Music on Hold

A 451-type card provides a music-on-hold for central office or PBX circuits. It works with up to seven lines to provide music to the distant party when the local party places the line on hold.

Music-on-hold can be provided using either the 451 or 498 KTU. The 451 is compatible with 400D, 400F, 400G KTUs. If music-on-hold is to be provided for a 400H KTU, the 498 KTU must be used.

The music source is typically connected to a 34A coupler. The output of the 34A coupler is then connected to the 451 KTU.

The 451-type KTUs are four inch cards with 40 contacts. The most common 451-type card is the 451B, followed by the 451A.

Below is the "Condensed Functional Schematic of 451B KTU (Music-On-Hold)" from issue 3 of section 518-215-401 updated in December 1977 from Volume II of the New Jersey Bell Key Systems Service Manual.