Akai GX-370D (1973)

Name: Akai GX-370D
Brand: Akai

Akai's $700 flagship open-reel deck arrived with three motors, a direct capstan drive, and a Compute-O-Matic system meant to end recording-level anxiety. The audio press called it a class-of-its-own machine in early 1973 (Audio, March 1973, p.65).

Overview

Akai's own advertising for the GX-370D opened with a confession: "Yes, it's expensive." That copy, which ran in the audio press in early 1973, put the deck "in the neighborhood of $700.00" and then called that "a high-class neighborhood." The machine underneath was ambitious. It is a 4-track, 2-channel stereo/monaural open-reel deck with three motors, and the capstan is driven directly by a servo-control motor instead of the usual belt-and-flywheel arrangement. The spec sheet claims a frequency response of 30 to 26,000 Hz ±3 dB at 7-1/2 ips with Akai's S.R.T. tape, and 30 to 24,000 Hz ±3 dB with regular tape. On paper, that puts it in the upper tier of consumer open-reel machines of the period.

The drive system wasn't the only thing that set the GX-370D apart. There was also the Compute-O-Matic recording level control. Akai's manual is candid about why it existed. Earlier automatic gain control circuits were popular because nobody had to ride a level knob, but they produced a flat, compressed sound. It was hard to tell whether a voice had been recorded from across the room or right up against the microphone. Compute-O-Matic worked differently. When the user depressed the set button, the recording indicator stopped at the loudest peak in the passage. That peak became the new "0" VU reference, and everything else was adjusted proportionally. In effect it was a one-button auto-calibration for each take, in a market where most decks still asked you to set levels by eye and ear.

The transport is a three-motor design, and the manual is specific about it. The capstan motor is a direct-drive servo-control unit. It combines the winding control of an ordinary eddy-current motor with a speed-detecting toothed cog-wheel in a single structure, which removes the belt, idler, and flywheel that would normally sit between motor and capstan. The two reel motors are eddy-current outer-rotor types, chosen for smooth tape travel and starting torque without stretching the tape. Tape speed deviation is measured by playing a 1,000 Hz test tape and reading a frequency counter at the beginning, middle, and end of the reel. Wow and flutter is specified at less than 0.07% RMS at 7-1/2 ips and less than 0.1% RMS at 3-3/4 ips.

Signal-to-noise is quoted as better than 50 dB, with a 48 dB alternative figure in parentheses. Total harmonic distortion is under 1.5% at 7-1/2 ips for a 1,000 Hz "0" VU recording, rising to under 3% at 3-3/4 ips. Cross talk is better than 70 dB in mono and better than 45 dB in stereo. Erase ratio is better than 70 dB, and the bias frequency sits at 103 kHz ±5 kHz. Inputs include a mic input at 0.7 mV into 10 kΩ and a line input at 70 mV into 150 kΩ. The deck handles reels up to 7 inches, runs both 7-1/2 and 3-3/4 ips, and supports reverse recording and playback.

The service documentation is unusually thorough for a consumer deck of this era. The manual walks a technician through frequency response measurement, signal-to-noise, tape speed deviation, wow and flutter (two methods - one using a pre-recorded test tape, one recording and playing back a 3,000 Hz tone), total harmonic distortion, cross talk between tracks, and erase ratio. There's a dedicated procedure for adjusting recording bias voltage via semi-fixed capacitors C-5 through C-8 on the oscillator board, targeting a 10 kHz output within +1/-0 dB of the 1,000 Hz reference. The manual also includes a torque-motor voltage chart showing the supply and take-up tensions in each transport mode - forward, reverse, fast-forward, and rewind - with separate figures for 50 Hz and 60 Hz operation.

The GX-370D sits near the top of Akai's open-reel lineup. It is above the GX-260D, which the same 1974 catalog pages describe as a four-head, three-motor stereo deck with reverse in both record and playback, and above the GX-210D, a two-speed 4-track stereo machine with Dolby. Its distinguishing features are the Compute-O-Matic system and the direct capstan drive, and Akai's marketing positioned both as premium features.

Anyone considering one today faces the usual questions for a fifty-year-old open-reel machine: head wear, capstan condition, and whether the eddy-current reel motors still hold their torque. The direct-drive capstan removes the belt-replacement chore that plagues belt-driven decks. But the servo-control circuitry is more complex than a simple AC motor, and the Compute-O-Matic system adds more electronics that can drift or fail. The manual's measurement procedures give a buyer a concrete checklist for evaluating a unit, and give an owner a way to bring one back into spec. Prices for clean GX-370D examples have moved well beyond the original $700 asking price, though the market for open-reel decks generally rewards condition and completeness over model prestige. A unit with the original S.R.T. tape, the service manual, and a recent head inspection is worth considerably more than a bare deck of unknown provenance.