Tech Folks - Please Step In

[ramegoom]

Looking to tap the masterminds here. I am a newbie, and this is my first intro into phonery.

This is my scenario:

I have a WE 551A switchboard which I have made fully functional by adding a 15 volt DC supply and a Ring supply. Everything works. I added a punchdown block internally with an interface connector, 25 pair. Then I attached a 25 foot long interface cable to yet another punchdown block, which now interfaces with 12 RJ keystones.

Further, I hard-wired various old phones; a 50's WE payphone, a space saver phone with an external bell, a "beehive" Leich phone, no dial with a crank, a Kellogg Masterphone with separate subset and crank, couple of 500 series desk phones, a 554 wall phone, an old 303, and a couple others - 11 in all. Every one of them work with the switchboard and all of them ring. The Kellogg is set up as an "incoming" line, so when I crank the phone, the incoming line 1 lights.

Now I want to add a dial tone so when anyone picks up any of the phones, they get the tone. Soliciting advice from an extremely helpful member here, I was directed to a site that published a schematic for a dial tone circuit. I then designed and printed a small PC board with a built-on voltage regulator and now have a source for a dial tone. It will fit easily inside the switchboard.

So.

What do I need to do to provide a dial tone on any phone that's plugged into the switchboard? And where on the switchboard do I route the dial tone signal? It doesn't have to actually work as if the CO sent the tone, so I can switch it off at will if needed, but I'd like to have it at least sound like a phone when you take it off-hook. Possibly attach it to line 2 of the incoming trunk?

Any help or comments deeply appreciated.

[RB]

Hi
I am no wizard, for sure.
I do not know if a 551 provided dialtone...a 555 did not.
The idea was, when you crank or pick up your phone, a light or a bell rang on the board.
The op connected to you, got your info, and dialed the call for you.
Now, if you are plugged into a trunk, then I believe you got dial tone from the Co.
but only on that one phone.
hopefully some of these brainiacs will chime in and correct if this is amiss...

[paul-f]

This cord switchboard service manual from the TCI Library may provide some good background on how calls were intended to be completed.

  http://www.telephonecollectors.info/index.php/browse/catalogs-manuals-educational-docs-by-company/western-electric-bell-system/manuals-by-date/5452-cord-switchboard-service-manual-nonmultiple-types-550-551-560-ibt-ocr-r/file

[RB]

yup, page 21 

[ramegoom]

This switchboard works well as an intercom and I can connect every phone together thru the keys on the keyshelf. So, I figure, if I also connect an outside trunk line and set up the dial tone circuit exclusively to that pair, then every phone will hear the tone as long as the operator is switched into the system. Looking for "effect", not really functionality. I can then switch it off at any time.

Or, I can place this dial tone circuit inside any phone, so that if you pick up the phone, you get a dial tone. That'd be slick for a phone that's a display-only. As long as I supply at least 10 volts DC to the circuit, it'll produce a tone.

Picture your collection where the phone is displayed, then someone takes the handset off the hook and they get a dial tone! Hmm....

Thanks for the manual, very interesting stuff there.

[RB]

I think you are on to something there.
If you insert the dial tone source at the trunk on the inside of the unit,
that would be a good place to get dt. Just like the real thing.
I do not know, however, if you can wire the dt into the trunk unit from the input side.
Trunk units act a little different than a station circuit. So I don't know what to expect.
The next issue...
You are adding an audio source to an existing system that has it's own set of rules.
Are the operating voltages compatible?
How about the impedance?

[ramegoom]

The original online schematic required a 7.5 volt source. I used an inexpensive converter module to provide that voltage and a wall wart supply to power it up.

My layout uses a "buck" type voltage regulator that can operate anywhere from around 9 volts up to around 30 volts.

On a common phone system, the phone itself runs on constant DC and the dial tone is added, so it seems to me that if the PC board used the phone's wiring to power it, and I attached the DT output to the same two wires, it'd play a tone when picked up.

