A short time ago, I was given some GMRS radios from a local ham friend who was cleaning out his house. One of the radios given to me was a Maxon SM4000 series radio, namely, the SM4450. From the factory, this radio operates from 450 to 470MHz. It was also one of the very first radios to be programmed via a computer and software.
Unfortunately, I am a ham operator and cannot use the GMRS frequencies legally. I am not licensed to do so and I am not seeking a GMRS license. I am, however, guaranteed a section of the UHF band. In the United States below Line A (see FCC definition), hams are allocated the 70 centimeter band on a secondary basis, running from 420MHz to 450MHz. Hams above Line A must refrain from using 420 to 430MHz, as this is a crucial frequency to the Canadian government. 70cm is unexplored territory for me. I have only made 2 contacts on 2 meters via a repeater and the simplex frequency.
To open up myself for 70cm exploration, I got the wild, but certainly not impossible, idea to realign this radio to operate in the 70cm band. In doing some rabbit-hole research, I not only found that this was possible, but it is simple too! Unfortunately, 70cm is very inactive around my area, but I'm hoping to change that with this task!
The SM4000 Series Radios
The Maxon SM4000 series radios were introduced for the GMRS, MURS, and business band markets in the 1990s. These radios were offered in VHF (SM4150) and UHF (SM4450) models. GE and Monogram (Ericcson) Radios also produced similar models. These were the exact same radios with a different name badge. The Maxons are considered to be the higher end of all the brands.
The SM4450 is a frequency-synthesized radio, meaning that it is not crystal-controlled or VFO-controlled. Instead, it utilizes an 8051 microprocessor/microcontroller with an Atmel 93C56 EEPROM, in combination with a PLL VCO for extreme frequency stability. From the factory, the radio is programmed to operate from 450 to 470MHz. Moving this to the 70cm band may seem daunting, but all it involves is realigning the PLL and, if deemed necessary by preference, adjusting the receiver sensitivity/front end. No extra components are needed or should need replacement.
Programming the radio involves connecting a specific programming cable to a computer running the MS-DOS operating system. DOSBox can also be used, but direct programming to the radio will not be possible. Instead, direct EEPROM programming will need to be done but is just as doable. This is what we will do as it is simply accessible and feasible for me. The programming cables are obsolete as of now.
Radios like these can be bought at ham swaps, surplus sales, and online marketplaces like eBay, usually all less than $100. I can't say what mine was worth since I got it for free, but I have verified that it works, turns on, and functions well.
Needed Tools
For this adventure, you'll most likely need the following:
- An electrically-insulated screwdriver
- A digital voltmeter
- Basic computer science knowledge and knowledge of the hexadecimal system
- A 50Ω, min. 50W dummy load rated for HF/VHF/UHF frequencies
- A simple wattmeter, if necessary, for TX power adjustment
- A 13.8V power supply capable of at least 10A
- An EEPROM reader/writer with compatible software (Usually under $20 on eBay or Amazon)
- A hex editor (Hex Workshop works just fine)
- Spare Atmel AT93C56 EPROMS
- A low-power or weak-signal (<= 1W) UHF signal generator (if necessary for RX sensitivity alignment)
Breaking In
First, place the radio right side up and use a flathead screwdriver to pry the top lid off. Next, locate the microprocessor - the largest chip on the board. Near it, there should be an 8-pin socketed chip, an Atmel 93C56 or similar. Extract this chip from the IC socket carefully.
A note on the EPROM chip: Replacement chips are easy to source since EPROM chips are still being used in similar technologies. Sources such as Digi-Key or Mouser stock replacements for around 50 cents a piece. Although EEPROMS are guaranteed a long read-write lifecycle (~1,000,000), they will eventually fail over time as they age. Even if the original still works, consider purchasing additional EEPROM chips, especially if you plan on going through several R/W cycles. Look up Atmel AT93C56 EEPROM on these sites to find availability.
Programming
Before starting, head into your C-drive on Windows (C:\) and make a new folder. You can call it dos, Maxon, etc. Download the Maxon SMP4004 software from here (https://hamfiles.co.uk/index.php?page=downloads&type=entry&id=radio-programming%2F-multi-purpose-multi%2Fmaxon-4150-rss, you'll need to make a free account to download) and extract the folder to the dos folder you created in the C-drive.
After opening the exe file in DOSBox, it will ask you to select a radio model. Select "SM-4450EX." Don't worry about the EX suffix. It should be just fine.
