Maxtrac 900 Mods Revisited First, some information on the vco module that is in the 900 Maxtrac. The module consists of two vco's. The left side of the vco is for receive and transmit, (repeater freqs) and the right side is for talkaround transmit. The normal range of the radio is 896-902 mhz transmit, and 935-941 mhz receive. The left side vco runs between 895.850-902.000 mhz for repeater transmit and receive. The right side of the vco runs between 935.000-941.000 mhz for talkaround transmit. The right side vco is pretty straight forward as it operates on the frequency that is program. If the programmed transmit frequency is 935.1125 mhz, that is the frequency the vco is operating at during tx mode. This is true of the left side vco frequency for transmit frequencies. If the programmed frequency is 896.0125 mhz that is the frequency the vco is operating at in tx mode. The receive vco frequency would have been easier to explain if the radio used a 39.00 mhz I.F. You would take the transmit vco frequency and just add the 39 mhz I.F. to it and viola your there. They actually use a 39.150 I.F. frequency, which shifts the transmit vco frequency down 150khz for receive frequencies. So the range of the left vco is 902.000 - 895.850 which equals 6.15mhz, and the right side vco is 6.00 mhz. In the following paragraph we will be referring to the left side vco (Repeater operation.) only. For operation in the ham band here in Phoenix we are using a 25mhz split, and if it were easy to do, changing the radio to a 25.150 mhx I.F. would keep the left vco with the same range of 6.15 mhz. But that would be the hard way. As it turns out, with a transmit frequency of 927.8375 mhz, and a receive frequency of 902.8375 mhz, the vco range will be 927.8375 - 39.150 = 888.6875, 902.8375 - 888.6875 = 14.15 mhz. The vco on most all maxtracs will handle a 14.15 mhz spread, and then some. The trick is getting it centered so both transmit and receive work. Now on to the fun. Both vco's need to come down in frequency range. In the original mod sheet you were to change the vco capacitor to move the range down. Well I tried it, and ten years ago when I could see I might have got this done, but not anymore. There needed to be an easier way to do this, and give you more control to put the vco right where you wanted it. Located just above the capacitor on both vco's is a stripline, which is laser cut by the factory to center the vco's. To move the vco frequency down, you would need to add more of the stripline back. Now I tried adding solder to the stripline and this was easier than the capacitor trick, but still didn't give easy control of where the vco ended up. We are now using a conductive pen, which dispenses highly conductive silver in an epoxy type base. (Thanks to wa7cjo, Jim for this very helpful tip) The pen is sold by Circuitworks (pnCW2200MTP) and sells in the 11-15 dollar range. You can now just dab the pen on each stripline, and let it dry. On the left vco, while measuring the vco voltage, slowly scrape away the silver trace you have applied, (a scribe works well for this operation) until you get the receive vco voltage down to around 2 volts on your lowest receive frequency. This should keep the left vco below 7 volts on transmit. The right vco is easier, since it is only used for talkaround transmit. While keying the radio on a talkaround frequency, it is only necessary to get the vco voltage between 2 and 7 volts since receive is still being done by the left vco. The vco's on average seem to go out of lock at around 1.5-1.6 on the low end and around 7.5 to 8.00 on the high end. This seems to vary from radio to radio. The other nice thing with using the pen is if you don't like how it is going, use some acetone on a cotton swab, and just clean the stripline off and start again. You will find that the receive sensitivity is around .6 - .9 microvolts. Changing the two gigafils in the front end will return the radio to rated sensitivity. Exact replacements are available from Toko. The part number is 6DFB-915E-10, which are available from Avnet for 26 dollars each in 10 plus quantities. Have also uploaded a picture file, which may be of some help. mm900pic.zip approx 350k. Joe n1kq