Onkyo Integra T-406 (1978)
The Onkyo Integra T-406 is a servo-lock AM/FM stereo tuner from 1978, priced at ¥45,000 in its home market. It was a Japan-only model. The FM tuning range is 76-90 MHz, the Japanese broadcast band, and it runs on 100 VAC. Onkyo was pushing hard on tuner technology at the time, and the T-406 has some genuinely interesting engineering in it.
The headline feature is the servo lock tuning system. You don't have to find the exact center of a station by ear or meter. The T-406 detects the point of minimum distortion and best stereo separation, then feeds that back to the local oscillator through a high-gain op-amp IC. That locks the tuning to within ±5 kHz of the optimum point. A red indicator glows when you're within ±50 kHz of the lock range. Once released, a green indicator confirms the lock. Touch the tuning knob to re-tune and the lock releases immediately, returning you to manual tuning. It goes after the old analog tuner problem of drift and mistuning.
The rest of the design backs it up. The front end uses a dual-gate MOS-FET in the RF stage with a frequency-linear four-gang variable capacitor. The IF section uses a wide-band linear phase filter, selected by a differential gain direct-viewing method, and it feeds a high-performance differential IC amplifier. The MPX section uses a PLL IC with a strengthening filter to suppress distortion. Onkyo also dealt with overmodulation, which was common in FM broadcasting then. The fix was an AF amplifier with a large dynamic margin, giving distortion of 0.15% or less at 150% modulation and 0.3% or less at 200% modulation.
The front panel has a deviation meter (a third meter, showing modulation level) and an air check calibrator. The calibrator emits a 440 Hz tone at 50% modulation for setting recording levels. The long-scale dial has 250 mm of travel and is driven by a large flywheel. There's a multipath output terminal for hooking up an oscilloscope to align antennas. Recording outputs come both variable and fixed, and the fixed one is 450 mV, meant for direct connection to a tape deck. A dual varicon and triple-tuned IF filter serve the AM section.
Specs: signal-to-noise ratio is 77 dB mono / 73 dB stereo, capture ratio is 1.0 dB, and stereo separation is 50 dB at 1 kHz. The tuner uses 4 ICs, 19 transistors (one FET), and 21 diodes. It measures 435 × 108 × 386 mm and weighs 6 kg. The Japanese-market audio database audio-database.com lists the key specs in detail. No period review from a Western publication appears to exist, since this tuner was never officially sold outside Japan.
- Type: Servo lock AM/FM stereo tuner
- FM tuning range: 76–90 MHz (Japanese band)
- Price: ¥45,000 (circa 1978)
- Power: 100 VAC, 50/60 Hz, 14W
- Dimensions: 435 × 108 × 386 mm; 6 kg
The Integra series was Onkyo's premium line in this era, and the T-406 sits in that spot. Documentation in English is essentially nonexistent. Its collectibility today is hard to gauge. It shows up occasionally on Japanese auction sites and used-audio dealers like Hifido, but no consistent market price has emerged. It is a feature-rich analog tuner from the years just before quartz-synthesized digital tuning took over.
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