Along with my Maxon SM4450, I was given a Motorola SM50 GMRS radio. I actually thought that it didn't work at first until I realized that the power leads were installed backwards on the SAE pigtail! I can confirm that it now works and puts out RF quite well.
Well, after lots and lots of trial and error (and with the confidence I gained through rebanding and reprogramming the 4450!), I got my Motorola SM50 UHF radio realigned on 440MHz! After modifying the .MDF file for the RSS software, I was able to program it to 440MHz to 480MHz. Of course, I won't need all this bandwidth as I'm only allocated to the lower 10MHz!
I programmed this radio using a CP2102 connected to the radio, and I used DOSBox to emulate DOS. It is recommended to use an actual DOS computer, but I found this method to work best for me, despite it having lots of hiccups along the way. I did find a workaround for it though!
The Interface Cable
This website gave me a basic start to constructing a programming cable for this specific radio: https://www.george-smart.co.uk/scrapbook/gm300_cables/. The GM300 is another style of Motorola radio, but this interface will work just fine for the SM50.
A CP2102 is used as they are relatively available and inexpensive. I bought a 2-pack for less than $5 on Amazon. eBay is probably cheaper. A 1N4148 diode is placed between TXD and RXD, with the anode side pointed toward TXD. With TXD and RXD tied together, this goes to pin 2 of the radio connector, a standard RJ45 connector, on the RXD pin. Ground goes to pin 5 on the radio, which is also ground. Here is the diagram below:
As you can see, it's not that difficult! Plus, you'll get bragging rights for making your own programming cable. For those of you who color code, the white-blue cable is ground, and the solid orange wire goes to RXD.
Of course, an FTDI chip or other USB-to-TTL chip can be used, as long as it has TXD, RXD, and GND. Pinouts may be different depending upon different chip designs, so be sure to take that into account.
Programming the Radio
I decided to use DOSBox since it was the only thing I had available to me. I also have a copy of Motorola's software, and unfortunately, I can't provide it here. It is copyrighted software and posting it here would be in direct violation of any copyright, so long story short, get your own copy. I know it's over 30 years old and it SHOULD be shareware, but sadly, Motorola is just too protective of their very old software. You'll need the Motorola RSS R05.00.00 software, dated 1998. I have R03.00.00, dated 1994 - it worked just fine for me.
The first step is to plug in the radio to your computer. In the DOSBox config file, you'll need to add a serial port, set to the COM port your programming cable is plugged in to. Once this is done, you can boot up DOSBox and read from the codeplug. Wait until it is finished reading and then save the file to the disk. You'll have to create a new directory if you haven't already. Go ahead and close DOSBox once the file has been saved. You can't program serially after reading or writing once to/from the radio. I know it's stupid, but, well, it works. Trust me - of course if this happens to you.
You'll need to restart DOSBox and a new execution of the RSS software. Once you've opened it up again, you can write the file to the radio from the archive/saved file. If all went correctly, you should have a radio that works on 440!
Here are some helpful sites that I found SUPER helpful:
* https://www.repeater-builder.com/motorola/sm50/sm50.html
* https://www.george-smart.co.uk/scrapbook/gm300_cables/
* http://www.batlabs.com/sm50.html - Covers the VHF model but should work for the UHF model.
* https://n5bew.us/index.php/articles/2-meter/29-motorola-radius-sm50-amateur-conversion - This one was of big help!!!
This didn't take as long as I was thinking it would, and within 30 minutes, I had a fully-programmed and functional radio. The SM50 models are not too difficult to find - many are being sold on eBay for a bargain price, some commercial departments might have them in surplus, and a few hams may be selling them at hamventions.
I know that this is a very brief write-up with not a lot of detail, but for the full programming guide, I recommend visiting N5BEW's website, the last one I linked above. It is my hope that Maryville will have a fully operational AllStar node on 70cm, since UHF is very inactive around my area, by at least the end of the year. With the small ham population here, I think this would help garner some more activity on 70cm for both the locals and other hams passing through along highway 71. And if it doesn't catch on (I hope it does though!), I can always use this node for myself!
Well, that's about it from me for now! My ham station is close to being set up, and with it I plan to install the AllStar node very soon. Stay tuned for updates as they come!!!
EDIT: I found a REALLY COOL cheat to expand your radio beyond 440 to 480MHz, to cover 430 to 450 (technically 430 to 490)! Here's how to do it.
First, download a hex editor such as Hex Workshop. Next, memorize this sequence: 94, 11, 5C, 12. This sequence of numbers correlates to 4500 (1194 - you read it backwards, little endian), or 450.0MHz. 125C would equate to 4700, or 470.0MHz.
To open the bandwidth up, you'll need to generate a checksum. Hex Workshop does this if you go to Tools >> Generate Checksum and click OK. Look for the value in the Checksum/Digest column. Mine was B8CB, yours may be different. Opening the range involves setting the radio in increments of 10, so we'll need to set our bounds at 10MHz less than the lower frequency and 10MHz more than the highest frequency. This translates to 440 to 480, 430 to 490, etc. I'm not sure how high these radios go, so be careful and make sure the radio can handle it!
Anyway, the hex sequence for 430 to 490MHz is: CC, 10, 24, 13. This correlates to 10CC (4300 in decimal) and 1324 (4900 in decimal). This should open the radio up to the ham band and keep it in the GMRS band should you decide to revise the radio back to stock.
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