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__ T_Y_P_E_l_60_8-_A_1 M _P_E_D_A_N_C_E_B_R_I_D_G_E
_
b. Noisy or Erratic Balance. If the instrument is
idle for an extended period, surface contamination of the
wire-wound DQ or CGRL vernier adjustments may cause
an erratic behavior of the null indicator. To remedy this
situation, rotate these controls back and forth several
times to clean the brush track.
Misalignment of the centade (CGRL adjustment)
may cause a change in its value as it is rocked in a de-
tented position.
The rotor of this adjustment should be
set so that the centade changes value halfway between
the detented steps (see paragraph 5.3.2).
c. Inability to Obtain Balance. If the bridge does
not seem to balance at all, several things should be con-
sidered before the bridge is assumed defective.
(1) Is the unknown component connected cor-
rectly?
(2) Is the unknown what it is thought to be?
(Large inductors can look like capacitors at 1 kc.) Try
another unknown.
(3) Are all the panel switches set properly?
(4) Are the jumpers between the BIAS terminals
and between the EXT DQ terminals in place?
(5) Is the correct bridge being used?
(Low Q
inductors and high D capacitors should be measured on
the R s and G p bridges, respectively (refer to paragraphs
2.4.1.1 and 2.4.2.1).
d. Low or No Meter Deflection when Bridge Un-
balanced.
(1) Is the GEN LEVEL control on (clockwise)?
(2) Is the DET SENS control on (clockwise)?
(3) Is the function switch set properly (and in
a detented position)?
(4) Check the oscillator and detector (see below).
5.5.2
OSCILLATOR AND DETECTOR CHECKS.
The
oscillator output can be measured from either BIAS ter-
minal to either EXT DQ terminal when the bridge is set
to R s , L s , or L p . If there is no output when the function
switch is in the INT AC position and the GEN LEV con-
trol is on (clockwise), the oscillator is not operating
properly. (Note: the output will be very low with a low-
impedance unknown.)
The test point voltages given in
Table 5-2 and the diagram (Figure 5-16) should enable
anyone skilled in the art to locate the difficulty. One of
the first things is to try
to
remove the plug-in frequency
board and bend up (slightly) all the terminals to ensure
contact.
34
To check the detector, insert a signal between the
LOW UNKNOWN terminal and ground. Be sure the func-
tion switch is set properly and the DET SENS control is
clockwise.
5.6 TABLES OF TEST VOLTAGES.
The following tables give voltages as an aid in
trouble-shooting.
Table 5-2 lists dc voltages at tran-
sistor
terminals
on
the oscillator-detector-amplifier
etched board.
Table 5-3 gives voltages from the UN-
KNOWN terminals to chassis for the Rand G bridges.
J 1 in this table is the left-hand binding post, J 2 the
right.
All voltages are as measured with a vacuum-tube
voltmeter, and are dc voltages from the terminal desig-
nated to chassis, except as otherwise indicated.
Line
voltage for measurements should be 115 volts.
TABLE 5-3
DC VOLTAGES ON UNKNOWN, RAND G BRIDGES
Centade at maximum
GRN LEVEL fully clockwise
For location of 53, 401FR, see Figure 5-1.
RANGE
MEASURE
VOL TS DC
FROM
TO
1100 mD S3,401FR
J2
3.5
11.12
S3,401FR
J2
3.5
110 .12
S3,401FR
J2
3.5
1100 .12
Jl
Chassis
30
11 k.12
Jl
Chassis
35
110 k.12
Jl
Chassis
180
1100 k.12
Jl
Chassis
340
1100 nU
Jl
Chassis
340
11
jJ-U
Jl
Chassis
180
110
jJ-U
Jl
Chassis
40
1100
jJ-U
S3,401FR
J2
35
11 mU S3,401FR
J2
30
1l0mU S3,401FR
J2
3.5
1100 mU S3,401FR
J2
3.5
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