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panel. One screw each is located
and then isolate the trouble within that cir-
at the top, bottom, right, and left
cuit by voltage, resistance, and continuity
sides of the tuning unit dust
(1) Resistance measurements. The use
(c) Hold the front panel securely and
o f resistance measurements to lo-
c a t e trouble will prevent further
carefully slide the dust cover off
d a m a g e to the equipment if short
t h e tuning unit frame assembly.
(2) Voltage measurements. Most trou-
c i r c u i t s are present. To assist in
b l e s either result from abnormal
the localization of such faults,
v o l t a g e or they produce abnormal
t r o u b l e s h o o t i n g data include the
v o l t a g e s . Normal voltages at tube
n o r m a l resistance values as meas-
sockets and key terminal points in
ured at tube sockets and at key ter-
the main unit are shown in figures
m i n a l points. The normal resist-
41 and 43 through 45. A convenient
ance values at any point in the main
p o i n t for measuring power supply
unit can be determined by referring
voltages is at the feedthrough ter-
t o the resistance values shown in
minals which project from the
f i g u r e s 41 through 45, or by the
filter box assembly, mounted on the
r e s i s t o r and capacitor color codes
(fig. 96 and 97). Before making any
exterior rear of the main unit (fig.
4 5 ) . Compare the actual readings
r e s i s t a n c e measurements, turn off
taken with the normal readings
the power and remove the main unit
s h o w n .  Tube socket voltages
d u s t cover by following the pro-
checks for the electron tubes
cedure described in paragraph 40b,
housed within each tuning unit can
item 12, of TM 11-6625-351-12. A
be Wade only by use of Cable As-
convenient point for making overall
sembly, Special Purpose, Elec-
r e s i s t a n c e measurements of a tun-
t r i c a l Branched CX-6681/URM-85
ing unit is at the multipin connector
( p a r a 61e). In addition, the dust
l o c a t e d at the rear of the tuning
c o v e r of the tuning unit must be
unit chassis. With a tuning unit re-
r e m o v e d ((1) above). Figure 46
moved from the main unit and
i l l u s t r a t e s a typical tuning unit
placed on the workbench, this con-
operating outside of its tuning unit
n e c t o r permits a checkout of the
c o m p a r t m e n t and receiving proper
proper resistance value for the 20
a c and dc potentials from the re-
terminals  w i t h i n  the connector
c e p t a c l e mounted on the rear in-
housing. If trouble is indicated by
terior wall of the tuning unit com-
l a c k of correspondence between
p a r t m e n t . With a malfunctioning
a c t u a l measurements taken at the
tuning unit set up as shown in
multipin connector and the normal
figure 46, compare the voltage
r e a d i n g s listed in paragraph 68,
readings taken with the normal
f u r t h e r localization of the trouble
readings shown in figures 47
must be obtained by measuring re-
through 50.
sistances at the various tube
(3) Troubleshooting chart. The t r o u-
sockets within the tuning unit. To
b l e symptoms listed in the chart
gain access to the tuning unit tube
( p a r a 67) will aid in localizing
s o c k e t s , remove the tuning unit
dust cover as follows:
(4) I n t e r m i t t e n t troubles. In all these
(a) R e m o v e t h e f o u r s c r e w s t h a t
secure the rear of the tuning unit
t e s t s , the possibility of intermit-
t e n t troubles should not be over-
dust cover to the tuning unit frame
looked. If present, this type of
trouble often can be made to appear
(b) Remove the four screws that se-
b y tapping or jarring the equip-
cure the tuning unit dust cover to
m e n t . Check the wiring and con-
the rear of the tuning unit front

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