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900 MHz Motorola Maxtracs

Here's some notes on my Motorola Maxtrac modifications for the 900 MHz ham band.  It's been a few years since I worked on some detail might be lacking.

The radios were modified by performing several changes:
  • Replace the receive RF filters with those designed for the 902-928 MHz band
  • Replace the varactor diodes in the VCO to allow the radio to tune a range over 30 MHz
  • Tune the VCO striplines so that the VCO tune range covers the right ranges for transmit and receive functions
  • Add hardware transmit power control (in the form of a 10 turn pot here) since software control doesn't work in the ham band
  • Replaced the firmware IC with the right firmware for conventional use
  • "Tweak" the Motorola programming software to allow programming out of the normal commercial range and into the ham bands
So there's a number of references out on the web for converting these radios to the ham band.  The unique solution I have is how the VCO is modified for use in the ham band.  The power control, firmware mods, software mods, filter changes, are all documented other places.  Some pictures are shown of my changes.

The whole purpose of the VCO change is to impact the tuning range.  Some other solutions include a DC-DC converter that doubles the available tune voltage to the VCO.  This uses the stock varactor diodes and gets the desired capacitance range by changing the tuning voltage range.  My solution was to change the varactor diode so that the tune voltage range can remain the same (no DC to DC converter needed), but the varactor changes.  The part number is shown below.  You'll also find some plots of the stock varactor and the new varactor (C vs V curves).

It has been a long time (and these pics are old pics of the modifications...not recent ones) so not much else is fresh in my mind.

The above picture shows a trace that is cut to facilitate manual transmit power control then a resistor added.  Err...looking at some more notes, this might be the "pin shift" modification.

More modifications for the power control - the added pot controls the transmit power.  The pictures show the before, then the two surface mount 56k resistors removed, then the pot (and leaded resistor) added.  

Here's VCO modification - the original varactors and the new.

Here's some graphics on the varactors.  One is a plot of the factory T8 diode.  The other of the MACOM diode used to replace it.

Some notes I found from a webpage I did back in 2007...

The heart of a Voltage Controlled Oscillator (VCO) is the varactor diode.  The varactor is a voltage variable capacitor.  The greater the reverse bias on the diode, the smaller the capacitance realized from the varactor. 

In the 900 MHz Maxtrac radio, the stock varactors are labeled as "T8", shown above.

The make and model number of these diodes is unknown.  The service manual for these radios simply has a block on the schematic labeled VCO, the details of what you see inside above are non-existent in the Motorola manuals (that I know of). 

I have characterized the diode as shown above (plot on the left).

The suggested replacement diode, MACOM MA4ST1330-1141T, has a C vs V curve as shown above (plot on the right).  As you can see, the MACOM part achieves the same capacitance range over a much smaller voltage range.