I have been using the TM-271 for my mobile APRS operation for over 18 Months now. Here is a quick article on how you can enable the radio for APRS Voice Alert. It is all about unsquelching the 1K2 packet audio path.
Unfortunately I forgot to shoot pictures of the mod. All I have are the notes and pictures I made while planing the mod. You can get the full schematics out of the service manual.
For APRS Voice Alert operation we need the attached TNC or Tracker to be fed with the receive audio and, in parallel, we want the speaker to remain silent unless the correct tone is being received.
You do all this at your own risks! if you hurt yourself or someone else or cause any harm to yourself, it’s all on you.
The squelch circuitry of the TM-271
Audio muting is done using two FETS Q252 and Q253 which have their gate directly driven by the CPU. Audio signal first flows through Q252 than is branched to the PR1 packet output and to Q253. After leaving Q253 signal is fed to the volume controller and the audio amplifier.
To unsquelch PR1 we need to defeat Q252.
Defeating the squelch on packet output
From what we have seen before, we might consider different options:
- Unsolder Q252 and short the pads of the source and drain pins
- Cut the trace between R25 and C249 and rewire it to R247
- Short Q252 drain and source
- Force Q252 gate to 0V by cutting the trace and wiring it to ground.
I went for option 3, simply because it is reversible and I did not feel comfortable un-soldering SMD in that place of the PCB where lots of components are squeezed. It has some minor side effects , but I can live with that, more on this later.
The easiest way to do short Drain and Source on Q252 is to put a solder blob between R249 and R246. See pictures below.
This modification will drop the DC Bias on Q253 from 2.7 to 2.5V. This is because R249 and R246 end-up being in parallel. As a matter of fact the resulting value is around 13kOhm. I did not notice any using caused by this voltage drop.
In a quiet environment the squelch might no be closed “tightly” because Q252 has been made useless, you might hear the noise at very low volume. Since I am using the radio in the car, I do not care.
Some might argue that I could have simply used the PR9 (9600bps) audio output as it is unsquelched. The reason I did not do this is that I was using a Tinytrak 4 at that time and the decoding on the PR9 output was not that great.
I have been running the TM-271 like this for 18 Months at the time of writing; It performed like a champ, it is a very sturdy radio, built like a tank, and the receiver is very selective.