sundance-communications.com/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems 1A2 MOH

Need MOH witnout a MOH card?

Most of the 400 series cards have a large resistor, sometimes blue, about 120 ohms that bridges the T/R to keep the line busy when on hold. Unsolder it and replace it with one winding of a small audio transformer that is as close as possible to the resistor value.(+-20%) Feed audio into the other winding. Also you can use a low impedance winding in series with the same resistor.

Check the loop current with the resistor and again after your changes to make sure it is high enough so the CO doesn't drop the held line.

The input audio must be speaker/earphone level, not line level. On a multiline system, series or parallel the audio input side of the transformers as required for impedance matching the source.

It may be desirable to add a capacitor across the audio source winding to clip some of the high end that won't make it down Ma Bell's pipe.

We've used this for years and it works well.

Len

Thanks for the info.

I will say, it seems like a lot of work. Good for a hobbyist, it would be a killer on the job. If I ever suggested that when we were doing 1A2 regularly, the boss would have fired me.

Sam

Silversam.

Right you are! Years ago I put together several 1A2 systems from 'abandoned' gear, and MOH cards were not readily available. Modifying the KTU's in the shop was quick. Sometimes things are done, just because, they can be done.

Len

would the held parties be able to talk to each other? if the music was low would you get enough signal through the common sides of the transformers?

Here is a hint. If you listen closely you can hear the interrupter crkt on hold on any 1a2 crkt. This is the path.

That is not a hint; that is a misconception.

The sound heard is not the interrupter circuit; it is the interrupted (winking) lamp supply during a held condition on 1A1 and 1A2, which has nothing to do with the line hold path.

It is cross-talk from unbalanced lamp leads within the KSU, KTU(s), cabling, cords, and tel sets.

On a system not equipped with MOH circuits, there is no direct or indirect connection of line T&R's.

That line card modification is comparable to the circuitry of an ITT 400E or Teltronics/Brand-Rex card. All parties on hold will be bridged onto the same audio path and YES, they will be able to hear each other while on hold. Without isolation transformers/capacitor/diode networks, like those provided by separate ITT 403A card, there is no electrical separation between held parties.

The hold wink "clicking" was definitely caused by imbalance, especially on larger systems when split pairs were used for lamp grounds. This often happened when button/buzzer signaling was present and grounds were "borrowed" as signal leads. Ring transfer keys also led to this "borrowing". I was told that insufficient KSU grounding led to this when 400D cards were being used, but I never recreated that condition myself.

On some ITT 564 sets where they used a 16pr cable and only 1 pair for all the set grounds we used to get large weird problems. Increasing the number of ground pairs improved the situation.

Sam

On a similar note:

Ever wonder why the W/Or pair in every 25-pair cable was twisted more tightly than any other pair?

(Let's not always see the same hands, class.)

Looking through the various 1A2 diagrams, I've noticed that some show the lamps wired individually to their own pairs, while others show them with a common lamp ground on white/green. Which was the more common arrangement?