Next, select Option 6: Create New File. Here, you will be prompted to enter in your desired frequencies. Once you have filled all 16 frequency slots, use the arrow keys to arrow over to the rightmost menu and select Program/Print/Save. In one of the frequencies, I highly encourage you to program 446.000MHz, which is the national FM calling frequency for 70cm.
As a side note, these have CTCSS and DCS options if you plan on doing repeater work. Adjust the TX and RX frequencies as necessary.
Next, you will need to locate the generated file in the C-drive folder you created. Locate the .CHN file and open it in a hex editor of your choice. THIS IS A VERY IMPORTANT STEP!!! You will need to keep the data starting at 5F and going to 15E, ALL INCLUSIVE! This should amount to 256 bytes of data (in 8-bit sections, or 256x8), which is the maximum memory our EPROM can hold. Once you have edited this file, save it to a known location and exit.
Now, plug in your EEPROM programmer and open the modified binary file in the programmer's software. When it asks to specify the chip to be programmed, select Atmel AT93C56(x8). We are programming it in 8-bit sections, not 16-bit. Begin the burning sequence and when finished, remove the chip and plug it back into the radio's IC socket.
To test if the radio reads the data correctly, simply turn on the radio. At this point, it should beep periodically and display "Er03" on the display. This is normal - "Er03" means that the programmed frequencies are out of the PLL's frequency range and it needs realigned. We will do this in the next step.
If you get "Er01" or "Er02" messages, it may mean that the data is incorrect or corrupted, or that the EEPROM is dying. Reflash the memory and try again, and if it persists, use a different 93C56 chip. Don't put the top cover back on yet - we still need to realign the PLL circuit.
As a side note, "Er04" means that the antenna connection is faulty or the SWR is too high. This will prohibit the microprocessor from allowing transmission. Yes, it is that smart of a radio!
Realignment of the PLL Circuitry
Realignment of the PLL is fairly straightforward. All you will need is a non-metallic, insulated flathead screwdriver and a multimeter set to DC voltage. You will also need a 50W minimum dummy load.
Find the silver metal encasing at the rear part of the radio's PCB and unscrew the 4 gold screws. Remove the enclosure to find the PLL circuitry. Find the variable capacitor (TC13) and a resistor with a few wire turns on one end - you'll put the DVM's positive probe in these turns. Connect the ground probe to the radio's chassis.
Turn on the radio and tune to the lowest frequency that is programmed in the radio. In my case, 430MHz is the lowest frequency. I won't be using the full 70cm since I don't plan on doing ATV. Take the insulated screwdriver and turn the variable capacitor until the voltmeter reads 1.5V (+/- 0.05V) exactly. The radio should also stop beeping if done correctly and will display Ch01.
Next, we'll align the transmitter section. Connect the dummy load to the antenna output and turn the radio to the highest frequency programmed. In my case, this is 450MHz. Press the PTT on the mic and read the voltage. It shouldn't exceed 7.5V. If this matches up, you now have a fully functional 70cm radio!!!
As an extended measure, you can also realign the receiver's sensitivity, though I would strongly recommend taking it to a radio technician if you aren't comfortable doing this. These radios should operate just fine for local communications as is, but if you're someone who wants to chase FM DX, you'll need to adjust this accordingly. As a basic and rudimentary adjustment, you can use a low-power (<1W) UHF signal generator, like an audio signal fed into a Baofeng handheld. On the bottom side of the radio, there should be several audio transformers. Adjust each of these until the received signal is at the loudest point at the squelch opening.
On these radios, care must also be taken not to overdrive the finals. While these are only 30-year-old radios and can top out at 40 watts, I recommend turning the power down using VR3 to about 25 or 30 watts for cooler operation and longevity of the finals. 25 - 30W is more than sufficient for a base UHF transceiver and will cover a fair distance depending upon the location.
Digital Mode Interfacing
If you plan on using this radio for digital communications, building a digital modes interface is beyond essential. Below is the pinout for the microphone cable:
Note that there is no RX audio on the microphone. To retrieve this, you'll need to use the AUX plug on the back of the radio, NOT the EXT. SP. plug. According to this pinout, shorting pin 5 to ground triggers PTT, so this shouldn't be too difficult to do with a relay or optocoupler. As with HF SSB modes, make certain that your audio levels don't overdrive the radio. FM runs at a high or full duty cycle. To avoid RF interference along audio lines, make sure to place audio transformers along these lines.
