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... A New Professional Audio Instrument - Perfect for Stereo and Monophonic Operation

The new RT-21A professional tape recorder is a high quality recording and reproducing instrument. Designed specifically for stereo operation, this new recorder offers excellent frequency response with very low distortion and cross talk. Advanced mechanical design combined with all transistor circuitry makes the RT-21A perfect for highest fidelity operation.

The unit has stabilized electronic functions, high signal-to-noise ratio, good frequency response, low flutter and wow. Rapid tape threading and ready access to all components makes the RT-21A easy to operate and maintain.

It utilizes dual-half-track or full-track, as well as the quarter-track heads, thus allowing stereo and monaural applications. Provisions have been made for an optional fourth head.

The basic tape recorder is supplied in two sections: the tape transport panel, and the amplifier module with control panel assembly. Both can be mounted in a standard 19-inch wide rack, console cabinet, or portable carrying case. A remote-control unit containing duplicate control-chassis functions is also available.
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The control panel features an interlocked record operation. Magnetic tape may be easily threaded in the RT-21A Tape Recorder without removing the head cover.

Tape guiding and lifting is accomplished with small sapphire rods. Solenoids will lift the tape away from all magnetic heads whenever the machine is in the fast-forward or fast-reverse mode of operation to minimize head wear.

Quiet operation of the tape lifters is achieved through the use of nylon and stainless steel for the two impact surfaces. Air damping is provided for slow release of the pinch roller from the capstan shaft to prevent excessive noise.

The continuously variable cue speed permits the operator to listen to the audio during final cueing. The tape is lifted off the erase and record heads during the cue mode.

Three safety features are included in the equipment: fail-safe braking system, tape break switch, and record interlock. The fail-safe brake will be engaged when a power failure occurs, thus avoiding tape breakage or spillage.

Should the magnetic tape break, the tape-break switch will cause the brake solenoid to drop out, thereby stopping the reel motors. The record interlock system requires that the tape be stopped before the record function may be started.

Another relay interlock in the recorder prevents selection of a low-speed mode after a high-speed mode without first going through the stop mode. This prevents breakage or spillage which would result if the start mode could be selected after the fast-forward, fast-reverse, or cue mode.

The speed selector switch, in addition to changing the number of poles used in the capstan motor, also selects the proper equalization of the record and playback amplifiers. The result of the various interlocks is a practically fool-proof operation of the tape recorder.
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To accomplish low flutter and wow, the supply reel motor has almost constant torque in a direction opposite to that of the moving tape. This provides the required tension in the tape for proper contact pressure between the magnetic heads and the tape.

Although the supply reel motor provides nearly constant torque, there are other factors contributing to the flutter and wow. In order to minimize these variations in tape speed, a tape-driven stabilizer is located directly under the supply reel motor.

The high inertia of the flywheel, when combined with low friction of the preloaded, precision ball bearings, results in stabilization of tape speed variations. The associated supply tension arm minimizes tape slippage over the stabilizer head.

Although the tape transport is designed to handle both 7-inch and 10 1/2-inch reels, using reels of the same size permits the most satisfactory brake operation. Low concentric hub and knob assemblies were designed for mounting either the 10 1/2-inch or 7-inch reels to the supply and take-up reel motor shafts.

With the hub-and-knob, collet-type assembly in place, the 10 1/4 inch reel can be slipped on and off easily without removing any part of the assembly, while for use of the 7-inch reel, the hub is not required.
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The capstan motor is designed to provide maintenance-free operation. This hysteresis-synchronous motor has permanently lubricated precision ball bearings of quality compatible with the low flutter requirements of the machine.

The motor operates from 115 volts-ac, 60 cycles (the unit can also be supplied for 50-cycle operation) with a synchronous rotor torque of 12 oz-in minimum. The capstan shaft is made of stainless steel with a hard-chrome plate to provide a hard, smooth surface for tape handling. All motors are balanced under 0.004 oz-in through the speed range of 0 to 1000 rpm.

