Sony M690
A car head unit with a motorized face and a MiniDisc heart — the kind of thing that made the late-'90s dashboard feel like a cockpit.
Overview
The M690 is a Sony in-dash car audio source unit — specifically the MDX-M690, a MiniDisc head unit that also plays CDs and pulls in FM, MW, and LW radio. It's a 12-volt car battery unit (negative earth), sized at approximately 178 × 50 × 182 mm for the chassis and 182 × 53 × 160 mm at the mounting dimensions, and it weighs in at roughly 1.5 kg. The front panel is motorized: the manual documents a front panel open/close motor (M601) driven through a BA6288FS IC, with dedicated open and close detection switches. That's the party trick — the faceplate swings out of the way so you can load a disc, then folds back down. It's also the thing that will eventually break, and collectors should go in expecting that.
The MD section is the headline spec. Sony rates the MiniDisc player at a 90 dB signal-to-noise ratio, a frequency response of 10 – 20,000 Hz, and wow and flutter below measurable limit — the last figure being the whole point of MD as a format. On the CD side the numbers mirror it closely enough that the manual groups them together. The tuner is a proper three-band European-market arrangement: FM covers 87.5 – 108.0 MHz with usable sensitivity of 8 dBf and selectivity of 75 dB at 400 kHz, stereo signal-to-noise runs 66 dB against 72 dB in mono, and harmonic distortion at 1 kHz sits at 0.6 % stereo and 0.3 % mono. Separation is quoted at 35 dB at 1 kHz with a 30 – 15,000 Hz FM frequency response. MW tunes 531 – 1,602 kHz at 30 µV sensitivity and LW covers 153 – 279 kHz at 40 µV. Intermediate frequency is 10.7 MHz/450 kHz across both bands.
Amplification is a maximum of 52 W × 4 into 4 ohms, with speaker impedance rated for 4 – 8 ohms and outputs on Sony's sure-seal connectors — which means anyone buying one today should confirm the harness is included, because those connectors are proprietary and a bare unit is a paperweight until you find the right plug. Tone controls are bass ±8 dB at 100 Hz and treble ±8 dB at 10 kHz, and the loudness contour adds +8 dB at 100 Hz and +2 dB at 10 kHz. There's a mono subwoofer output, front and rear audio outputs, and a full set of control leads: power aerial relay, power amplifier control, telephone ATT, illumination control, plus BUS control input and BUS audio input connectors.
The adjustable items list is where the M690 shows its tuning ambitions. A high-pass filter offers off, 78 Hz, or 125 Hz; a low-pass filter offers 78 Hz, 125 Hz, or off — so you can shape a subwoofer crossover without external hardware. The equalizer volume is adjustable in 1 dB steps from −10 dB to +10 dB on the M670 and M690 (the M770 gets a wider −12 to +12 dB range). The supplied card remote is the RM-X111, and a rotary commander RM-X5S was available as an optional accessory. Sony also lists an ecosystem of optional add-ons: 10-disc changers (CDX-848X, CDX-646), 6-disc changers (CDX-T68X, CDX-T67), the MDX-65 six-disc MD changer, the XA-C30 source selector, and the XT-100DAB DAB tuner unit. One important limitation, straight from the manual: this unit cannot be connected to a digital preamplifier or an equalizer.
There's a MiniDisc gotcha worth flagging for anyone buying used. The troubleshooting section notes that an "LP" message in the display means MDLP tracks can't be played back — an optional MD unit that doesn't conform to MDLP playback simply won't work, and the fix is to play the MDLP tracks on the M690 itself. In other words, the head unit handles MDLP, but some of the changers it talks to don't. If a seller reports an LP error, that's what it is.
What the M690 is not, at least in any documentation available here, is a piece with a documented production year, original price, or collector market. There's no service history, no known failure pattern beyond the obvious motorized-faceplate risk, and no independent review data to lean on. It's a late-format car MD unit — MiniDisc in the dash was a short-lived proposition, and the M690 sits at the tail end of that window. The appeal is narrow and specific: someone who still has MDs, wants a period-correct Sony dash, and is willing to hunt down the sure-seal harness and the RM-X111 remote. Everyone else should look elsewhere.
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