Sony TC-900
A four-pound battery portable that made point-and-record real, and left a trail of hinge, motor and amplifier faults behind it.
Overview
The TC-900 weighs just over four pounds with four U2 torch cells inside. Sony built it to make tape recording something you did on the move rather than at a desk. It is a half-track mono mains/battery portable with automatic gain control, and it sold in Britain for £30 9s. Studio Sound (March 1967, p.21) called it an "inexpensive network of components" and found its simplicity almost beyond belief.
The controls are few. A single rotary knob handles playback, fast forward and rewind. A second knob, to the left of the playback volume and tone controls, interlocks with the play selector to engage record. Tape speed changes from 3¾ to 1⅞ ips by unscrewing a capstan sleeve. That is all of it. Owners report the appeal is exactly this: point the microphone, press the button, and trust the AGC to sort out the levels.
The AGC circuit is clever 1960s transistor design. Studio Sound (February 1967, p.4) walked through it. Part of the signal fed to the recording head is tapped off, rectified, and fed through a 50µF/20k time constant to regulate the base bias of a 2SB381 transistor. That transistor's collector voltage is held by a 30µF capacitor switched into the coupling between the first and second stages. The gain control responds to signal level above a threshold set by the p-n-p shunt transistor's working voltages. It is not a laboratory-grade limiter, but it is simple, and it works.
Power is flexible in a way that mattered in 1967. The TC-900 runs from four U2 cells or from mains — and switching between the two is automatic, accomplished simply by plugging the mains cable into the recorder. Rated output is 1W, bias frequency is 30kHz, and spool capacity is 3-inch reels. Battery life is quoted at 20 hours with four high-power torch cells, which is optimistic by the standards of most portables of the era but not absurd for a machine this frugal.
The tape path is where the cost-cutting shows. The TC-900 uses a permanent-magnet erase head that swings into operation against the uncoated side of the tape. Studio Sound (March 1967, p.21) was blunt about this: the technique of using a magnet or DC-powered erase head is "all too popular in Japan" but "seems an unforgiveable thrift when AC recording bias is incorporated." The 30kHz bias does remove the worst of the background hiss, and the magazine noted the TC-900 has a low level of background noise compared with most low-price portables, with motor interference well smoothed. The erase head is still a compromise.
The supplied dynamic microphone is adequate for general indoor and outdoor speech, and the AGC behaves happily with it. Substitute a better microphone and the circuit may object. Studio Sound (March 1967, p.21) suspected that any substantial variation in microphone output level would upset the AGC. A fixed-threshold automatic system is tuned for the microphone in the box.
Reliability is the TC-900's soft spot. The first unit Studio Sound tested arrived with a broken hinge on the lid protecting the deck. The lid releases by pushing back against a flexible tin hinge and, though it seems removable, is a permanent feature for practical purposes. That same machine then developed a loose motor bearing that caused severe screaming when running, plus a fault in the recording amplifier that laid a continuous crackle on the tape. The replacement unit behaved itself over months of use, but had slightly worse wow and flutter, with speed fluctuation audible even on light music. A later reader's letter to the same magazine (September 1967, p.39) described a loud bang and occasional squeal when using the remote-control switch on the TC-900 microphone. It was traced to a stiff switch and, likely, moulding flash on the plastic slider. Easy to cure, but it says something about the build quality.
None of this makes the TC-900 a bad machine. The point-and-press simplicity is real, the AGC works, and the automatic mains/battery switching is a small pleasure every time. But it is a portable built to a price, with a permanent-magnet erase head, a plastic remote switch that can bind, and a lid hinge that has been known to arrive broken.
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