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Azure
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Posted: 26 April 2018 at 11:02am | IP Logged Quote Azure

Grogster wrote:
I though a crystal was a crystal was a crystal - so long as it's frequency is accurate.


Yes and No, it depends on the application for most general use that would be a fair statement.

From my memory using different Tx/Rx Freq crystals was for one way communication. The crystals were matched (or mismatched depending on how you look at it) by the IF frequency.

Each Crystal was still marked with the crystals actual frequency, just they used a different frequency in the Tx unit and the Rx unit.

This meant for the remote controls back then you had to have a Tx/Rx pair for every channel you were going to use in your remote. Hopefully the Rx unit was easy to access to swap the Rx Crystal, sometimes they were tucked away for replica or small scale models.


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PeterB
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Posted: 26 April 2018 at 11:19am | IP Logged Quote PeterB

My experience with radio control was very limited but I seem to remember Xtals could be marked e.g. T27.xxx for Tx and R27.xxx for Rx and the two frequencies, although marked the same, were different by the IF. I think there was also a letter code.
My experience was in two RC tanks for a nephew who was only 50 at the time.
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Boppa
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Posted: 26 April 2018 at 11:30am | IP Logged Quote Boppa

Correct PeterB, also used was a grey heatsrink for rx and a black one for tx
The marking was 'usually' for the nominal frequency, the actual frequency the crystal ran at could be a number of different actual frequencies, depending on use, it could be at the actual frequency, it could be (from memory here, so actual numbers may be wrong) 455khz out, 10.7or 10.8??mhz out, or it might be used at a multiple so if using 2nd harmonic could be half the cases written frequency
There were also different 'cuts' which had an effect as well, but that I am hazy on (was a long time ago I regurly played with them, back in the old 18/23 channel cb's and 3 channel dse/tandy walkie talkies, then remote controled planes for a while)

One trick we used to do to get 'private' channels on crystal controlled two ways was swap the rx and tx crystals on one channel, that way standard sets couldnt pick up our tx's (good for fox hunts lol)- also worked when you were in congested airways in the city
(are you PeterB from SSSF by any chance?
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PeterB
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Posted: 26 April 2018 at 11:37am | IP Logged Quote PeterB

I have often wondered if it was possible to swap Xtals and hence move the channel by twice the IF.
I am not PeterB from SSSF, well I don't think so. What is SSSF?
Peter

oops. Move the channel by the IF. I hope that's correct.
Peter

Edited by PeterB on 26 April 2018 at 11:40am
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Boppa
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Posted: 26 April 2018 at 11:39am | IP Logged Quote Boppa

Was an old forum now closed that had a PeterB as well (was run by the ABC for Dr Karl)
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Grogster
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Posted: 27 April 2018 at 10:20am | IP Logged Quote Grogster

UPDATE:

Phil over at the PICAXE forums has received two of my clone modules, and has tested the crystals - they are stunningly bad......

An extract from an email that he sent me:

Phil from PICAXE forums wrote:
28.5MHz and less than half the amplitude of the signal across the T300 crystal!


Please visit the PICAXE thread for an image he has posted comparing the genuine with the clone crystals.

I have asked him in an email if I can repost his image here, because you can't get the full-size image from the PICAXE forums, unless you are logged in, and many members here won't be members of the PICAXE forums. As soon as he says that is OK, I will re-post his image here.

EDIT: Permission granted from Phil to re-post his scope image, so here you are:





For a crystal that is supposed to be 30.000MHz, this is not good....

Edited by Grogster on 27 April 2018 at 11:18am


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Grogster
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Posted: 02 May 2018 at 9:41pm | IP Logged Quote Grogster

Just a little update for this thread:

reginald on the PICAXE forums wrote:
I had three HC-12 30mhz crystal variant left whch did not communicate well(or at all) over distance with my T300 type. I obtained new crystals from farnell type

7M-30.000MEEQ-T - Crystal, 30 MHz, SMD, 3.2mm x 2.5mm, 10 ppm, 10 pF, 10 ppm, 7M Series stock No 2308724

Swapped these today and now all three boards are happy chatting away with no errors. One was a USB type plugin which failed and on opening up it had a HC-12 board piggy backed on the USB board which I had swapped with 30mhz type (before I new the difference) and wondered why it didin't work. Shows not all crystals are equal.


I have ordered some new crystals, but they are not here yet. I got mine from Element-14 here.

I am now very hopeful that I can revive some of my dead ones by recrystallizing them.

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CaptainBoing
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Posted: 02 May 2018 at 10:52pm | IP Logged Quote CaptainBoing

good stuff! keep us in the loop
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paceman
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Posted: 08 May 2018 at 12:17am | IP Logged Quote paceman

Grogs, you're famous - this HC-12/crystal problem thread made "Hackaday" - here
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Posted: 08 May 2018 at 5:17am | IP Logged Quote CaptainBoing

... which completely justifies the in-depth detective work undertaken by the entire thread. Just look at the coverage and potential pain you have saved others.

good stuff!
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JohnS
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Posted: 08 May 2018 at 5:43am | IP Logged Quote JohnS

Congrats to Grogster!

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Grogster
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Posted: 08 May 2018 at 9:55am | IP Logged Quote Grogster

@ paceman - thanks very much for the link! Crikey!

Nice to see most of the comments support what we have found out thus far. Usually, the comments section on blogs can get nasty if people still think you are wrong. For the record, I am perfectly prepared to accept anyone else's point of view, and if they can demonstrate that the testing done thus far was fundamentally wrong, I will listen.

There. That should keep everyone happy.

My replacement TXC-genuine crystals have arrived, and when I get a spare moment, I plan to replace the crystal on one module with the TXC one, and see if I can replicate the results from reg on the PICAXE forum thread.

I am quite hopeful that I will. (fingers crossed)

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