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.