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104 input transformer 5 which is connected in the ordinary way to the valve grid circuit. The filters 2 and 7 limit the transmitted frequency band upwards, which is necessary to enable balancing of the repeater to be done, this being the more difficult the wider the frequency band to be transmitted. The correction network 5 has an attenuation falling- with the frequency which counteracts the natural rise of the line attenuation with the frequency, so that the circuit gets a uniform transmission of all frequencies in the band. The output side of the valve delivers amplified telephone energy over the output transformer 6, filter 7 and the second differential transformer 1 to the other line LB. Renewed ringing current is obtained from the ringing repeater at RFB—RLB over transformer 1 to the line LB. For the reverse direction of speech the lower part of the repeater diagram applies, the ringing repeater then working in the opposite direction. The test equipment included in the station may be seen on Fig. 3. Immediately below the resistance lamps and fuses at the top of this rack there is a voltmeter on a panel with the switch for testing the station's filament and anode tensions. Below this panel is located the level measuring device which consists in principle of an amplifier with known amplification and a voltage measuring instrument. In order that this may always show on measuring a definite deflection the incoming voice frequency tension must be more or less attenuated by certain potentiometer arrangements. The handwheels for these are directly graduated in neper in the level of the incoming tension. With this instrument attenuation, overall line attenuation, level and amplification may be measured. The lowest panel is a voice frequency generator, which delivers the voice frequency current necessary for the above tests. It can be adjusted for any for the frequencies 300, 500, 800, 1000, 1400, 2000 or 2 400 c/s. The panel above the generator is a supervision panel enabling calls over the repeaters to be heard at the repeaters, cutting in for conversation with subscribers and ringing up other stations.
Fiq. 3 & 4 x « Rocks in detail left, front of test rack with, from top to bottom : fuse panel, voltmeter panel, level measuring panel, jack panel, supervisory panel and oscillator panel; right, back of mains supply rack with, from top to bottom: regulating resistance, rectifier, condensers and chokes There is a mains supply bay at the station for energy supply. Fig. 4. The incoming AC current, c. g., 220 V, 50 c/s, is transformed to suitable tension for the rough regulating transformers I and 2, see the diagram, Fig. 5, proceeding to the metal rectifiers arranged in full wave bridge circuit 5 and 6. The former of these provides anode tension 130 V DC for the valve anodes, the latter filament tension 24 V DC for the cathodes. Fine adjustment of the tensions is done by the two resistances 3 and 4, operated by two dials on the front of bay. From the rectifiers the anode and filament currents pass through filters for smoothing and are finally fed direct to the repeater valves after passing over a switching relay. The two filaments in a repeater are connected in series with a regulating lamp. If for any reason the AC current fails, thus putting the rectifiers out of action, the switching relay couples in the reserve feed so that operation proceeds uninterruptedly. The reserve feed consists of an accumulator delivering 24 V DC to the filaments and a rotary converter delivering 130 V and driven by the battery. Each repeater consumes 0.25 A filament current, so that with mains supply the disconnection of a repeater signifies an appreciable 105
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Page 35 and 36: \ Hotel Switchboard K. G. H A N S 5
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Page 39 and 40: \ The sensitivity of the apparatus
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