The capstan shaft drives the tape directly, using 24 poles at 3.75 ips and 12 poles at 7 ips. This provides optimum performance, particularly at 3.75 ips where flutter and wow are normally excessive with motors having fewer poles and speed-reduction mechanisms. As a result, the RT-21A will handle 1/4-inch-wide tape on 7-inch or 10 1/2-inch reels with flutter and wow of less than 0.25 per cent at 3.75 ips, 0.15 per cent at 7.5 ips, and 0.1 per cent at 15 ips.
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The RT-21A braking system is designed to provide an inexpensive, maintenance-free brake. It applies greater braking torque to the supply reeling hub than to the take-up reeling hub. Simple construction results in fewer parts than other braking mechanisms of this type which provide differential braking action.

The brakes use compliance loops combined with conventional spring-biased band-type brakes. The bands are arranged in mirror-image fashion on the two brake hubs and are interconnected by a floating rigid-bar assembly.

When the brake bands are brought into contact with the brake hubs, the rotational energy of the brake hub, which supplies the tape, is transmitted from the brake band associated therewith through the rigid-bar assembly to the other brake band, which operates with the brake hub that takes up the tape.

The band assemblies associated with the reeling brake hubs that supply and take up the tape expand and contract so that the bands and the interconnecting bar assembly are displaced. The brake band associated with the reeling take-up brake hub is self-relieving, since in its displaced position less braking is applied; the brake band associated with the supply brake hub is self-energizing in its displaced position, since more braking is applied.

Differential braking action is thus produced. As greater braking torque is applied to the reeling supply brake hub than to the reeling take-up brake hub, proper tension is maintained in the tape to prevent the throwing of tape loops.

When the tape is emptied from the supply reeling hub or when the supply reeling hub is stopped, forces are no longer supplied through the rigid bar assembly to the brake band associated with the take-up reeling hub.

Accordingly, the supply hub brake is not displaced toward the take-up reeling hub and full braking torque is applied to the take-up reeling hub. The take-up reeling hub is then rapidly brought to a stop. This minimizes the whipping and tearing of the end portion of tape as it is reeled onto the reeling take-up hub.
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The electronic circuits are completely transistorized and mounted on printed circuit boards. Each record-reproduce unit and each control unit is self-contained on a modular chassis. These are called the Amplifier and the Control module, respectively. The modules can be removed from the recorder with little effort.
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The control module contains the transport function (which will be discussed later) and a 30 volt regulated dc supply that supplies power to the circuits in the amplifier module. The supply consists of a solid-state bridge rectifier, filter capacitors, and regulator. The ac power is obtained from the secondary of the main transformer in the relay chassis on the transport. The regulator uses four transistors and one voltage-reference Zener diode.

The operation is of the usual series-regulator type: The output voltage is compared to a voltage reference, and the error voltage is amplified by dc amplifiers. This in turn causes an increase or decrease in the voltage drop across the series regulator in a direction to oppose a voltage change across the output terminals.
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The record channel consists of three printed-circuit boards, a gain control, two transformers, an equalization network, and a metering circuit. One transformer is used for matching either a 150- or 600-ohm line to the input preamplifier.

The first and second printed circuit boards are identical amplifiers and are connected via a gain control on the front panel. The preamplifiers use a low noise n-p-n transistor in the first stage and a p-n-p transistor in the second stage in a direct-coupled, second - collector-to-first-emitter, negative-feedback pair. This feedback, together with local emitter negative feedback, provides for good temperature and gain stability, along with good transistor interchangeability.

The third printed circuit board is a unity-gain, parallel-connected series amplifier using four transistors; it is connected to an output line-matching transformer.

The record amplifier will deliver 100 milliwatts into 600 ohms. The output transformer also drives a parallel rc equalization network, bias trap, and the record head. The rc equalization network provides for a constant record current plus the necessary peaking that compensates for the high-frequency losses in the recording process. Proper tape equalization is automatically selected by the speed change switch.

An 80-kc bias and erase oscillator is also included in the amplifier module. One output of the oscillator is fed through a capacitor which resonates with the inductance of the erase head to provide for maximum erase current.

The other output is fed through a large series resistor directly into the record head where it is added to the audio signal being recorded. (Wir nennen das die HF-Vormagnetisierung.) The oscillator and record amplifier are isolated from each other by an 80-kc parallel-tuned lc bias trap between the output of the record amplifier and the output of the bias oscillator.
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The playback channel consists of the playback head, three printed-circuit boards, a gain control, a transformer, and a metering circuit.

