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Using a Maxon SD-125 on M17

 

All-
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Here are my notes and information to share regarding the Maxon (or sometimes sold under the name Midland) model SD-125 UHF data radio.? It is a programmable unit, the U2 model covers 440-470mhz although it will program out of band and work down to ~ 430mhz if desired.
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My goal here is to show what I did to modify my unit for direct discriminator and modulator access to use as an amateur digital radio hotspot.
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These radios show up on ebay and other flea market sites.? I tested the ones I have on several amateur digital modes.. Dstar, P25, NXDN and now M17.? I have not tried DMR.? I can confirm that the SD-125 works great for M17 use.
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The radio can be operated at 1 watt output or 5 watts output.? However be aware that the unit was not built for lengthy continuous transmissions.? I have found running it at 1 watt in stock form, even though it gets warm to the touch is still within what I would consider to be reasonable temperature limits and I have even let it transmit during hour long Dstar nets with no concern for overheating.? Mounting a heat sink for additional cooling would be advantageous and make the radio look even more jazzy.? For my own unit I bypassed the PA module entirely and use the VCO output buffer transistor at 10 milliwatts RF output.? I find that it covers my ham shack with plenty of RF and the unit stays cool indefinitely.
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A brief radio programming mention, setting the radio for narrow band operation works best.
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The picture above shows where I removed the two wires from the edge connector (the arrows) and attached extension wires.? The white wire (tx audio to the modulator) goes to pin 7.? The red wire (rx audio from discriminator) goes to pin 10.
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DB9 pin 1 = modulator = edge connector pin 2
DB9 pin 2 = disc. = edge connector pin 5
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A correction I would make if doing it over again would be to pick the DB9 connector pin 7 for modulator.? It is a "no connection" pin.? By removing pin 1's connection from the top board edge connector takes away the ability to program the unit.? I can of course resolder the white wire back to the edge connector but that's a hassle and I didn't think that through.? My bad.? Fortunately I have no real need to re-program the unit again any time soon so I'm okay with this.
?
When using pi-star or WPSD under mmdvmhost radio settings, TXInvert=1 and RXInvert=0.
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A final comment and I'll wrap this up.? Because these radios were built some 20 years ago I'm finding that they're all way off frequency.? Be sure and check the transmitters on the bench and align via the VCO trimmer in the metal can on the bottom board if necessary.
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73-
Steve
KA4YMY
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Nice write-up, Steve!

Are these radios strictly?half-duplex, so you'd need two of them for a low-power repeater/high-power duplex hotspot?

Jim
N1ADJ

On Sat, Aug 31, 2024 at 8:59?PM Steve - KA4YMY via <ka4ymy=[email protected]> wrote:
All-
?
Here are my notes and information to share regarding the Maxon (or sometimes sold under the name Midland) model SD-125 UHF data radio.? It is a programmable unit, the U2 model covers 440-470mhz although it will program out of band and work down to ~ 430mhz if desired.
?
My goal here is to show what I did to modify my unit for direct discriminator and modulator access to use as an amateur digital radio hotspot.
?
These radios show up on ebay and other flea market sites.? I tested the ones I have on several amateur digital modes.. Dstar, P25, NXDN and now M17.? I have not tried DMR.? I can confirm that the SD-125 works great for M17 use.
?
The radio can be operated at 1 watt output or 5 watts output.? However be aware that the unit was not built for lengthy continuous transmissions.? I have found running it at 1 watt in stock form, even though it gets warm to the touch is still within what I would consider to be reasonable temperature limits and I have even let it transmit during hour long Dstar nets with no concern for overheating.? Mounting a heat sink for additional cooling would be advantageous and make the radio look even more jazzy.? For my own unit I bypassed the PA module entirely and use the VCO output buffer transistor at 10 milliwatts RF output.? I find that it covers my ham shack with plenty of RF and the unit stays cool indefinitely.
?
A brief radio programming mention, setting the radio for narrow band operation works best.
?
?
The picture above shows where I removed the two wires from the edge connector (the arrows) and attached extension wires.? The white wire (tx audio to the modulator) goes to pin 7.? The red wire (rx audio from discriminator) goes to pin 10.
?
DB9 pin 1 = modulator = edge connector pin 2
DB9 pin 2 = disc. = edge connector pin 5
?
A correction I would make if doing it over again would be to pick the DB9 connector pin 7 for modulator.? It is a "no connection" pin.? By removing pin 1's connection from the top board edge connector takes away the ability to program the unit.? I can of course resolder the white wire back to the edge connector but that's a hassle and I didn't think that through.? My bad.? Fortunately I have no real need to re-program the unit again any time soon so I'm okay with this.
?
When using pi-star or WPSD under mmdvmhost radio settings, TXInvert=1 and RXInvert=0.
?
A final comment and I'll wrap this up.? Because these radios were built some 20 years ago I'm finding that they're all way off frequency.? Be sure and check the transmitters on the bench and align via the VCO trimmer in the metal can on the bottom board if necessary.
?
73-
Steve
KA4YMY
?
?
 

Hi Jim! ?You are correct, these radios are half duplex. ?Essentially just a portable handheld in a box.
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A few weeks ago I configured a pair of these using a low power 20 watt notch only duplexer in order to find out how they would behave in such an arrangement. ?Running 1 watt on the transmitter box using a j-pole antenna just outside the ham shack window I achieved reliable service at least a mile out. ?The receiver front end does not offer any sort of pre-selection or selective filtering so it won't be good in high noise RF environments without additional help but for something portable in the field such as a demo setup at a hamfest table or teaching class.. ?Oh yeah, they would work great for that purpose.
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I meant to include this in the last email. ?It's a pinout of the 18-pin connector that the red (pin 10) and white (pin 7) wires are soldered to. ?For those interested you can check out what other signals are available including RSSI which is interesting.
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Steve
KA4YMY
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Thanks a lot for the detailed infos…
Is the programming software available easily?
Can separate RX and TX QRG be programmed ?
55&73
Wernef
 

Hi Wernef!
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Resources for manuals and other related material can be found online.? A good site to start with is here:
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https://tarpn.net/t/builder/builders_radios_maxon_125.html
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A one-transistor programming interface is required.? It's easy to build, in fact I made my own using a piece of perfboard.? A 2N2222 transistor is still your friend, even in the 21st century :)
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And YES, different receive and transmit frequencies can be entered.? You can also choose to enter just a receive only frequency and leave the transmit field blank and so on.
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Steve
 

le.petite on ebay must be wondering what just happened with?all his $15 SD-125s.? Glad I was able to snag two of them.


On Sun, Sep 1, 2024 at 12:05?PM Steve - KA4YMY via <ka4ymy=[email protected]> wrote:
Hi Wernef!
?
Resources for manuals and other related material can be found online.? A good site to start with is here:
?
?
A one-transistor programming interface is required.? It's easy to build, in fact I made my own using a piece of perfboard.? A 2N2222 transistor is still your friend, even in the 21st century :)
?
And YES, different receive and transmit frequencies can be entered.? You can also choose to enter just a receive only frequency and leave the transmit field blank and so on.
?
Steve



--
Joe Hamelin, W7COM,?Portland, OR, +1 360 474 7474
 

Thanks for the excellent link.
I am just wondering as the link stated the original wiring for RX and TX signals on the 9pin connector are already “flat audio”… but apparently they are not as you made the modifications….
55&73
Werner

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