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What kind of 72Mhz crystals are these?

Old 03-06-2015, 10:00 AM
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rustyrivet
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Default What kind of 72Mhz crystals are these?

I'm confused with the bare metal crystal on the left of the photo. The plastic gray cased crystal on the right is the standard JR crystal that I'm familiar with

I bought the gray cased crystal from a seller here on the classifieds, and he included the bare metal crystal on the left with my order too, (as a bonus). I'm guessing that it is the same crystal but without the plastic gray casing??? The bare metal crystal says "single conversion" on it, and I was wondering if it is exactly the same as the one in the plastic case. (??) Or is the one in the plastic case a dual conversion? Or are all crystals the same single/dual conversion, and it's the receiver that determines single or dual conversion aspect of it? Or, can the frequencies be slightly different, and one crystal might work at a longer range then the other, so it would come in handy to have a bare spare crystal that might work better??

I have never disassembled the crystals from out of the gray plastic case, and was wondering if there is any reason to ever start swapping them in and out of the plastic cases.


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Old 03-06-2015, 11:04 AM
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Rodney
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The frequecies for dual conversion and single conversion are different for the same channel. The crystal for either is usually 1/5 or 1/3 the actual frequency being transmitted, the frequency being multiplied electronically by the local oscillator. A single conversion transmits a frequency either 455 Killocycles higher or lower than the channel frequency, a dual conversion transmitter transmitts 11.7 Mhz higher or lower than the channel frequency. You must use the proper crystal, single conversion for a single conversion receiver, dual conversion for a dual conversion receiver as well as have the proper positive or negative difference. Also, different brands of transmitters use different local oscillators so the cut of the crystal can be important (is it a Colpits or a Hartly oscillator or something else) slightly different ways of tuning the crystal and multiplying the fundamental frequency. As to your question, the grey plastic is just what you thought, a holder for the type of crystal case you have on the left. While a crystal may look simple, the actual details can be quite interesting, such as is it a Y cut, C cut or a hybrid. Different cuts require different oscillator circuitry.
Old 03-06-2015, 11:48 AM
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Not quite correct. Any transmitter will transmit on frequency and the transmitter doesn't care at all whether the receiver is dual or single conversion. A dual conversion receiver converts the on frequency transmitter signal to the first IF frequency which is in the 10 - 11 MHZ range. The second conversion further converts it to the fairly industry standard 455 Khz. Obviously, single conversion receivers convert the received signal one time directly to the lower 455 IF. So single conversion crystals are very different from dual conversion crystals.

The crystal in the gray holder shown in the photo is a JR crystal and can be easily identified as such because of the circles and dot below the frequency. These denoted the frequency tolerance of that particular crystal. For the record, JR produced very few dual conversion receivers as they used a special process (they called it ABC&W) in their receivers to maintain the required narrow band and interference rejection characteristics w/o using dual conversion. In later years, only a few of their more expensive, PCM receivers were dual conversion. I know there are exceptions so please don't take me to task on this if you are a JR fanboy.

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Old 03-06-2015, 12:35 PM
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Rodney
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Mia culpa, Truckracer, you are so correct. I spoke in post 3 before engaging brain as I know better. It is the receiver crystal that have to be offset from the transmitted frequency, 455 KC difference for single conversion and 11.7 MHz for dual conversion
Old 03-06-2015, 12:39 PM
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Truckracer
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Rodney, don't feel bad! Quite often my fingers don't type the words I'm thinking then I have to go back and edit at a later date. Product of age for me though my brain has always been faster than my fingers.
Old 03-06-2015, 02:27 PM
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rustyrivet
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"Any transmitter will transmit on frequency and the transmitter doesn't care at all whether the receiver is dual or single conversion. A dual conversion receiver converts the on frequency transmitter signal to the first IF frequency which is in the 10 - 11 MHZ range. The second conversion further converts it to the fairly industry standard 455 Khz. Obviously, single conversion receivers convert the received signal one time directly to the lower 455 IF. So single conversion crystals are very different from dual conversion crystals."



Guys, please have patience with me, but both posts are technical in nature and neither succinctly tells me if both these crystals in the photo are the same from JR and will work in most common JR receivers. If I described a device that was designed to make molecules deflegrate rapidy while covering a large expanse of area with intense heat through fission and the destabalization of radioactive isotopes, I may not know what you're talkng about until you told me "Rusty, such devices have been placed on top of a missile or dropped from a plane, and they usually work very well."

I have P720, NER 649, and R700 receivers, all are FM and some PCM. NONE of my receivers says anything about dual or single conversion on them, nor does the plastic cased crystals. And I can interchange any of my gray cased crystal in them. From what I have read on Google in the past is that is no PRACTICAL difference with most modern JR 72mhz receivers and they are all made to operate the same as far as single/dual conversion goes...except the crystals that come in the red case which is designed for a special JR receiver. Are you saying that the crystal that came in the gray case would be marked from the factory with a sticker that said "dual conversion" on it.........and it really is different then the bare crystal in the photo that has a sticker that says "single conversion" on it?
Old 03-06-2015, 02:52 PM
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The gray cased crystal will work in all recent production (before 72MHz went away for the most part) JR receivers "except" the special dual conversion receiver which probably uses the red cased crystal you refer to. I have no idea whether the crystal with the tag is the same or not. Plug it in and see if it works. If it works do a range test and if you get the same range as a known correct crystal, assume it is ok to use. If it doesn't work ..... well then you know it is different and not for the receivers you have.
Old 08-30-2015, 07:21 AM
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Great information!
I now understand why a different crystal for single and dual conversion, and that crystals are not interchangeable between brands.
Now I just need to find a ch 48 FM dual conversion for another Futaba R127DF receiver.
Thanks
Jayd
Old 08-30-2015, 01:23 PM
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Jim Schwagle
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http://www3.towerhobbies.com/cgi-bin...2+MHz+crystals

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