Dave, Keep in mind that the Midland 13-509 was built for the Amateur Radio market, and the transmit crystals are only specified to maintain .001%, which is 10 PPM. There is no temperature compensation in the crystal circuit, and you may make the drift worse by using an NPO capacitor. If you have the time and the test equipment to do it, you can determine a crude temperature compensation by finding out how much the TX crystal drifts for a given change in temperature, then using a capacitor whose TC has an equal but opposite effect. This capacitor will definitely not be an NPO type, which is stable over a wide temperature range. In fact, you want an "unstable" capacitor that exactly balances the crystal drift.
Commercial radios of the same vintage often used bare crystals with a color dot on the side of the can, and you were instructed to install the appropriate color TC capacitor with that particular crystal. Not perfect, but adequate. You might also consider replacing the bare TX crystal oscillator with a small TCXO unit from any of several sources, including ICM. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of na6df Sent: Monday, May 22, 2006 10:48 AM To: [email protected] Subject: [Norton AntiSpam] [Repeater-Builder] midland 13-509 tx freq stability... cap change? Working on my midland 220 box, with new international crystals. Crystals are standard delivery, not "rushed", so they should be pretty stable. They always are in my other rigs... Question: Is it worth swapping out the fixed value cap that is paralleled across the ceramic trimmer on the transmit side? Mine seems to drift around a bit more than I like. I have to assume the stock cap is an NPO type. Schematic does not state capacitance of this cap. Anybody know what it is? RF Parts sells NPO's, but is it worth it? Better ideas, if any? tnx and 73, Dave NA6DF Yahoo! Groups Links Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
