Re: [Repeater-Builder] Micor Recrystalled Element Problem

[Kevin Custer]

Randy Nelson wrote: I am moving a 463/468 MHz Micor repeater to 444/449 MHz. I ordered new xtals from ICM and the receive xtal is OK. However I can't get the xmit xtal to pull onto freq. It will pull within 3 KHz above 444.350 MHz. This isn't the first Micor I have changed rocks in myself. I've done several in the 450-470 MHz range without any problems and no realignment callbacks to the tower site. This isn't my "first time." I have a Micor test meter and used it to align the exciter and everything tuned as designed. I called ICM and they doubted it was their error and it must be my channel element since the frequency move was quite far. But for $30.00 they would "fix" the problem. Does anyone have any technical docs on the Micor channel elements and the require capacitor changes? The channel element is KXN1052A. I would rather replace the required cap with the correct value and get this repeater on the air instead of waiting another 3 weeks for ICM. I have read the Repeater-Builder webpage regarding Micor elements and it doesn't address the need to replace any components inside the element when moving from 463 to 444 MHz. The first thing I'd check is to make sure the variable capacitor (the netting capacitor) isn't broken.  It is easy to break the ceramic rotator on the netting cap; which results in the element not being able to go low enough. At any rate, here is some information on Motorola Micor Channel Elements: http://www.repeater-builder.com/micor/micor-element-info.html The KXN-1019B and KXN-1052A are the same element with only a few exceptions, so the schematic and layout available (for the 1019B) from this page will help you with your 1052A.  Changing the capacitor in parallel with the netting trimmer will change where the crystal resonates.  Realize the capacitance in parallel with the trimmer and the type of capacitors used there affect the loading and temperature stability of the element. Please realize that the frequency stability of the element is only as good as the crystal installed in it, and the thermal compensation that was done at the time the crystal was installed in the element.   If you are serious about your repeater, especially if it is going to be located in a hilltop building that has no environmental controls, do yourself a favor and when you purchase the crystals for your repeater send the elements back to the crystal manufacturer and pay to have them compensate the element to the new crystal.   The elements will hold 2ppm or 5ppm only if the individual element is matched to the crystal - i.e. if the crystal tends to swing upwards in frequency as the temperature rises, the element has to swing lower, and the same amount.   Likewise, if it swings down in frequency, then naturally the element has to swing upwards.   Another way to think of this behavior is "frequency tracking with temperature". Note that the compensation components in the element are dependent on the characteristics of the individual crystal, and those characteristics have to be measured after the crystal is made, and the components selected by hand to match, and then installed in the element.   Hence, this has to be redone whenever the element is recrystaled.   Many of the cheaper suppliers simply install the new crystal into the element and ship it back.   This can result in real problems if the new crystal swings the opposite of the old one, and as the temperature causes the crystal to drift off frequency the compensation components push it even further off frequency. Also note that there is a difference between a receive crystal and a transmit crystal.   A crystal with the *wrong* cut may not exhibit enough *rubberiness* for full deviation to occur.   This situation happens when crystal vendors cut a crystal for absolute stability and don't know that it will be used in an FM transmit element, like the KXN-1052A. ICM and Bomar are the only companies that I know of that actually test the modulation capability and insure modulation symmetry in elements.   They are also the only companies that I know of that actually change the temperature compensation components (capacitors and resistors) to *properly* TC the element. When you are ready to recrystal an element and are calling around to get prices, you need to ASK what each company consider "compensating".   Considering what you are going to spend on the entire repeater paying a little extra for a professional top quality rock house to manufacture the crystals and do the compensation is definitely worth it.  Now you know why ICM wants $30 for this effort.  Realize that it is NOT only to get the crystal/element on frequency, but to make sure it stays there in varying temperatures as well! All new crystals drift during the first few months of operation (most noticeable on 900MHz, and quite visible even on 2m).   This is called "crystal aging", and IS QUITE NORMAL.   Even if your brand new UHF channel element drifts as much as 7 kHz (on the transmit frequency) during the first 4 months, it does NOT mean that the crystal is defective.   Just plan on setting the frequency a few of times in the first 6 months of continuous operation.   Once it ages it will settle down.   As a personal example, one of my UHF receivers drifted 11 kHz in the first 5,000 hours of operation (four adjustments of about 5 kHz, 4 kHz, 2 kHz and 400 Hz across roughly 6.5 months), and hasn't budged 800 Hz in the 20 years since.   Since then, crystal manufacturers have adopted a technique called "pre-aging", where they "age" the quartz crystal material in an oscillator circuit under elevated temperatures for a time period.   Then they make the crystals using these pre-aged blanks.  This technique produces a crystal that might drift 3-5 kHz total. Hope this helps... Kevin YAHOO! GROUPS LINKS  Visit your group "Repeater-Builder" on the web.    To unsubscribe from this group, send an email to:  [EMAIL PROTECTED]    Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.