Patent GB-785441-A (GB)
Title: Improvements in or relating to multi-channel magnetic recording and reproducing apparatus
Inventor: CLARK FREDERICK
IPC: G11B15/46
Filed: 1953-12-22
Abstract: 785,441. Control circuits for tape-recorders and reproducers. THERMIONIC PRODUCTS, Ltd. Dec. 22, 1953, No. 35622/53. Class 40 (2). [Also in Group XXXIX] In a magnetic taperecording mechanism comprising a plurality of tape transport decks in the event of the deck in use becoming faulty or running out of tape a start signal is extended over a closed loop circuit to the first deck which is available for operation and if no further deck is available an alarm is given. Tape transport mechanism, Fig. 2A. The supply and take-up spools SS, TS are driven by induction motors, operating in opposition to maintain the tape taut, the direction of movement of the tape being controlled by the voltages applied to the motors. Thus for recording and play-back the take-up motor voltage exceeds the supply motor voltage and for rewinding the latter voltage exceeds the former. For controlling the speed during recording and play-back the tape also passes over a capstan 20 to which it is held by a pinchwheel 21. The capstan is driven by a synchronous motor. The movement of the pinchwheel into position closes a capstan head gateswitch operable to ensure that no attempt is made to record or playback with the capstan head open and that no attempt is made to rewind with the capstan head closed. For recording only the tape is passed round a diverter pulley 18 so positioned that towards the end of the run the tape makes contact with a contactor 24 consisting of a post comprising two vertically aligned insulated contact pieces. Five minutes running time from the end the tape is provided with a metallic strip for bridging the contact pieces as a signal to the recording equipment. A limit switch 23 held open by the tension of the tape, so that if the switch is closed the equipment is not ready for use. The idler pulley 22 operates a velocity switch which delivers pulses at a prescribed frequency when the tape is running at the correct speed for recording. Recording equipment. Figs. 3 and 4 show a master recording set and a subsidiary recording set. Further subsidiary sets may also be provided. Assuming that the master set is ready for use, i.e. its head-gate switch HG is actuated to its upward position and the limit switch S5 is open, then relay M is released and operation of key KS to its start position causes positive potential to be applied to the righthand grid of valve V1, thereby operating the anode relay A which lights the "unit running" lamp LPR. Relay A operates M which locks and also feeds the 20 kc. bias current lead to the left-hand grid of valve V1. If the bias current is present the emergency relay E operates to disconnect the emergency lamp LPE. Relay A also operates relay S which connects power to the spool supply and take-up motors SSM, TSM and the capstan motor M. Relay S also operates RLB to energize the solenoids SN1, SN2 which remove the brakes from the motors. Pulses now received from the velocity switch VS cause repeated operation and release of relay V to alternately charge condenser C5 and discharge it into condensers C3, C4. The charge thus maintained on the latter keeps the righthand grid of V1 positive thereby holding anode relay A. If the tape stops running C3, C4 lose their charge, A releases (lighting the "attention " lamp LPA) followed by S which stops the motors and applies the brakes. During the slow release of S a 30 ms. pulse is applied over lead P10 of a closed chain circuit feeding the righthand grids of valves V1 of subsidiary recording circuits. In the first of these which is ready for use (A and M both unoperated) the anode relay A therein operates and recording continues on the tape associated therewith. When the tape has run to within five minutes of the end relay T operates over the 5-minute contacts 24, locks, and lights a warning lamp and during the operate time of T battery is extended from contacts 24 to wire P10 of the chain circuit to start another recorder. In each recording circuit the emergency relay E is released if no 20 kc. bias is available and if one of the power supplies fails (e.g. due to a fuse blowing) B, R, C or H release to interrupt the cathode circuit of the corresponding valve V1 so again releasing E to provide an alarm. In addition, the bias supply to the left-hand grid of V1 in the master circuit is fed through parallel circuits containing serially related contacts of A and T in all recording circuits so that if no recording circuit available (T up or A back) the bias does not reach the valve and E releases. Relay E is arranged to operate in a charge-discharge circuit for capacitor C2 connected to the lefthand grid of V1 in such a way that E operates and releases intermittently to flash lamp LPE. All the E relays also control an external alarm circuit. Play-back and rewind circuit, Fig. 5. With control switch WK in the neutral position if the head-gate switch HG is open, HC operates and prevents play-back from subsequently taking place by opening a locking circuit for L. If the switch HG is closed then with WK in the neutral position L operates and locks so that when switch WK is thrown into the "play" position P operates to energize the motors provided the limit switch is open, i.e. held off by the tape. If, however, the limit switch is closed, HC operates to open the locking circuit for L which releases P. Relay P, moreover, cannot operate if the head -gate switch HG is open. When P operates, RLB follows to cause solenoids SN1, SN2 to remove the brakes from the motors. With WK in the wind position, then, provided the head-gate switch HG is open and the limit switch is open, HG ard W operate the latter modifying the voltages applied to the supply spool and take-up motors to effect rewinding. Relays S and RLB operate to remove the motor brakes. Power supply arrangements, Figs. 3 and 4. An emergency power supply is provided which is first tested when the power is switched on. Thus, when the start key KS is thrown, MP operates to the emergency power supply followed by CP and HP which changes over the supply connections to the regular supply unit. CP holds MP over contacts of the individual supply alarm relays B, R, C and H. On a power failure one or more of these releases followed by MP, CP and HP which bring in the emergency supply. Fuse alarm relays B, R, C, H are also provided in the play-back circuit, Fig. 12. Specification 695,150 is referred to.