Now, you may be wondering what digital modes you can use on 70cm, and I have wondered that myself. According to the ARRL band plan, anything is free game - the entire 70cm band employs the use of both voice and digital modes. This includes PSK31, RTTY, SSTV - the list goes on! However, this is an FM wideband radio (almost 25KHz of BW), so using PSK31 or another narrow-bandwidth mode would be a HUGE waste of bandwidth. Using RTTY could work, but the best mode suited to this band would be MT63 or 8PSK. Both were created, especially 8PSK, with the intent to be used for FM bandwidths and conditions. If I recall correctly, MT63 can go up to a 2KHz bandwidth! These two modes are also great for side-by-side use with FLDigi and its emergency communication forwarding system, Flarq. Digital voice is also useful on these frequencies and include modes like DMR, C4FM, and even D-STAR.
Winlink with VaraFM and PACKET are also other great modes that can be used on 70cm, and many amateurs have node stations set up for this purpose. While it's more common on 2 meters, APRS also has its use on 70cm. In the future, I want to establish an AllStar node using another old business band radio that I have that can work on the 440MHz band. AllStar is another cool thing to look into - across the world, nodes that use VoIP allow amateurs to talk to one another without using HF, albeit it does rely on an internet connection to work.
Legality Disclaimer
Many of you may wonder why a ham would convert one of these radios to a specific amateur VHF/UHF frequency band, and I will explain why this works.
A few years ago, the FCC declared the Baofeng import radios as illegal for use on amateur bands. Despite this, many hams still own these HTs. The reason was simple - they could be programmed to operate on frequencies other than amateur frequencies. This opened up a risk to accidentally, or even purposefully, transmit on GMRS, FRS, or public safety and police radio bands. Baofeng has since made claims that they have revised this to pass for sale in the US.
Baofengs are considered Part 90 radios, meaning that they have passed several rigorous tests for use on GMRS and other public service bands. Amateur equipment, on the other hand, needs to only pass Part 15 rules, which is meant for the mitigation of harmful interference. In layman's terms, hams don't have as many (or if any) regulations when it comes to testing RF equipment. This is why Part 90 rigs are legal to use on ham bands, AS LONG AS YOU KEEP THEM WITHIN THE HAM BANDS. These radios become illegal if they interfere on other radio services like the nearby GMRS service.
So, after realigning the radio, it is YOUR RESPONSIBILITY to ensure that this radio will stay in the 70cm band. If it strays out into frequencies that us amateurs can't use, it's defective and should NOT be used on-air. Spurious emissions wreak havoc on all operators, regardless of license privilege, service, etc. It's not worth a suspended ham license and a hefty fine if you get caught, so please use good judgement and common sense while moving this radio to another frequency band. Be polite.
Antenna Problem... or Solution???
UHF radios work much different than HF radios. They utilize what is called "line of sight," meaning that the signal will only reach to the visible horizon. However, in urban environments, UHF works wonders, which makes it a great band for heavily-populated areas. However, it works even better if the radiating element, like VHF, is placed away from objects.
The key is to get your antenna as high and as clear from as many obstructions as possible. I recommend erecting it at a height of 10, 20, or even 30 feet from the ground, and for a DIY project, a simple 1/4-wave ground plane antenna can be fabricated. Each element is only 6.5 inches long, so this antenna will be very low profile and very simple to construct and tote around.
Conclusion
Unfortunately, 2 meters and 70cm are starting to become "use it or lose it" bands. Maryville has had little 2m and no 70cm activity in the past 5 years, and I'm afraid that the FCC, the ITU, and the other telecommunications governing bodies may come for our bands.
This isn't a complaint but rather a plea to all of you reading this. I ask that you make 2 meters and 70 centimeters relevant in your areas. Yes, there is so much more to HF than there is with VHF and UHF, but VHF and UHF can also be fun and intriguing bands, especially 70cm. It's a vast, unexplored territory for me so I am very excited to begin using it.
Recently, our only 2 meter VARA FM and Packet Winlink node went off the air. I'm not sure if the operator took it down for maintenance or if it is out of commission for good, but this is not good news. Raspberry Pis are starting to go down in price so I'm hoping to build an APRS gateway soon for our area with my UV-5R. If I travel, I will definitely take it with me.
No comments:
Post a Comment