In some KSUs like the 551 A and B's yes it needs to be on the last card slot. Some 551 C's the 401 card can be in any slot. Below is from 551 BSP 518·215-407 .
Manual Intercom
3.11 This feature is provided by installing a 401A
KTU in the KSU. In early production models
of the 550/551C KSUs, only jack J4 was factory-wired
with the talk (A) battery and ground required for
operation of the 401A. Later models (indicated by
an asterisk [*] stamped after the code on the KSU)
are wired to accept a 401A KTU in any of the
four jacks.
3.12 The early models can be modified to accept
a 401A in another jack by disconnecting the
green wires from terminals 3 (A grd) and 18 (A
bat) of J4 on the back of the KSU, and transferring
them to the same terminals of the desired jack.
3.13 On the 550/551A and B KSUs (MD), the
talk battery terminals are factory-wired at
each KTU jack, but only the BR-W/W-BR pair on
J4 is wired back to the talk battery terminals on
the connecting block. The pairs from the other
jacks are dead dressed. To use a jack other than J4 for the manual intercom KTU, disconnect the
BR-W/W-BR pair from the back of the connecting
block, and connect the pair from the other jack
as shown in Table B. Insulate and store the
disconnected pair.
Last edited by morephones; 05/02/20 09:33 PM.
