NOTE: These modifications are for a 900MHz Maxtrac. Those are radios whose model numbers start with either D27 or D37. You cannot convert an 800MHz Maxtrac (D35 or D45 models) to 900MHz.
Below you will find a revised procedure for converting a 900MHz Maxtrac into the Amateur Radio band. The original method involved changing out a couple of surface mount capacitors in order to change the VCO range. The revised method is an update of the first which just uses a conductive ink pen to adjust the VCO range by changing the stripline characteristics. This should make it simpler to accomplish.
You will still have to change out the receiver front end filters for proper receive sensitivity.
Various part numbers for the front filters are:
You may find that the Murata filter has been discontinued. The TOKO filters should be an equivalent with the same specifications. The 6DFB should be about the same physical size as the original, it is available through AVNET Electronics. If you don't mind playing with small components, you could also use the 4DFB, it is a SMD dielectric filter with the identical specifications, and it is available from DIGIKEY.
If your radio doesn't already do conventional operation, you will probably want to order the conventional firmware part number FVN4019A from Motorola.
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, ie. repeater frequencies, and the right side is for talkaround transmit. The normal range of the radio is 896-902MHz transmit, and 935-941MHz receive. The left side VCO runs between 895.850-902.000MHz for repeater transmit and receive. The right side of the VCO runs between 935.000-941.000MHzz for talkaround transmit.
The right side VCO is pretty straight forward as it operates on the frequency that is programmed. If the programmed transmit frequency is 935.1125MHz, that is the frequency the VCO is operating at during TX mode.
This is also true of the left side VCO frequency, but only during transmit. If the programmed frequency is 896.0125MHz, 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 39.00MHz 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 39.150MHz I.F. frequency, which shifts the transmit VCO frequency down 150kHz for receive frequencies. So the range of the left VCO is really 902.000-895.850MHz which equals 6.15MHz, and the right side VCO is 6.00MHz.
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 25.150MHz I.F. would keep the left VCO with the same range of 6.15MHz. But that would be the hard way. As it turns out, with a transmit frequency of 927.8375MHz, and a receive frequency of 902.8375MHz, the VCO range will be 927.8375 - 39.150 = 888.6875, 902.8375 - 888.6875 = 14.15MHz. The VCO on most all Maxtracs will handle a 14.15MHz spread, and then some. The trick is getting it centered so both transmit and receive work.
Start with a 900 Mhz Maxtrac. If is is conventional then fine, if not then order FVN4019A conventional firmware.
Install the firmware if not already conventional, you may or may not have to blank and re-initialize the logic board to work properly with the new firmware.
Remove the two Murata filters in the front end and replace with a set of the above substitutions so that the radio will pass 915MHz +/-13.0MHz frequencies.
Locate the VCO and using a LARGE solder iron heat the top cover and pry it off. Now looking into VCO you will see that the small board has components on left and right halfs and they are a mirror of each other (the left and right). Looking in two corners you will see the VCO striplines that you will be working on.
The striplines are laser cut by the factory to center the VCO's. You need to move the range of the VCO's down in frequency. In order to do this, 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. The pen is sold by Circuitworks (pnCW2200MTP) and sells in the $11-$15 range.
Using the pen, dab it on each stripline, and let it dry.
Now, with the ink dried, perform the following. On the left VCO, while measuring the VCO steering line voltage (see the service manual for the required test point), 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 2VDC on your lowest receive frequency. This should keep the left VCO below 7VDC during 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 1.5-1.6VDC on the low end and around 7.5-8.00VDC 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.
Now put everything back together. Programming of the radio will require Maxtrac conventional software and when the program tells you the frequency you entered is out of band, ignore it and continue. You can always hack the band limits in the RSS to make this message go away.