Resistance measurements

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

measurements.

(1) Resistance measurements. The use

cover.

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

trouble.

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

assembly.

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