I wanted to use my TNC-X as a APRS TNC. The TNC was easy to build and John Hansen was great help with the few problems I had. I have a buddy that showed me a OEM GPS board (basically the GPS receiver with a NMEA encoder, a COM port and not much else). You can get them for $10-$20 on eBay.
My first thought was to get a few and put one in a box of its own and connect it to the TNC-X. When I got the boards I realized each of them would fit into the TNC-X case. I thought it would be even better to have the TNC-X be a standalone APRS codec that I only needed to connect to a radio and a GPS antenna. The pictures and notes here describe the project to add a GPS receiver to the TNC-X
The smallest GPS receiver I bought (a Furuno GN-80) was about 1.5”x0.7” and would be easy to fit into the TNC-X case. and the bigger one (an Axiom Sandpiper II) would be a tight squeeze. Both GPS receivers have a SMA connector for a GPS antenna. The Furuno has the SMA separated from the PCB by a short length of flexible coax. The Axiom has the SMA soldered onto the PCB. The Furuno requires 3.3V DC power, the Axiom 5V. I chose to squeeze the Axiom into the TNC-X because it operates on 5V (same as the TNC-X) and because I would get some support of the GPS PCB from the antenna connector.
The mod was simply:
Investigate a variety of positions for the GPS board looking for one that fits without interfering with the TNC-X or its connectors
The only options are either to have the GPS antenna connector exposed on the same side as the TNC-X connectors or the same side as the TNC-X LEDs
There's more room on the LED side but I thought it would be tidier to use the connector side
Because the GPS board length is the same as the TNC-X case depth there's no way to put the GPS board into the case and shuffle it around looking for a good fit
I spent a lot of time with as much of the GPS board in the TNC-X case as possible
I first thought I could get the antenna connector between the power and USB connectors. I even drilled the pilot hole but I was dreaming.
Eventually I thought I could fit the board into the case if the SMA was exposed through a hole at about the same height as the top jacks about a half inch from the top jack on the right side (as you look at the connector side of the TNC-X case)
Removed all TNC-X components from the case
Drilled the hole for the SMA
Re-inserted the TNC-X PCB
Inserted the GPS board (required careful handling because it barely fits)
Rotated the GPS board to make room for the radio jacks
Powered up the TNC-X and verified that the GPS board (not yet powered up) was causing no side-effects (e.g. Touching components on the TNC-X)
Removed all components again
Identified locations for +5V and Ground on the TNC-X PCB that would be close to the location of the interface connector on the GPS board (I chose pins from the USB connector and I only plan to power the TNC-X via the USB connector)
Soldered 3” of thin insulated wire onto the selected places on the TNC-X board
Re-inserted all the components (including the GPS board) into the case leaving the added wires accessible above the TNC-X PCB
Soldered the added wires onto the power and ground pins on the GPS board interface connector
Soldered two thin insulated wires to the NMEA output and ground pins on the GPS board interface connector
Soldered the other end of those wires onto the auxiliary COM port connector on the TNC-X
Connected a GPS antenna to the SMA
Connected the TNC-X to a USB bus
Connected the TNC-X auxiliary COM port to a PC COM port (the installation of the GPS board makes the auxiliary COM port connector a test point in the GPS to TNC-X serial data path)
Verified there was telemetry from the GPS board (i.e. NMEA data)
Connected the TNC-X to a radio
Verified broadcast of valid APRS traffic
Closed the case,
Smiled
I am grateful to John Hansen for helping me with a few details. In particular, I was observing the PTT not dropping on the radio when the TNC-X red LED was extinguished. John pointed out it was likely due to RF pickup on the audio leads due to them being close to the antenna on the HT I was testing with. He was correct and I resolved that with a remote antenna (my final plan anyway).
I concede the mod is not high quality. The GPS board was forced in. No effort was made to protect the boards from touching due to vibration, shock or the nut securing the SMA coming loose. It was assumed that the USB bus could supply sufficient current to drive the TNC-X and the GPS board (very likely since there are GPS receivers powered only via a USB bus). I will probably upgrade the 7805 on the TNC-X to a TO-220 package or, maybe a LM317 since the GPS board has a quite restrictive voltage tolerence.
Pictures of the modified TNC-X are below. Click on a picture to view a larger version (very large, about 3MB each)
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