problems. That's the only way I can think of for getting the receiver as flat as possible. I don't see why it wouldn't work, if the wires were exactly the same length .... but what do I know!

What do you guys think?

Thanks
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Old Jun 15, 2004, 05:48 PM
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All you can do is try it. In general it's not a brilliant idea because even adding a couple of fairly short wires will change the capacitance the crystal sees and can in theory throw it out of tune.

Some receivers don't seem to be much bothered by changes like that but others can simply stop working. I don't know any way to tell in advance which will be which.

Steve
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Old Jun 15, 2004, 06:05 PM
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Thanks for the reply Steve. Yes that's what I had been thinking about too, hopefully it shouldn't make too much of a difference though. I'll try it tomorrow and of course perform a range-check.

Thanks
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Old Jun 15, 2004, 10:14 PM
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Crystals can be obtained with very small diameter wire directly soldered to either side of the circuit board, compared to the usual thicker diameter pins plugged into the Rcvr's factory provided socket...However you end-up electrically connected, make sure you've got a very well supported physical attachment.....You absolutely need that little, fragile piece of quartz solidly working with no chance of a bad connection......... kw
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Old Jun 16, 2004, 03:51 AM
HeliAP'er!
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Thanks KillerWatt! (Love the name! )

Well, here's how I've done it:


Larger image

I'll be doing a range check just as soon as I have the time.

Cheers
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Old Jun 16, 2004, 04:04 AM
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Quote:
Originally Posted by Hogster
Well, here's how I've done it:

[SNIP]

I'll be doing a range check just as soon as I have the time.
Well don't do it! Seriously, those leads WILL pull the center frequency off a little. And do remember crystals are very fragile, so support well and isolate from vibration.

I don't want to sound a killjoy, but consider your position if you should happen to injure someone.

"Yes your honour, I know I shouldn't have done it like that, but I thought it was worth a try!"

Just my 2p worth,
--
Steve
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Old Jun 16, 2004, 04:15 AM
HeliAP'er!
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Point taken Steve. But IF the range check is flauless, is there really anything to worry about? If I did an aerial-up range check and walked the full length of the field I fly at (must be at least 300m) and all if Ok, should I be concerned?

I hear what you're saying though and I appreciate your advice.
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Old Jun 16, 2004, 03:26 PM
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Quote:
Originally Posted by Hogster
Point taken Steve. But IF the range check is flauless, is there really anything to worry about? If I did an aerial-up range check and walked the full length of the field I fly at (must be at least 300m) and all if Ok, should I be concerned?

I hear what you're saying though and I appreciate your advice.
Well the point is that even if the crystal frequency is pulled slightly off centre it won't affect the range test. But somebody on an adjacent channel in the direction the crystal is off by could well interfere with your receiver.
The xtal freq would have to move by a significant amount as a proportion of channel separation before the range was affected.

When I used to design RF kit I took inordinate care to keep xtals, their tuning capacitors, and active components VERY close together - in terms of mm. And never allowed any related components to move in relation to each other. If you saw the internals of a crystal you'd understand just how fragile they are.

So I can't say with any certainty that what you propose won't work, but that you are taking an unnecessary risk - maybe with somebody else!

Surely you can come up with an arrangement that doesn't involve modifying the receiver and case?
--
Steve
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Old Jun 16, 2004, 04:39 PM
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I've been considering it and I think I'll just mount the Xtal as per usual. I can make a canopy that fits over the majority of the electronics, and just cut a hole to allow the Xtal to poke through. The risk (as you say) probably isn't worth it.

I'll be converting it back to its original arrangement tomorrow.

Hopefully I haven't *rs*d up the crystal by soldering wires onto it.

Thanks
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Old Jun 17, 2004, 07:46 AM
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Gone back to the proper method now



Thanks for the persuasion gents. I'll just make a hole in the canopy for the crystal to poke through.
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Old Jun 17, 2004, 10:35 AM
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Quote:
Originally Posted by Hogster
Hopefully I haven't *rs*d up the crystal by soldering wires onto it.
Oh dear, I have! Starts glitching at 20ft even with the aerial up. Guess I've learnt my lesson the hard way. That's a fiver down the drain.
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Old Jun 17, 2004, 02:58 PM
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Get the little Berg 4 channel. You could fit two of them in there.
RD
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Old Jun 17, 2004, 04:47 PM
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Bergs aren't available in 35MHz. Sucks MAJORLY. I hear Schulze receivers are almost as good as Bergs so if I can actually find any spare money I may go with one of those instead!

I'm trying to make the most of the equipment I currently own ... and if that means using a full-range Hitec Rx, then so be it! Perhaps when I've actually got a steady income of money things will be different .... but possibly not!
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Old Jun 17, 2004, 05:51 PM
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Hogster,

What are your requirements for the receiver ? if only 6 channels then perhaps the 610 receiver from JR, 7 channel then either the R700 PPM or R770PCM receiver. They use end on xtal insertion.
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Old Jun 17, 2004, 06:02 PM
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As stated previously the added capacity added to the leads will detune the RX. It could possibly be realigned but it would not be a good idea. Yes, some Xtals came with wire leads that where soldered to the PC board,but that has not been the case for many years with most manufacturers opting for the socket approach. Xtals are one of the weakest links in the RX as they are fragile. Careful packaging in the aircraft (lots of foam loosely packed) will help prevent breakage in a crash.
As stated on prior posts, I like to mount the RX behind the servos. This way the RX antenna can easily be routed out the fuse away from servos and there is a lot of room for foam to protect the RX.
Most installs are what I call "the meat in the sandwich method" Engine,tank,RX pack,then the RX(the Meat) and then servos. In a crash the meat(RX) gets crunched with these other components.
Sinbad
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