Finding an affordable single-band FM radio for 6m is a challenge, occasionally these 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 and a breeze to work on - after tapping the discriminator / modulator the radio was bench tested using an 857D as a reference with a pair of NinoTNCs and works on 4k8 beautifully.

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 Recieve it's (apparently) very selective & sensitive, and the second IF filter is 20KHz wide so this TRX might even work on wider bandwidth modes. Anecdotal testing 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 FTM-1500.

• Pin 11 of IC3 is the output from the discriminator
• The junction between C25 and C26 is the output of the de-emphasis filter
• 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)

1. “Flat” audio in / modulator input
2. “Filtered” audio in / audio input to pre-emphasis & mic preamp circuit
3. Ground
4. “Flat” audio out / discriminator output
5. “Filtered” audio out / output from de-emphasis circuit, before squelch pull-down transistors

After initial tapping

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 & 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.

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.

Detail of PA module

Horrifyingly there was no thermal paste anywhere in the radio! I suppose they had a dry day at the Alinco factory. By its condition this radio has barely been used, if at all, so hopefully no harm has been done - I liberally applied that horrid white Servisol thermal transfer 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.

Wire routing

The tap wires were routed around the front central case mounting post 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 test point when adding the green wire. Silly billy. Nonetheless it was an easy fix :)