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Marconi TF 868B Other
TF 868B
Universal Bridge
GENERAL The Universal Bridge Type TF 868B is a direct-reading instrument which measures values of inductance from 1 (j-H to 100 henrys, capacitance from 1 ¡x|xF to 100 nF, and resistance from 0-1 ohm to 100 MQ. The instrument employs a single dial for the measurement of inductance, capacitance, and resistance values. Changing the setting of the lcr and range selector switches automatically changes the dial calibration together with the bridge circuit conditions to suit the component under test. For inductance and capacitance measurement the bridge is energized by an RC oscillator-amplifier that can be switched to 1 kc/s or 10 kc/s. The out-of- balance bridge voltage, after amplification and detection, is displayed by a moving-coil meter on the front panel. Measurement of inductance and capacitance is normally made at 1 kc/s; however, the use of 10 kc/s is an advantage, for example, when evaluating low-Q inductors. When measuring resistance, a d.c. voltage is applied to the bridge. The out- of-balance bridge voltage is interrupted at twice the supply frequency by means of a vibrator (or chopper) before being applied to the amplifier- detector circuits. This system gives a high degree of sensitivity without the use of a high potential across the component. A control, phase balance, is provided for balancing out the resistive component when inductance and capacitance are being measured. This control has two scales which are calibrated in Q and tan 8 respectively. The scale in use is determined by the setting of the q-tan S switch. The Q scale is calibrated from 0-1 to 10, and the tan 8 scale from 0-001 to 0-1. Q is normally used for inductors and tan S for capacitors. Q, also known as magnification factor or storage factor, is the ratio of reactance to resistance in a series circuit, or susceptance to conductance in a parallel circuit. Tan 8, also known as loss tangent, dissipation factor or D, is the reciprocal of Q. Power factor, another term commonly used to express capacitor losses, is the ratio of resistance to impedance, or conductance to admittance, and differs from tan S by less than 1 % when their values are less than 0.15. For convenience, an instruction plate is fitted which interrelates values of Q and tan S together with effective series and parallel values. The plate also gives summarized instructions for operating the Bridge. The test terminals are located on top of the instrument; the flat top provides a useful insulated platform for supporting the component to be tested. When measuring inductive components, the usefulness of the instrument may be extended by the use of the D.C. Choke Adaptor, described below. 1.2 OPTIONAL ACCESSORY D.C. Choke Adaptor Type TM 6113 With this Adaptor fitted to the Bridge terminals, an inductor in the range 100 mH to 100 henrys can be measured at 1 kc/s while a d.c. current up to 200 mA is passed through it from an external source. The essential function of the Adaptor is to isolate the Bridge from the d.c. supply, while at the same time preventing the external circuit from appearing as an undesirable load across the test terminals. When making a measurement, the introduced error is not likely to be greater than 3 %; this can be ignored, or eliminated by a simple substitution method.

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Service and User Manual
Manual type:
Service and User Manual
2.59 Mbytes (2711902 Bytes)
OM 868B
01 January 1960
Scanned document, reading partly badly, partly not readable.
Upload date:
06 January 2019
11 2.4.4 Phase Balance ... .12 2.5.3 In Situ Measurements ... .14 3.2.2 Press for R x 10 Switch . . . .15 3.2.3 Phase Balance Control ... .15 3.2.5 Q-tan § Switch ... .15 3.2.8 Range Switch ... 15 3.3 CIRCUIT DESCRIPTION ... 16 3.3.1 Bridge A.C. Supply (Oscillator-Amplifier) . . .16 3.3.2 Bridge D.C. Supply ... 16 3.3.3 Measuring Bridge Circuits . . * 16 3.3.4 Amplifier-Detector ... 18 3.3.6 D.C. Choke Adaptor ... 18 4 MAINTENANCE ... 20 4.1 REMOVAL FROM CASE . . .20 4.2 MAINS INPUT ADJUSTMENTS .20 4.3 WORKING VOLTAGES . .20 4.4 REPLACEMENT OF VALVES . .20 4.5 ADJUSTMENT AND LOCATION OF PRESET COMPONENTS . 21 4.6 SCHEDULE OF TESTS . .23 4.6.1 Apparatus Required .23 4.6.2 Insulation . . . .23 4.6.3 Oscillator-Amplifier . .23 4.6.4 Measuring Bridge . . .23 4.6.5 Amplifier-Detector . . .24 4.6.6 Calibration of Main Balance Dial . . .24 4.6.7 Checking the Phase Balance Dial Calibration . 25 4.6.8 Calibrating the Phase Balance Dial . . .25 5 COMPONENT LAYOUT ILLUSTRATIONS GENERAL VIEW FROM REAR . . . Fig. 5.1 UNDERSIDE OF CHASSIS—AMPLIFIER-DETECTOR . Fig. 5.2 UNDERSIDE OF CHASSIS—OSCILLATOR-AMPLIFIER AND POWER SUPPLY ... Fig. 5.3 RANGE SWITCH ASSEMBLY . Fig. 5.4 BALANCE DIAL ASSEMBLY (EXPLODED VIEW) . Fig. 5.5 6 DRAWINGS COMPLETE CIRCUIT DIAGRAM . . . Fig. 6.1 MEASURING BRIDGE CIRCUITS Fig. 6.2 D.C. CHOKE ADAPTOR ... Fig. 6.3 7 SPARES ORDERING SCHEDULE SOS 1 ILLUSTRATIONS IN TEXT UNIVERSAL BRIDGE CONTROLS . . . Fig. 2.1 TEST TERMINAL VOLTAGE AT BALANCE FOR INDUCTORS Figs. 2.2 & 2.3 TEST TERMINAL VOLTAGE AT BALANCE FOR CAPACITORS . Fig. 2.4 Q-TAN S CONVERSION CHART . Fig. 2.5 FUNCTIONAL DIAGRAM OF TF 868B Fig. 3.1 SIMPLIFIED RESISTANCE BRIDGE Fig. 3.2 SIMPLIFIED INDUCTANCE BRIDGE Figs. 3.3 & 3.4 SIMPLIFIED CAPACITANCE BRIDGE . . Fig. 3.5 SIMPLIFIED CIRCUIT OF D.C. CHOKE ADAPTOR . . Fig. 3.6

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