atcomsystems.ca/forum Forums Other TDM Phone Systems 1A2 and Comkey Telephones & Systems "A" lead question

Alow me to start off by saying I am not a tech, but can very somewhat technical. I am planning to install a Valcom V-9941A single zone talkback paging unit. It will be used with a talkbackhorn and a dedicated phone line (3rd line of a 3 line tele) I want to use the "inhibit" feature to allow it to disconnect talkback speakers when a second telephone is picked up, but says I need to have a single line telephone modified for "A" lead control. I have no idea what this means. Can I make any phone "A" lead control capable? The link is for the manual of unit I am using. Note the inhibit option Installation instructions for V-9941A

I can assure you that a three-line phone does not offer A-lead functions whatsoever.

Let me back up and describe what A leads are. Basically, they are the two ends of a switch contact that are closed when the line is seized. When the call is complete and the handset is restored, these contacts go open.

Valcom units rely upon this pair of contacts (along with a 5.1K 1/4 watt resistor in series) to detect that two phones have gone off-hook. In doing so, the resistance across the A lead connection contacts is effectively cut half when two stations go off-hook. This causes the page control to release the connection to the paging speaker(s), resulting in a private conversation.

Unfortunately, there's no way to perform what you are seeking without phones that provide a true per-station contact closure when the line is seized. While there are A-Lead adapters out there, they won't work in this application. Sorry for the bad news.

Thanks for the quick reply. Just to be clear.... The 3line tele is in the house. The talkback horn is located in the garage. I want to add a telephone (no dial?) in the garage so a private conversation can be had if needed. Right now my setup is is with a Valcom V-2900 which is vasically a door control. Works great. Want to have the telephone in the garage for privacy if needed. 2900 does not support this. I do not have a phone yet for the garage. Anything I said help me? Thanks again

Perhaps I'm not clear on the functionality you're trying to attain, but....

How about a single line telephone in the garaga (with or without a dial). Using either true A-lead modification or an a-lead adapter short out or open the speaker leads when the handset on the single line phone is lifted.

Sam

Why not? Put an adapter on each telephone. Seems easy to me.

I thought about that, but since it is only a 24 volt battery feed at best from the Valcom unit, I didn't think that two of them would respond well on the same circuit. An off-hook condition would drop the line voltage down to the point that a second adapter may not have enough voltage to operate.

I appreciate all the posts. Thank You. So there is no way to modify a single line, lets say, $10 analog handset to be "A" lead capable? When valcom made this a requirement, what phones were they assuming someone was going to use? Also, what adapters is everyone referring to? Can't find any info on the internet about them.

You will need to use an industry-standard 2500 or 2554 set. Most new stuff today isn't designed to allow for A lead modifications since they are rarely needed anymore. In the days when these types of paging adapters were commonplace, most phones did have A leads or easy field modifications to provide them.

The only reliable source for A lead adapters that I can think of is Mike Sandman ( www.sandman.com ).

A lead control requires freeing up a switchook contact so that you can use it to control the 400 card.

Sam