Crystal AM or FM?

[mogul]

Hello,
I recently picked up a couple of old 27Mhz FM receivers. They came with a bunch of crystals, but I have no idea whether they are AM or FM crystals. I was just wondering if there was anyway to check?

I've searched the net, but can't seem to find any site that addresses this particular question, so I thought I would try here.

Thanks in advance.

[Coelacanth]

Do the crystals have any numbers printed on them? Without that, even I would be interested to know how to determine their frequency & AM/FM'ness.

[mogul]

The crystals themselves don't really have much written on them. See photo.
Just the word "Koyo" and then the frequency. I guess the only way to tell whether it is AM or FM is trial and error .
I was a little worried about damaging something using that approach.

[ASC6000]

don't forget about the 27mhz trick of reversing crystals, using the tx in the rx and the rx in the tx, creates a unique frequency

[slotcarrod]

The crystals themselves don't really have much written on them. See photo.
Just the word "Koyo" and then the frequency. I guess the only way to tell whether it is AM or FM is trial and error .
I was a little worried about damaging something using that approach.

You will not damage them. You can only assume they will be FM! But you know how assume is spelled right?

[slotcarrod]

don't forget about the 27mhz trick of reversing crystals, using the tx in the rx and the rx in the tx, creates a unique frequency

WHAT?????

Can I have some of what you been smoke'n?

[klavy69]

don't forget about the 27mhz trick of reversing crystals, using the tx in the rx and the rx in the tx, creates a unique frequency

WHAT?????

Can I have some of what you been smoke'n?

Thats a new one for me too. I've heard guys say it before though and that is what threw me here...
Todd

[Lonestar]

was always told it worked from people who were playing with their buddies both having the same RTR on the same frequency, and one swapping tx/rx xtals to be able to drive at the same time - never tried it.

will give it a shot this weekend on one of my runners. worst case, no signal

only drawback I can think of is electronics operating outside of their tuning window, so more glitches

stay tuned
Paul

[jwscab]

theoretically, the operation of the xtal's should not matter whether rx or tx. it is possible that the manufacturers in fact did use two different types of xtal for the transmit and receive and here's why:

a crystal converts electric energy to mechanical energy, or vice versa. they are sliced literally from a piece of crystal, and the geometry of the slice dictates the natural resonant frequency. depending on how the crystal is cut, you can 'pull' the crystal in it's natural frequency by capacitors in it's oscillator circuit, or voltage applied to the crystal. So every crystal that you buy has specific characteristics for it's 'pullability' and loading power (the voltage applied to it)

for an AM system, you basically change the gain of the frequency in the transmitter with your control signal, which the receiver deconstructs by using a crystal at the same frequency, the receiver basically 'filters' the crystal frequency from the control signal, which it then sends to the servos. so in this case, the crystals are pretty much the same, and should swap back and forth, rx to tx and operation should be identical.

in an fm system, the crystal is 'pulled' from it's center frequency back and forth by the control signal to modulate the center frequency in the transmitter. so this crystal needs to have characteristics to allow it to be pulled back and forth over a wider range than a crystal that is just a free running reference.

on the receive side, the crystal acts like a reference, so it does not need to have the same characteristics, it is free running and needs to be very accurate with little drift.

it's possible that a manufacturer would use the same crystal cut for both transmit and receive but if the wider operation of the crystal on the transmit costs more, the manufacturer could save cost by going to a cheaper receive crystal.

so long winded, but, if you swap the crystals in an FM system, you could potentially change the center frequency slightly, and it might be enough that it would be a 'different' enough frequency that you can operate two systems that close together.

[slotcarrod]

Over the last 30 or so years of idiots (self included) and idiot customers placing TX crystals in the RX and vice versa with no good results. I have NEVER seen this work in AM or FM systems!

[ASC6000]

Did it for years with an XL2P. It only works with AM 27mhz if I recall. It definitely does not work with FM 75mhz. Found that out when I switched from the XL2P to the CS2P. I never tried it with am75 or fm27. Works like a champ, I did it almost all the time, just in case someone in the pits turned their radio on. Be warned it does create a NON authorized frequency.

[tamiya]

You will be transmitting at 455kHz lower than original TX frequency.

Works with AM but detunes your system so might not work with newer narrowband units.
But with old AM gear that had high RF output, Reversed freq was very handy.

Especially when we used to race heats with 10-12 cars running simultaneously.