How can I build a device for making a stand-alone phone ring and operate for conducting tests purposes?

I work in a small-town electronics store. Quite regularly, we hold people bring contained by telephones or answering machines that are out of order properly. Unfortunately, the telephone system within the store is business digital, which means we cannot directly connect the customers devices to it for trialling purposes.

So, what I am wanting to do is construct a standalone device into which I can connect the phone and/or answering machine that will allow me to clear the phone ring, and also allow the phone to operate when it is taken off the hook ("Operate" as within hear sound on the chain, not necessarily call someone).

Now, I hold already figured out how to get the phone ring, that was the unproblematic part (A twosome of transformers to step household 120VAC down to 10VAC, then subsidise up to ~90VAC). The must-do tasks that still remain are:

-How do I make it stop ringing when I pick up the phone?
-How do I put an audio signal on the splash that makes the phone work?

I'm tentative at this, so gentle/simple answers please.


Answers:    The phone ring is usually 20 Hz, but 60 Hz will work fine as you note. When a phone is taken off-hook, the switching equipment at the telco premises see this and removes the ringing voltage an applies 48 VDC for the voice part. The phone impedance off-hook should be almost 500 ohms. On hook is about 1200 ohms if I am remembering right. If the phone rings, you no longer inevitability to keep the 90 VAC applies. Remove it and connect the 48 VDC. You necessitate to watch your polarity. One process will work and the other way won't. If the impedance change correctly, and the audio works, the phone should be OK.
If you connect two phones and the 90v trfr in series beside each other and you pick up both receiver, you will have something close to an intercom system.
With cost of electronics so low these days, and cost of labor so high-ranking, I'm surprised that people would be ready to pay for repair fairly than replacement. Still, check out this site for an off-line telephone tester, No phone chain necessary.

http://www.uashem.com/pageid-305.html...

Since you've already mentioned 120v ac, brand name sure you use a 120v to 24v CT (12v-0v-12v) transformer rather than the 230v one shown contained by the schematic.

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