MOH tie in on a 1A1 system.... where does it go??

[Babybearjs]

I have a MOH module that I was testing and am ready to wire into my system.... the problem, where does it tie into... so it works across all lines.... or do I have to have multiple MOH's??

[AE_Collector]

On 1A1 not 1A2?

Was MOH ever offered on 1A1 key systems?

Terry

[Dave F]

MOH (Music On Hold) was never offered on 1A1, but it can be done.  Most 1A1 systems use either a 230B KTU or a 202D KTU to provide the hold function.  On 230B and 202D KTUs, CO lines are put on hold when the coil of the H relay is placed across the phone line by other relays in the KTU.  This happens when you depress and then release the hold button while:

1) a line is in use,
2) the phone is off-hook, and
3) the corresponding line button is already pressed.

The H relay coil provides resistance low enough to keep the line off-hook and "on hold".  The DC current provided to the H relay coil from the phone line keeps the H activated, and contacts on the H enable the wink-hold flashing of the line lamp in the phone.

To add MOH to a 1A1 system, all you have to do is properly apply the audio signal to the H relay coil.  You will need separate transformer-isolated MOH audio outputs for each CO line.  You could use a 498A KTU (from a newer 1A2 system) for that, or you could build an equivalent circuit on your own.  Connect each separate MOH output in series with a 1uf non-polarized (non-electrolytic) capacitor across the coil of the appropriate H relay.  Running the audio through a capacitor, rather than directly, insures that nothing you add will interfere with the DC operation of the relay, and only the AC audio will pass through.  When the H relay activates and the line goes on hold, the music will also be applied to the CO line.  It's been quite awhile since I've played with 1A1, but that should do it.

The circuit diagram for the 202D and 230B KTUs can be found in BSP 518-114-403 issue 3.

The 498A KTU is discussed in BSP 518-215-401 issue 3

DF

[Babybearjs]

I was re-reading the 1A1 practice and it referred to the adapter the allows for the use of the 400 series line cards... and the MOH module used in the 1A2 system..... because I have such a small system, I'm going to try using a unit I bought a few years ago... wish me luck!