ance arm resistors, R 7 to RlO, are close tolerance wirewound type and are
switched sequentially in series to give eleven values each increasing by
1000. The switching sequence is 0,100,2.00,300,300+ 100,300+ 2.00,
600,600+ 100,600+ 2.00, 600+ 300,600+ 300+ 100.
3.2 BRIDGE ENERGIZING SOURCES
A single transistor, VTl, is used far the internaI l kc/s oscillator
in a tuned collector circuito The tuned circuit comprises capacitor Cl
and the primary of TI, an extension of which provides the feedback applied
to the base via C2.. The output is taken from the secondary of the trans-
formerj on the six lower impedance ranges an intermediate tap is used to
provide a source e. m. f. of approximately 170 mV, the two upper ranges
using the full output of l. 5 V.
When a jack plug is inserted into the EXT A. C. socket the battery
supply to the oscillator is disconnected and the residual charge of C 3 is
short circ}.1ited via R4. An external a. f. signal connected to this jack is
fed to the bridge via the secondary of T l.
The internaI 9 V battery is used to energize the bridge far resistance
measurements at the R ffiT D.C. position of switch SA. A source of higher
voltage may be connected betweenthe DET+VE and -VE (chassis) terminals
far better discrimination when the internaI battery is disconnected at the
R EXT D. C. position.
3.3 DETECTOR
The dètector has been made unselective to enable external a.f. source s
to be used far energizing the bridge, its frequency response being sensibly
flat from 2.0 c/s to 2.0 kcfs. The input capacitor C5, attenuates the lower
frequencies somewhat to prevent surges damaging the transistors.
An emitter follower input stage provides the high input resistance
necessary to maintain sensitivity on the high impedance ranges, VT2. being
a silicon transistor in arder to minimize noise.
The output of the emitter follower drives a three stage amplifier via
a logarithmic potentiometer. This is part of the SENSITIVITY contrai
which consists of two ganged potentiometersj RV3 is a gain contrai far the
amplifier and RV 2. shunts the meter so that a sensitivity contrai is operative
during d. c. resistance measurements. The amplifier comprises VT3,
VT4 and VT 5, alI similarly biased by d. c. feedback from their collectors.
The last stage has a variable emitter resistor RV'6 to enable the overall
sensitivity to be preset.
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