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The Maxar 80 is very simple to interface with any 9600 baud modem or modem/TNC such as the DRSI. There are two versions of the Maxar being used on 9600 baud. One is the common voice communications radio. The other is a "data" radio that lacks audio circuitry for both transmit and receive. 

If your radio has a volume and squelch control, its the standard Maxar. If it has no controls and has parts "missing" off the PC board, its the "Data" radio version. As far as RF goes, there is no difference between the radios. Note that the Data radio version was used for very slow speed AFSK, not 9600 baud packet! If you try to use the audio connections on the rear Molex connector of the radio it will not work. 

You need to make four connections to the radio: 

  1. 1. Receive audio direct to the discriminator output.
  2. 2. Transmit audio direct to the varactor circuit.
  3. 3. DC ground to prevent ground loops
  4. 4. 13.8 volt to power the TNC 
For the audio connections use small diameter shielded wire. Ground only the end at the radio. leave the modem end ungrounded. Ground the two units together using a separate ground wire. This will prevent ground loops between the radio and the modem. Powering the modem/TNC off the radio also helps prevent ground loops between them. 

The connections for the 9600 baud receive audio is the same for both radios. Set the radio with the component side up and the heat sink facing away from you. On the right front side you will see a large shielded coil. Its bigger than all the rest. Most are wrapped in red tape for some strange reason. This is the discriminator coil. 

Behind it is the IF amp/limiter/FM detector IC. Mine has the number M2078 on it. Locate pin 6 on the IC. Trace out the PC board from pin 6. You will see first a disk ceramic capacitor to the left of the IC. Then a 10K (brown/black/orange) resistor, then a 2.7K resistor (red/violet/red). The end of the 2.7K resistor that faces the heat sink is connected to pin 6 of the IC. This is a good place to connect the discriminator audio lead to the modem with out soldering directly to the IC lead. 

Next you have to remove one capacitor from the receiver. Just to the left of the discriminator coil is a large .0022 mylar capacitor (it may be marked "222"). Unsolder this capacitor and remove it. Its job was to roll off some of the radios high frequency response. This was needed on voice to reduce the background hiss. On 9600 baud packet, you need all the high frequency response you can get! 

The transmit audio is just as easy. With the radio sitting as before, locate the deviation control for channel one. With the radio on channel one, the deviation control is the small black pot located in the center. There are two of these pots (one for each transmit channel on a two channel radio). The pots are right in the front of the small shielded enclosure in the rear of the radio. 

Locate the pot in the center. It is just to the front of a small shielded coil. Unsolder the pot and remove it. Note that the pot has three pins. The single pin to the rear (closest to the shielded coil) is the connection to the varactor. Connect the transmitter audio here. This is the pin that went to the wiper of the pot. 

The "Data" radio version of the Maxar connects exactly the same way except that the radio does not have the pots installed. You will find there is a small white wire jumpering two of the connections where the pot would have been located. Remove the jumper. Connect the transmitter audio lead to the rear position of the jumper. Note that there is a hole drilled in the board at the center of the mounting position for the pots. This hole makes a good landmark to locate the correct place to make the connection. 

Ground connection can be made anywhere on the metal part of the chassis. In the forward left hand corner of the PC board, you'll find several red/white wires connected together on the PC board. This is the common 13.8V connection point. This is the best place to make your TNC power connection. 

Frequency adjustment will be covered in the next message. Any questions? Contact me by voice or packet. 

David Metz, WA0AUQ 
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