======Modifying an Alinco DR-M06TH 6m transciever for packet usage======
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Finding an affordable single-band FM radio for 6m is a challenge, occasionally these Alincos turn up on eBay so I nabbed one tried my hand at modifying it. As it turns out they're well built, old-skool analogue architecture and a breeze to work on. The [[https://www.manualslib.com/products/Alinco-Dr-M06th-3260732.html|service manual]] is widely available online and is superb.

The PA puts out ~20W and can be adjusted using VR1. The deviation is adjusted with VR2 and is set ~5KHz from the factory. On receive it's (apparently) very selective & sensitive, and the second IF filter is 20KHz wide so this TRX might even work on wider bandwidth modes. Using an IC-705 as a reference receiver the wideband PLL phase noise looks to be very low and there are no horrendous key-up transients a-la FT-1500M.

After tapping the discriminator & modulator the radio was bench tested using an 857D and a pair of NinoTNCs; it works on 4k8 IL2Pc beautifully. The main issue with testing was those Aerial Facilities dummy loads are **superb** at 50MHz!

Ticking away happily as GB7WIN's 6m port! Live long and prosper little Alinco!

Mods:
- Inputs for TX audio, pre/deemphasis and direct modulator inputs\\
- Ouptuts for RX audio, pre and post filter\\
- 6 pin mini-DIN connector on the rear\\
- Selectable "1200/9600" audio output\\
- 2nd IF filter replacement (6KHz -> 3KHz)\\
- Second receiver SDR tap\\


=====Schematic detail=====
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  * Pin 11 of IC3 is the output from the discriminator
  * The junction between C25 and C26 is the output of the de-emphasis filter and before the squelch pull-down transistors Q4 and Q16
  * The junction of VR2 / R24 / R25 / R123 is the modulator input, via the deviation pot VR2 and coupling capacitor C40
  * CP11 is the input to the mic preamplifier, pre-emphasis filter & modulator drive buffer amplifier
  * PTT line is taken from the front panel connector, Pin 2 (3-o-clock position from the rear)

=====Tap points=====
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  - "Flat" audio in / modulator input
  - "Filtered" audio in / audio input to pre-emphasis & mic preamp circuit
  - Ground
  - "Flat" audio out / discriminator output
  - "Filtered" audio out / output from de-emphasis circuit, before squelch pull-down transistors

=====Finished mods=====
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**After initial tapping; the board is littered with test points so I soldered to them**

{{:radios:dr-m06th:dr-m06th-audioout2.jpg?nolink&960|}}\\
**I decided to add a "filtered audio out" as I was using the Yaesu 6-pin Mini-DIN connector, so why not make the "1200" pin work too**

=====Rear connector=====
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**Rear connector & switch**

I had a 6-pin Mini-DIN socket lying about; after removing the external speaker jack drilling out the hole allowed the connector to fit in. After pulling the wires through & soldering the connector I pushed it back into the hole & epoxied it into place. Pinout is the same as Yaesu use.

Alinco had also left another dimple in the casting, presumably for a different model of radio, so I drilled that out and installed a mini toggle switch to allow changing between "flat" audio in and "filtered" audio in.


=====Other tweaks=====
{{:radios:dr-m06th:dr-m06th-reardetail.jpg?nolink&960|}}\\
**Detail of rear connector area**

To route all the taps to the rear connector I poked them through the external speaker jack holes. A small bit of wire was also soldered between the tip and tap pads to route audio back to the internal speaker, normally this would be done within the speaker jack when nothing is plugged in.

{{:radios:dr-m06th:dr-m06th-pamodule.jpg?nolink&960|}}\\
** Detail of PA module **

These radios (I've got two) don't have any thermal compound between the PA and heatsink so I liberally applied that horrid white Servisol paste to the PA and linear regulators. After holding the radio in transmit for a few minutes at full power the heatsink becomes appreciably warm, but not remotely hot - and the PA module is the same temperature so good thermal contact was achieved. After dialling back the RF power from 22W to 10W and leaving it in transmit for 10 minutes the heatsink had reached equilibrium and was ~40C with no forced airflow. At 10W the radio drew 1.6A at 12.2V, so that's ~10W thermal into the heatsink which it certainly appears appropriately sized for.

=====Finished article=====
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**Wire routing**

The tap wires were routed around the front central case mounting post before soldering to reduce the chances of pulling a trace off the board. The eagle-eyed viewer will notice an extra wire in the area of the "flat audio in" wire - yes, I pulled off the orange wire's pad when adding the green wire. Silly billy. Nonetheless it was an easy fix :)

=====Filter replacement=====
The radio is set up to receive +-5KHz FM voice, but on packet we're using +-2.5KHz deviation. I replaced the second IF filter (CFW455F, -6dB BW +-6KHz, -40dB BW +-12.5KHz) with a tighter filter (CFW455H, -6dB BW +-3KHz, -40dB BW +-9KHz).

=====SDR Tap=====
To give us a bit more information about what is happening on site I opted to fit an SDR tap to the radio and feed it into an RTL-SDR Blog v4. Initial experiments show that with an S9 signal into the receiver and 45dB gain set on the RTL a -30dBm signal is reported by the SDR, and in full power transmit a -10dBm signal is reported; well within the maximum input power of the RTL dongle.

The SDR tap board employed was the PATV type with no LPF as this board is being fed directly from the input LPF in the radio.

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**Receiver front-end**

Q1 is the input LNA and feeds a low-pass filter consisting of L4, L5 and L6. The hupRF board was tapped into the pole of L6 to provide a second receiver.

{{:radios:dr-m06th-huptap.jpeg?nolink&960|}}\\
**SDR Tap board installed**

The whole thing just fits into the radio underneath the main PCB; liberal application of Kapton tape and some sticky foam on the underside of the case holds it in nicely. I opted to drill yet more holes in the casting to fit a rear SMA connector to prevent mechanical accidents :)
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de G5RKT, Feb '26