The head is connected between the bias network and the base of the low-noise n-p-n input stage of an equalized preamplifier. The dc current through the head is less than 1,5 microamperes and does not result in magnetization of the head or erasure of a recorded tape. Other than this difference in the input connection, the design of the preamplifier is of the same design as the two preamplifiers in the record channel.

This type of input connection eliminates loading of the head by the relatively low-resistance base bias network. The input stage itself does not appreciably load the head, even at the high frequency end of the spectrum because of the dual voltage feedback which results in a high input impedance. The feedback network also contains the equalization necessary to reproduce a standard NAB recorded tape.

The second printed-circuit board is a three-stage feedback-voltage amplifier. The first and second amplifier boards are connected via a gain control on the front panel. The three-stage amplifier is of the capacitance-coupled type, with each stage having its own bias network.

Local feedback is used in each stage combined with a feedback network from the third to first emitter. This type of feedback and biasing provides for good temperature and gain stability along with good transistor interchangeability.

The third board is identical to the third board of the record channel, as is the output transformer; therefore, the playback channel will also deliver 100 milliwatts into 600 ohms.

The output of the record and playback channels can be monitored with the VU meter on the front panel. Provisions have also been made so that the bias and erase current can be monitored with the same VU meter.
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The record half and full track heads have a 500-microinch gap. The two playback heads have a 100-microinch gap. Also available is a playback head with a 200-microinch gap for 7 1/2" and 15 ips.

The distance between the centers of all the head gaps and the head plate on which the heads and tape lifter assemblies are mounted is a controlled, fixed dimension. "Inasmuch" as the tape path is determined by the two take-up tension arms, the head-plate is adjusted to provide alignment of tape over head gaps by three knurled nuts located behind the head-plate.

After this adjustment is made it should never have to be changed; the only adjustment thereafter to the heads is azimuth. This is accomplished by two screws, one on each side of the head mounting block. These screws provide a fine rotating mechanical adjustment to the heads to obtain maximum efficiency.
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The RT-21A is a high quality recording instrument designed specifically for broadcast use. Many of its advanced performance features have been borrowed from the RCA family of television tape recorders. Now that stereo broadcasting is feasible, the RT-21A is an excellent low-cost method of originating stereo programs.

Diese Broadcast News beginnen mit einer Verkaufs-Anzeige für obiges Bandgerät. Aus dem obigen Text kann man entnehmen, daß es so ziemlich das erste professionelle Studio-Tonbandgerät von RCA war.

Die Anzeige auf der 1. Seite :
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"Convenience" features make operating the RT-21 Transistor Tape Recorder a pleasure. Many new RCA developments are included in this truly professional audio recorder which is the first audio recorder to include continuously variable cue speed control.

Remote control of all operating functions greatly improves operator flexibility. Basic recorder is supplied in two sections-a transport tape panel and a control panel, permitting custom or standard rack mounting. A portable carrying case is also available. Duplicate record-playback amplifier is available for two or four track stereo recording.

You'll Appreciate These Convenience Features:
• Continuously variable cue speed control
• Easy access to all components
• Tape lifters DC solenoid operated
• Sapphires used to lift and guide tape
• Interlocked record operation
• 3 3/4 and 7 1/2 in/sec tape speed provided (15 in/sec available)
• Half track recording with full or quarter track optional. Rack or console mounting.

For complete information write to

RCA, Dept. PD-26i, Building 15-1, Camden, N.J. - In Canada: RCA VICTOR Company, Ltd., Montreal. - Another fine new product from the Broadcast and Television Equipment Division of RCA. - The Most Trusted Name in Radio
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FIG. 1. This is the stereo version of the RT-21A Tape Recorder with two record/play amplifiers mounted side by side. The control unit is on the right. The tape transport unit (above the amplifier panel) is very functional. Straight line tape threading is accomplished without disturbing the hand cover.

FIG. 2. The rear of the RT-21A provides easy access to the transport panel for routine maintenance. The amplifier and control chassis are easily removed.

FIG. 3. Mounted in this rugged carrying case the RT-21A can be used for recording at remote locations or it can be easily moved within the station.
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