I bought a TM-271A on eBay hoping to do this mod to add the MiniDin6 interface: http://n1vg.net/
Unfortunately it turned out that TM-271A are not all US version. The US version is TM-271A(K) whereas I have TM-271A(M3). So now I have to be careful not to transmit OOB.
The bigger problem though is that the PCB doesn’t match the above. Here is my PCB:
As you can see, this is quite similar to the PCB above, so I’m not sure whether what I bought is a knockoff product or a genuine Kenwood product that has just had a PCB revision.
Unfortunately the MiniDin6 pads are no longer obvious. There are 6 small pads in the same place as the old pads, but in a different configuration. Those could probably be used, but I could use some help.
Numbered top to bottom, line 1 is clearly GND. Line 3 is at 5V when squelched and close to GND when unsquelched, so this could be the SQC line (although the polarity is reverse what I expected). Line 5 is 5V and gets pulled close to GND when pushing PTT, so this seems to be PKS.
The other lines I’m not sure. Line 2 and Line 4 seem to be floating in the mV range and I don’t see any change when unsquelched or changing frequency / volume. Line 6 is at 5V under all tested conditions.
I would have expected all the data lines (2 receive, 1 transmit) to appear floating, so I’m not sure what is what here. Any thoughts?
I’d give it a try. We need GND, PTT, RIG_AFIN, RIG_AFOUT.
GND is easy, just ring to the chassis
PTT: I would use a resistor to 1-2K to short all pins to the ground through it and see when radio switches to TX.
RIG_AFOUT: You can use scope or Digirig’s audio input to look for radio’s audio output. Digirig decouples DC with the capacitors on input/output so even if you stick it into some DC voltage source that not going to cause any issues. There may be two outputs (1200 & 9600)
RIG_AFIN: Look for audio input by feeding audio from Digirig to all unknown pins while in TX, monitor the air with another radio on the same frequency. I’d do it through 1-2K resistor for a good measure.
GND is definitely line 1; you can see it connects directly to the ground plane. But given how small these pads are agree there’s probably easier places to get it.
PTT and SQC I’m pretty sure I’ve already found. SQC isn’t needed but it turned out to be in the same relative position as in the old PCB so I was hoping the other lines were also. But not so.
As an update, I was able to hook a little speaker up to GND versus line 4 and could hear the NOAA audio received, so pretty sure line 4 is “1200” / PR1. (Although not sure what 9600 should sound like.) I don’t hear anything from the speaker on line 2.
My Digirig should arrive tomorrow. I’ll try to hunt for the AFIN as you suggest. Is there any reason AFIN or 9600 OUT would sit at 5V though? Maybe it’s just a weak pull-up, I haven’t checked that.
I got confirmation from Kenwood that this is a fake. Buyer beware. It’s possible it could still work for my purpose, but I’m not currently planning to use it.
I know this doesn’t help you now because you already know it’s a fake, but I fell victim to this too when I purchased my first “TM-281A” on eBay. First off, it shipped an original TM-281 box that looked like it had been through the ringer. When I cracked it open and took the radio out, the operating manual looked like it had been photocopied from another manual and stapled together very sloppily. When I opened up the radio, suspecting I’d gotten a fake Kenwood, the circuit board inside did not even come close to matching what a genuine Kenwood TM-281 looked like based on their service manual for the model. When I examined the sticker on the bottom I also noticed that it was void of any FCC/regulatory markings at all and upon comparing to the sticker of a genuine product, it was confirmed to me that it was a fake. The funny thing is the faceplate looks like it’s real, with TM-281 badging and all, but when I reset the radio it identified as a TM-271 when it powered back on. I don’t know what they’re doing over there in China, but it’s not cool. I tried to dispute the purchase as a fake and unfortunately was denied, so buyer beware! I was hoping to turn it into an APRS-only unit for my truck but those plans diminished quickly and I was out ~$300CAD.
