Equipment information
General Radio Company 1212-A Test Set
Manufacturer:
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Model:
1212-A
Date:
1964
Category:
Group:
Description:
Unit Null Detector
Information
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.
Manual Type:
Service and User Manual
Pages:
18
Size:
13.71 Mbytes (14377188 Bytes)
Language:
english
Revision:
Manual-ID:
1212-0100-H
Date:
1964 01 01
Quality:
Scanned document, all readable.
Upload date:
2016 12 29
MD5:
0c7b8e45e090552be37b1a4795ec356f
Downloads:
617
Information
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