It's not just netting the crystal; there's a lot more to compensating a crystal than simply compensating for temperature. It is easy to assume that all ICM or Bomar does is to tweak a few capacitors so that the crystal can 1) be brought to the proper frequency with the trimmer centered, and 2) maintain a close tolerance over a wide temperature range, But the really good crystal houses also make component changes to 3) ensure spectral purity of the output, 4) ensure proper operation with the specified load, 5) ensure the static output level exceeds the minimum specification, 6) ensure the deviation is symmetrical about the center frequency, and 7) ensure that the channel element can be deviated full-range with a specified input.
As noted in the article on the RBTIP, items 3 through 7 cannot be performed without the channel element. I know of quite a few cases where a transmitter's signal was distorted when modulated but was rock-solid when unmodulated, and the problem was cured when the crystal and the channel element were sent back for complete compensation. As always, YMMV... 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of kk2ed Sent: Wednesday, April 23, 2008 9:46 PM To: [email protected] Subject: [Repeater-Builder] Re: Info Wanted: RE: Motorola Channel Elements parts I posted on this subject awhile back. Do a search on posts by my user name. Basically, there are two caps towards the edge of the circuit board (opposite of the trimmer/socket end) that vary anywhere from 5- 33pf. Install the crystal, then play with the value of these two caps. I have found that I usally need to reduce the capacitance a little, and the crystals net right on. I've never had to change an inductor. Pick up the small capacitor assortment that Radio Shack sells - it has about 50 caps in the 5-33pf range. I use them all the time to replace the original caps in the elements. Before you start changing the caps, first install the crystal and fire it up, and while watching your frequency counter or service monitor, set the warp trimmer to midpoint (halfway between the lowest and highest freq settings possible). Then start playing with the capacitor value until the crystal is as close to your desired frequency as possible. This simple step will ensure that you can adjust the crystal adequately up or down as needed in the future, as well as the initial setup. If the capacitor swap gets you within 2Khz or so, the trimmer should have enough "pull" to net you on center frequency. If I recall, the capacitor in question is usually red, and while holding the element component-side up and plug-in end towards you, the cap is to the far left corner. Speaking of elements, I need to call Bomar tomorrow - a one-year old 440MHz repeater crystal decided over the weekend to jump 25Khz high - right onto the input of another repeater! Same PL no less! (Here in the NE every other 25KHz pair flips low-in hi-out and vice versa) I think someone was going to take my original post about compensating elements and place it on repeater-builder, but I never noticed it there. Hopefully it will save others some money. It always amazed me that guys would embark on a large project building, wiring, and tuning a repeater, but the simple task of re-crystalling and netting elements always seemed to be intimidating. Most likely due to lack of info being published on the subject. Eric KE2D --- In [email protected] <mailto:Repeater-Builder%40yahoogroups.com> , "w4dg.geo" <[EMAIL PROTECTED]> wrote: > > > I recently ordered TX and RX crystals from ICM for the MICOR RT Station. > I specified the correct FC and FX and catalog # 167380 for MOT KNX1018C > TX (132-174). When these crystals were received they would not "net" on > frequency. The TX was over 20Khz. high and would not net any closer. I > notified ICM and their Customer Service Dept., agreed to replace the TX > crystal. When I received this "replacement "crystal, it was over 30Khz. > high! I sent another email and got a reply from the owner, Royden > Freeland, W5EMH, stating the following: > > "To get the crystal to tune properly you will need to change L1 to > 2.6-2.7uh and C6 to 1-10 pf depending on the crystal to get it on > frequency at close to center trim. Each element was compensated and > tuned for the original crystal so if you are changing frequency very far > there can be problems. Motorola generally used a crystal load of 32pf > in elements with trimmers and varied components within the element to > bring it on frequency. ICM will be happy to install the crystal and > compensate the element if you would prefer. We would charge $60.00 to > install and compensate your element and crystal". > > I have printed out the channel element schmetic and picture (from W3KCC) > repeater-Builders website and see that C6 is the trimmer and L1, but no > values are represented. Can anyone tell me what the current value of > the inductor L1 is? If it's currently higher in value, can it be > "rewound" for the 2.7uH inductance ICM references? I believe there is > enough room to insert a fixed cap of proper C in series to drop the > tuning range of the C6, to perhaps the 1-10pf. I have looked at Mouser > Electronics for a trimmer 1- 10pf and 2.7uH axil-lead inductor, > L1without success. > > If they are available, can you please stear me to where I can purchase > one or both? > > I always thought that you sent ICOM's back for temperature compensation > and not for simple exchance of crystals. I have never needed to to that > with any GE MASTR series TX & RX or Mocom 70 series or any EF Johnson > series that simply required new crystals. But again, that was 20 years > ago, which is a long time. > > Any advice would be appreciated as I don't need to spend another $120 to > "install the crystal and compensate the element". > > TNX is advance...Dennis - W4DG >
