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~ __ T_Y_P_E_l_60_S-_A_1 M _P_E_D_A_N_C_E_B_R_I_D_G_E
_
obtain the correct value
if it is
too high.
If R t
is
too
low, place small resistors in series with Rr; an easy
way of doing this
is
to connect them to L1 with short
leads.
b.
Ratio Arms
(R a ).
If Rt
is
of the correct
value, the ratio-arm resistors can be checked by the
series of measurements given in Table 5-1.
Note that
the same set of ratio arms
is
used on all bridges except
that there are two 1-k resistors (see Table 2-7).
The C
and G bridges use R14 as a I-k ratio arm and the Land
R bridges use the series sum of R7
+
R8
+
R9
+
RIO.
The ratio arms may be double-checked by measurement
of each external standard (when possible) on both the R s
and G p bridges.
c. Check of C t (Cl). If the lOok ratio arm resis-
tor is accurate, C t may be checked by measurement of a
O.l-f.1f capacitor on the 1IO-nf range. A positive error in
the reading indicates that C1
is
too small. If C1 is too
~mall,
it
can be easily set by additional padding. If C1
is too large, the small padding capacitors already there
may be replaced by smaller ones.
5.2.2 CHECK OF CENTADE ACCUR.!l..CY. It is not nec-
essary to check the total value of the centade, since if
it is
slightly in error, a correction of R t and C t will
give the correct bridge readings (see above). However,
the centade must have good linearity. It can be checked
ave
its range by measurement of a decade resistor on
the bridge as it is adjusted over the range.
A General
Radio Type 1432-K
is
recommended, and this should be
adjusted in 10-ohm steps to check each step of the cent-
ade. Actually, it is necessary to check only those cent-
ade positions that are divisible by three once the first
two steps have been checked (refer to paragraph 4.4).
5.2.3 CHECK OF CGRL VERNIER ADJUSTMENT (R2).
The vernier rheostat adjustment can be checked by meas-
urement of a 1-k decade resistor (Type 1432-K) on the
110- kD R s bridge range.
The shaft position of this po-
tentiometer should be set to give a correct reading at 1
(10 ohms on the Type 1432-K), and the padding resistors
should be adjusted for best accuracy over the rest of the
range.
5.2.4 DQ CHECKS.
D and Q scale checks can be made
by calculation of D and Q of series or parallel RC com-
binations of precision components.
Checking the two
capacitance bridges
is
much easier than checking the
inductance bridges, and checks on both are not neces-
sary since the DQ scales depend upon the same compo-
nents for both bridges (see Figure 1-2). Likewise, it is
easier to check the G p bridge than the R s bridge.
32
The fixed phase-shift error (±0.0005) can be checked
on the C bridge by measurement of capacitors with low,
known D values. The D error on the lowest C
ran~e
de-
pends somewhat on the position of the 1-M ratio arm
(R13). The
fixed Q
error on
th~
R s and G p bridges can
be set by adjustment of C3 (just below the standard
capacitor) to give a zero
Q
reading when a 1-k compo-
sition or film resistor
is
measured.
5.3
ADJUSTMENTS.
5.3.1 OSCILLATOR OUTPUT CONTROL (R529). This
control, on the rear of the printed wiring"board at the top
of the instrument, controls the maximum output level of
the internal RC oscillator.
It should be set to give an
un clipped output at anchor terminals 32 and 31 when the
GEN LEV control
is
fully off (counterclockwise).
5.3.2 CENTADE ADJUSTMENTS.
The mechanical ad-
justments of the centade should not be necessary unless
the centade assembly has been taken apart.
If adjust-
ment
i~
necessary,
it
should be done carefully and in the
correct sequence.
First,
adjust the position of the detent block so
that the digits of the counter readout are centered in the
window.
To do this, siightly loosen the hex-head nuts
on the rear of the subpanel and rotate the detent block.
Next, connect a component of known value to the
bridge and set the FULL SCALE RANGE switch and CGRL
control to the correct value. Then balance the bridge by
positioning the rotor of the centade.
This
setting should
be accurately made since the centade should change
value halfway between the detented steps.
Tighten the
rotor set screws. It is best to check the centade adjust-
ments at several points of its range.
Finally, loosen the centade knob (the larger knob)
and set it so that a zero reading appears when the knob
hits the stop, which is on the dress panel under the knob.
The pressure for the centade detent
is
adjusted by
the screw on the detent block directly behind the front
panel. The setting of this pressure
is
a matter of person-
al preference.
Too tight an adjustment will make the
control difficult to rotate, and too' loose an adjustment
will not give the necessary detent action to ensure that
the centade rests on a detented position.
5.3.3 ADJUSTMENT OF CGRL VERNIER CONTROL.
The procedure for setting the vernier CGRL control with
respect to the counter reading is given in paragraph 5.2.3
above. Tpe stop for this control, mounted on the front of
the plate holding the vernier rheostat (see Figure 5),should
be set to give a zero reading when the potentiometer is
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