Programming Uniden AMH-350 for APRS

This is a narrative post. If you want to see my python program that calculates out the diode matrix, skip to the end or click here,

I recently received this “Force Communications AMH-350” radio. Actually, it was an entire cabinet with a large power supply, an MFJ TNC2 tnc, and an old DOS PC running JNOS. These had active in a tower shed and turned off 3 years ago. The club wanted me to repurpose this packet system for APRS.

Once I plugged it in, the computer booted up to JNOS, but the radio and TNC did not turn on. The power supply had a plastic box on the back with a larger bussman 30A fuse. When I pulled it out, corrosion dust leaked out. I made a trip to the hardware store and replaced it. The radio turned on but not the TNC. On the front I found 3 smaller fuses and a note describing that “F3” ran the TNC. Pulled that fuse out and it was dead. A second trip to the hardware store got this fuse replaced. Then I plugged everything back in and turned on the power supply. Within 10 seconds, the “make it work smoke” had leaked out of the TNC2. This is probably why the F3 fuse had blown in the first place. This was disappointing, because there is new firmware for the TNC2 that makes it a decent APRS TNC, no computer needed.

The computer, I deemed too old to run a soundcard packet (using direwolf as my driver), so this left me with the power supply and radio. Grounding out the PTT line and using a frequency counter, it showed me “channel 2” was transmitting on 145.050. Channel 1 was not programmed at all.

A quick google search told me that Uniden bought Force Communications and sold this radio as a Uniden AMH-350. I found 2 other people looking for how to program it (one in 1994, and the other in 2004) with no response. I found someone selling the radio’s manual on ebay for $20. I offered them $10 and received the manual earlier this week.

The radio itself is programmed with a common cathode diode matrix, representing a binary value. Here is a picture of one back side of it programmed for 145.050. The manual provides a table covering frequencies from 148Mhz to 174Mhz in 5khz increments. Fortunately, it provides a formula on how to come up with your own frequencies. I ran through this formula multiple times getting different results from the book, till I realized the book was rounding some values UP or outright disregarding fractional parts. It also took a bit to wrap my head around binary “1” being disconnected (or cut) and binary “0” being connected. That felt backwards to me.

Eventually though, I was able to match the book, create a chart that matched the existing programmed 145.050 frequency (both Tx and Rx, which are programmed separately). Then, I wrapped the whole thing up in a set of python functions inside an ipython notebook. You can view this on ipython’s nbviewer or the direct gist.

I don’t have the radio programmed yet. I feel getting the diode matrixes out of “channel 2” and still having them useful for programming with is going to be difficult. I will need 7 diodes connected for each Tx and Rx slot, 14 total. I am attempting to program up channel 1. By the time I got to this portion, I was a bit tired and making mistakes, so I called it a night. Once I get to building out the programming board, I’ll post some more pictures.

Radio Musings

Completely unrelated to the post below, I discovered that some has forked a copy of my website on github. I’ve actually migrated from the static site to Mezzanine, but the repo is still in place. I was very surprised someone has forked it. I checked the user out and he recently forked a large number of repositories, all related to docker or coreos type things. It appears that “builtdock” is putting together some sort of platform as a service offering. I wonder if forking my site was an accident, or if they are planning to use it as a base of making their own static site.


I recently took my Raspberry Pi, connected a GPS, installed Xastir and have that setup as a sort of tactical display in my office. [](https://lh3.googleusercontent.com/–RicSliZ2kE/VDb9oB4auGI/AAAAAAAAT1M/eCkTaa8AIEg/w876-h1168-no/14%2B-%2B1 target=”_new”).

I have some interesting things I came across recently.

The first interesting project is from hackaday, a 0-30Mhz Portable SDR with transceive capabilities, opens-source design. It’d be interesting do this, but use a Pi+Touchscreen for the CPU portion.

http://hackaday.io/project/1538-portablesdr

The second one has been around for a while, and it’s called Earl. This is a crowd-funded, e-ink touchscreen android system with gps, built-in maps, and a VHF/UHF transceiver for GMRS/FRS/2m/70cm walkie talkie type capabilities. The radios are software defined, so something like DroidAPRS would be able to talk to the radio directly (perhaps with a bit of coding). I’ve been keeping and eye on this project (because I want one), but never thought it would actually materialize due to FCC rules concerning GMRS and FRS type radios. However they are now actively working through the FCC approval process and modifying the radio portions to get acceptance. I am hoping that enough get produced that some of these end up on ebay by non-hams that are disappointed in the radio communications aspect.