AT&T Merlin 820 Service And Maintenance Manual page 22

Basic Trouble Tests
Using the voice terminal with the intercom number of the voice terminal
Step 5.
known to be working (see notes made in Step 2), perform the action that
revealed the trouble.
The trouble persists,
The trouble no longer appears,
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IF
THEN
The trouble is caused by either the
control unit or the expansion
module inserted into the control
unit. Replace the cords as they
were.
Go to "Test D: 2-Line/5-Terminal
Expansion Module Test" in the
"TROUBLE ON ONE VOICE TER-
MINAL" section.
Trouble found. It is caused by
one of the cords/cables to the ori-
ginal voice terminal location. To
find out which cord/cable is defec-
tive, replace them, one at a time,
with other cords/cables.
Read the section on Maintenance
Exchange Procedures at the end of
this manual.