The concern I would have to address would be when the AC ring signal hits the same two wires; 90+ volts may attack the electronics, not sure if that's going to be a problem.

When I wired the Kellogg phone to Line 1 of the CO string, I had to supply 3 volts to the phone, using one of these buck regulators, and placed it inside the switchboard. Cranking the phone routes the AC ring voltage thru the line, parallel to its 3V DC source, and it works fine, no problems. These little two-dollar regulators apparently can take some abuse.

The output of the tone circuit uses a decoupling capacitor, so the ring voltage shouldn't affect the circuit. I'm going to do a bit of testing to see if it will work as a source to the CO line. Seems that once the tone is present on the incoming circuit, it would transfer to the extension station whenever the cord is attached thru the switchboard. Time to play.

[Key2871]

How are you going to get the dial tone to go away once the board, or phone is signalled. Are you going to use a relay or other circuit to sense that the call has been responded to?

[paul-f]

An off topic alternative would be to provide dial tone using a telephone line simulator or small PBX (see the many other topics on devices such as the Panasonic 616).

The lines provided would enter the switchboard just like CO trunks. Once connected to one of the provided lines, a user could dial an extension connected to the simulator or PBX. Some extensions could go back into the switchboard as CO trunks, where they could be manually connected to another station. That would open up opportunities for some creative demonstrations.

If you use a PBX, you'd also be able to demo sets that are not wired to the switchboard without the need for an operator.

[ramegoom]

How are you going to get the dial tone to go away once the board, or phone is signalled. Are you going to use a relay or other circuit to sense that the call has been responded to?

That's the next challenge. I would either install a switch to shut the DT circuit off, or maybe some sort of sense circuit that listens for the pulse from the dial to shut it down. Only exception would be whether I used a phone without a dial - then I'd have to use another means to shut the DT off, maybe look at AC from the crank, or listen for audio from the transmitter.

It would get a bit complicated but seems it could be done.

[RB]

I would go with what Paul said...
hang a 616 on the back, and connect through it.
easy solution... and completely functional too.

[Key2871]

Yea, I agree. Paul's idea would work out better, because as soon as a digit is dialed the Dt stops, and it can also prove ring voltage.
Other than that you would need to have it operated via relay(s) connected to the calling extension, or sencing a ring current etc. To disconnect the DT circuit.

[ramegoom]

Some quick questions:

What voltage does the phone line see when the phone is on its hook? is it between 10 and 40 volts?

And, once the phone is off the hook, what voltage is measured? Lower than 12 volts? Or does the voltage stay the same?

When dialing a phone, does it momentarily short the voltage present in the phone line?

I may be able to use the voltage differential to disable the dial tone. As it sits now, I can insert this circuit directly into the phone red-green wires and place a wall wart in it for power. When you pick the phone up, you can hear the dial tone.

[dsk]

The off hook voltage you see (measure in the phone jack) are typical 48 or 24 V with a pretty high deviation, a 48 V lead battery may have up to 57 volts under charging.  I have seen a PABX with as little as 12 V. 


When you go off hook the voltage drops considerably, usually in the relay coils, sometimes in other components. 

The voltage is actually not of less interest, it just have to be OK to squeeze a current trough the telephone for powering the phone, The current should be 18-35 milli-amps.  At that current the voltage drops to maybe 5 V. 

When you move the dial out of rest position, many phones shorts the line (and you get 0 V) when pulsing on return the voltage will go from full to zero for each pulse.

Regarding dial tone on manual exchanges: Here we never had any dial tone before we got phones with a dial.


dsk

[Key2871]

Just a thought here, but I have a 32- or 36 station intercom system that is small and provides a dial tone. You can put ring voltage in it for signaling. It would be smaller than a EKSU such as a Panasonic. It uses 24 volts, to operate, provides some features such as re-ring, it is a two digit dial type, not five or six etc.
I have a couple and I'm going to set up Onegin my house through my 1A2 KSU.