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Author Topic: Icom CR-338 Installation Question:  (Read 1834 times)
AB7RG
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« on: March 29, 2002, 09:34:20 PM »


I have a question regarding the installation of an Icom CR-338 High Stability Crystal Unit. In the manual it mentions hooking up a frequency counter to the frequency check point and adjusting the frequency to 64.00000 MHz with an adjusting pot in the transceiver. My question is, is this necessary? I do not have a frequency counter (good excuse to buy one though), and the radio is new and appears to be exactly on frequency already. I just purchased the CR-338 to make sure that there isn't any drift at all in a field environment using PSK-31. So has anyone installed the Icom CR-338 in their radio without using a frequency counter? It seems to me that if your radio is already spot on that no adjustment would be needed, just a simple replacement of the original internal crystal with the CR-338. Is there any other advice or experiences with the Icom CR-338?

Thanks for any help/advice!

73 Clinton AB7RG

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AC5E
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« Reply #1 on: March 29, 2002, 10:30:54 PM »

Clinton, I would gladly bet you a dollar to a doughnut that every thing that affects frequency in your rig is referenced to that crystal oscillator. Crystals have tolerances like everything else. Even a little shift in frequency will throw everything else off. That's why you want a high stability oscillator in the first place.

So do buy that frequency counter before you go any further. A good frequency counter, one with close tolerances.

And practice using it on your rig BEFORE you change the crystal. Because it's quite possible for a frequency counter to lead you far astray instead of putting you on the button. So you want to know what things should be before you start adjusting things.

73  Pete Allen  AC5E
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N0VVV
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« Reply #2 on: December 14, 2012, 03:18:04 AM »

I too have a question and a comment on this topic.  I have already installed the CR-338 high stability Crystal.  And my frequency is off by .05+.  Where can I purchase a frequency counter that goes to 64.00000 Mhz ?  99% of them seem to go to only 50 Mhz.  Also do I unplug what is in the port while checking the frequency ?  Or just tap into the port ?  Also to adjust it while the radio is powered would be a chore because you must have the radios bottom cover off and insert a screwdriver from the bottom up.  Which seems very extream to me in order to do this.  Please help.    Thanks.
 
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AA4PB
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« Reply #3 on: December 14, 2012, 05:38:20 AM »

Its the oscillator that determines the frequency error in the radio. You are replacing the original with a new one (the CR-338) so it will now determine the frequency error so the fact that the radio was accurate with the old one means nothing. That's why they recommend adjusting it with a frequency counter. You'll probably find it pretty close without adjustment however. Just check your receiver against WWV or some other accurate signal source.

Remember, the counter needs to be accurate if you expect the oscillator frequency (and thus the radio) to be accurate. That excludes the use of most of the inexpensive frequency counters. What you need is a lab grade HP or something along those lines if you want to "know" that it is accurate.

I've installed several and just checked the radio against WWV.

Actually, the original crystal units are fine for PSK31 work if the radio is in a reasonably stable temperature environment (i.e. inside the house). I might consider one if I were operating from a mobile where the temperature can vary all over the place.
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WA3SKN
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« Reply #4 on: December 14, 2012, 06:27:29 AM »

Good reason to get the high stability unit.  I would just check the frequency against WWV and unless it is off I would not make adjustments.  And I would look to borrow a lab unit that is accurately calibrated to make any adjustments... any local clubs in the area where you could ask?

-Mike.
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K5LXP
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« Reply #5 on: December 14, 2012, 02:37:57 PM »

There's two reasons to have the TCXO - accuracy and stability.  If all you're interested in is stability, you're already done.  Whether it says 14.070.00 or 14.070.35 is irrelevant.  If you want the dial accuracy to go with it, then you'll have to net the reference to a standard.

I recall the 64MHz netting is just a coarse adjustment anyway.  Any basic counter will be accurate enough for that.  Then use the fine adjustment pot on the back of the rig to net it exactly by zero beating WWV.  That can get you easily within less than 1Hz accuracy, better than your display is able to resolve.


Mark K5LXP
Albuquerque, NM
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AC5UP
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« Reply #6 on: December 14, 2012, 03:28:49 PM »

On the original Icom 756 it is possible to calibrate the radio EXACTLY on frequency with no tool more exotic than a small screwdriver. You may be able to adapt this idea to your rig.

By the numbers:

1)  Warm up the radio for 30 minutes or better.
2)  Set the RX mode to AM
3)  Turn on the internal 100 kHz marker / calibrator
4)  Tune WWV to whichever frequency is strongest and has the least QSB
5)  Adjust the master oscillator trim cap through the hole in the lower front right cover for exact zero beat *
6)  Turn off the internal 100 kHz marker / calibrator

*  When approaching zero beat the audio will go sub-audible. Observe the S-Meter. It will oscillate slowly then hold steady at exact zero. It takes a good hand to tweak to exact zero, but don't worry about being off slightly as the adjustment will drift a tiny bit with temperature. There is no shame in being off by three Hz.

Ten Hz, yes, the other guys in the net will start calling you Vinnie.  But three Hz? Not so much.   Tongue
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WB6DGN
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« Reply #7 on: December 15, 2012, 01:10:58 PM »

All the other posters have pretty well nailed it.  I'll just add one comment.  By convention, the measuring instrument (in this case the frequency counter) should have an accuracy of, at least, two times that of the unit under test.  That means that, if your new oscillator has a stability of +- 1.0ppm (parts per million), then your frequency counter should have an absolute minimum stability of +- 0.5ppm.  I haven't checked the current crop of frequency counters on the general market in many years but, when I last bought mine, that type of counter was pretty pricey.  If it were me in your situation, I'd do as several recommended and install it and check it against WWV or, if it tunes low enough, WWVB.  I wouldn't adjust anything that way but only make note of the comparison and mentally compensate your readings.  Then, if you have access at a later time to an appropriate instrument such as a late model service monitor that's been recently calibrated, you could adjust it using the factory method.
Tom
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N0VVV
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« Reply #8 on: December 17, 2012, 05:54:20 AM »

Thank Guys for all of your input.  I now have a good idea to correct my problem.  However, I have one more question.  I have located the frequency adjustment screw on my Icom-718.  My frequency reads Low on my Radio.  To be on frequency say at 7.255.00, I must tune to 7.255.09.  Which way should I turn the frequency adjustment screw ?  Clockwise or counterclockwise to correct my problem ?  And would a quarter turn in the right direction be to much or not enough ?   

                                                                                        Thanks Again
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K5LXP
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« Reply #9 on: December 17, 2012, 07:55:11 AM »

Set the dial to WWV and leave the rig on for an hour or so to stabilize.  Wait for the point in the transmission where there's no tone, just the ticks.  When that happens, turn the frequency pot to zero beat the tone until it goes away.  It might be easier to to with headphones on.  If you listen carefully you can hear just a flutter when you get within a few hertz of zero beat, and basically nothing or a slow swishing sound when you're dead-on.


Mark K5LXP
Albuquerque, NM
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