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General Radio Company 1212-A Test Set
Unit Null Detector
The Type 1212-A Unit Null Detector (Figure 1) is a logarithmic Amplifier used primarily as a balance indicator for ac bridge measurements from 50 cps to 5 m egacycles. With suitable input filters, the Unit Null Detector can be used with the Type 716-C Capacitance Bridge, Type 1661-A Vacuum- Tube Bridge, Type 916-AL R-F Bridge, Type 716-CS1 Capacitance Bridge, and up to 5 Me with the Type 1606-A R-F Bridge. Frequency coverage can be extended to 60 Me and sensitivity substantially improved by the use of the Type 1212-P3 RF Mixer ahead of the null detector. In addition, the mixer permits use of the Type 1212-A with the Type 1602-B Admittance Meter and Type 1607-A Transfer Function and Immitance Bridge. 1.2 DESCRIPTION. 1.2.1 GENERAL. The Unit Null Detector is a three-stage, wide-band, pentode Amplifier with a maximum gain of about 70 db. As shown in the elementary schematic diagram (Figure 2), a pair of germanium crystal diodes are connected back-to-back in the output of each stage to obtain a quasilogarithmic input-output characteristic. When the voltage on a pair of diodes is low the impedance is high and the gain of the associated stage is at maximum. Three of these stages are cascaded to produce the quasilogarithmic response over a wide range of input voltages. At fairly low input voltages the third-stage diodes begin to reduce the gain of the Amplifier. As the input voltage is increased, the second-stage and first-stage diodes also begin to reduce over-all gain. When the input voltage is increased beyond a few volts, the first stage draws grid current. This puts negative bias on the first tube, further reducing the over-all gain. Because of the crystal diodes, the second and third stages can never draw grid current. The output is usually distorted because of the clipping action of the crystal diodes. At low signal levels the distortion is negligible, but at high signal levels the output is approximately a square wave. Direct current is used on the filament on the first 6AK5 to reduce internal pickup of power-line frequencies. Regulated plate and screen voltages throughout reduce effects of line-voltage variation. 1.2.2 CONTROLS. The front-panel controls are the ADJUST ZERO control, a potentiometer which establishes electrical zero on the meter, the METER SENSITIVITY screw-driver adjustment, used to establish a full-scale reference with maximum voltage applied, and the mechanical zero adjustment for the meter. 1.2.3 CONNECTORS. On the front panel of the Unit Null Detector is a Type 874 coaxial locking-type input connector. A jack-top binding post directly below the coaxial connector permits input connection by means of either the Type 874 Coaxial Connector supplied or a Type 274 Double Plug. At the rear of the instrument, a pair of Type 274 Jacks permit connection to headphones for aural null indication. The power input connector is on the left side of the instrument, and the Null Detector and companion Unit Power supply can be attached to each other directly, without the need of intermediate cable. 1.3 Accessories. Supplied with the Type 1212-A Unit Null Detector are a multipoint connector matching the power input connector and a Type 874 Coaxial Cable Connector for the signal input connection. Available for use with the Unit Null Detector are the Type 1212-PI High- Pass Filter, the Type 1212-P2 1-Mc Filter (refer to paragraph 2.3) and the Type 1212-P3 RF Mixer (paragraph 2.4). Two fixed supplies have been designed to provide plate and heater power for Unit instruments. The Type 1203-B is a general-purpose, unregulated, 300-volt dc and 6.3-volt ac supply, while the Type 1201-B has a regulated plate voltage supply to minimize the effects of line-voltage fluctuations. Both supplies are primarily for bench use but can be rack mounted. An adjustable supply, the Type 1205-B if available, can also be used.

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Service and User Manual
Manual type:
Service and User Manual
13.71 Mbytes (14377188 Bytes)
01 January 1964
Scanned document, all readable.
Upload date:
29 December 2016
TABLE OF CONTENTS Section 1 INTRODUCTION ... 1 1.1 Purpose ... ... 1 1.2 D escription ... 1 1.3 Accessories ... 2 Section 2 OPERATING P R O C E D U R E ... 3 2.1 Power Supply ... 3 2.2 Operation ... 3 2.3 Use With F ilt e r s ... 4 2.4 Use With RF Mixer ... 5 Section 3 SERVICE AND M A IN T E N A N C E ... 6 3.1 General ... 6 3.2 Crystal-Diode Replacem ent ... 6 3.3 Tube Test Voltages ... 6 PARTS L I S T ... 1

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