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 Post subject: Telephone "hold coil"
PostPosted: Sep Sat 25, 2021 4:25 am 
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Location: Lincoln City, OR 97367
Greetings to the Forum:

I need some data on a telephone "hold coil". These were used in multi-line phone systems to prevent the call from being dropped when the subscriber switched away. It was switched in when the subscriber pressed the "Hold" button on the phone. The coil was of an appropriate resistance so as to hold up the connection, but had sufficient inductance to allow monitoring or "music on hold" to be impressed on the line without appreciable loss.

I am building a "Phone Patch" for radio and I need the inductance and resistance of this type of coil (or its inductance and series resistance if employed). I need to hold the line up from a DC standpoint while the patch hybrid (which is capacitively coupled) handles the two-way audio.

Any data or specifications for such a coil would be greatly appreciated.

Thanks,

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Jim T.
KB6GM


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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sat 25, 2021 5:52 am 
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Location: Montreal, Quebec
I don't think you need it. I don't remember them in phone patches.And there were definitely solid state circuits that did the same thing.

Legally, once they existed, you were supposed to connect "foreign equipment" to the phonebline through a DAA. Eventually, a DAA became part of the equipment. So a scrap modem would give you one, if yiu can trace the parts.

texas Instruments had a book in the eighties about telephones, I think Radio Shack sold it under their cover. It showed things like this, as did plenty if construction articles.


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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 2:45 am 
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Location: Aurora, IL
Don't know if this will help but here is a link to a circuit to put a line on hold and modified to add music on hold. In the notes it indicates a loop current draw of as little as 7ma being enough to hold the line. Normal central office battery supplied to your phone line is -48V with a maximum of around 50ma loop current.

http://avtvtech.com/hold/Telephone%20on%20hold.htm


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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 3:39 am 
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Location: Equinunk PA 18417
A patch I built years ago uses a 1.6k resistor switched across the line to hold the connection, and has worked on every POTS line it's been hooked to. Audio to/from the line is capacitively coupled into a 1:1 transformer to prevent dc current flow through the transformer (since mine saturated when it was part of the hold current), and the transformer helps prevent ground loops and keeps the line balanced. Audio level loss caused by the resistor is easily adjusted for. There are more elegant designs but this works quite well

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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 3:48 am 
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Location: S. Dartmouth MA 02748-1225 USA
I never used auto-patch in any of my Ham shacks. However from years ago I recall something called a "Murphy's Autopatch". Seems it used some sort of "special" transformer?

Is this what is to be built or something all together different?

I do have some sort of 1:1:1 telephone transformer around somewhere...

Chas

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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 4:29 am 
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Location: Montreal, Quebec
I haven't looked at a phonepatch schematic in a long time.

But I recall, some complication came from wanting vox operation. You didn't want the receive audio going out on the phone line triggering the vox.

I see one thing. Checking a sample phonepatch, Jan 1971 Electronics Illustrated, the transformer is coupled to the phone line, and you're suppised to use the phone in parallel, for dialing but also to load the line. So you can monitor on the phone.


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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 4:53 am 
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Quote:
So you can monitor on the phone.
I'm no expert on telephone jargon but is the ability to hear ones own voice in the receiver called "the sidetone"?

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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 5:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Minneapolis, MN USA
Not to discourage DIY but would a Heathkit HD 15 phone patch serve your needs ?
My unit has sat dormant for decades after assembly and a few test calls.
If you can use it either whole or for parts you are welcome to it for shipping cost only.

You can review the schematic and determine if suitable coils are present from the assembly manual:
https://w5rkl.com/wp-content/uploads/2015/02/hd-15.pdf


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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 5:10 am 
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Location: Montreal, Quebec
Sidetone in phones is an arrangement where some of what you speak into the microphone is heard in the earpiece. It's arranged so the level is low.

A phone patch connects your receiver to the phone line, so someone at the other end can hear it. And when they speak, it goes out on your transmitter. I'm talking about using the phone at the same time. Not only does it load the phone line to keep the connection going, but you can monitor what's being said at the other end, so nothing forbidden goes out over your transmitter.


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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 5:22 am 
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Location: Montreal, Quebec
A 600 ohm resistor.

So you have a double pole switch between the phone line and the patch. A 600 phm resistor across the line after the switch. Then a coupling capacitor to the transformer.

Flip the switch, and audio can go in and out on the phone line, and the resistor loads the line.

But way to dial.


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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 6:58 am 
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Location: Lincoln City, OR 97367
Greetings All!

First, thanks to all who responded. Some very helpful things were said. I suppose I should better explain what I am trying to do so that I don't waste anyone's time.

I take care of an LPFM (Low Power FM) radio station in Newport, OR. Of course, like most such operations, they run on a shoestring budget, so much of the technical support hardware comes out of my pocket (and relies on my inventiveness, what little I have). In this case, they wish to do a call-in program, with one or more participants in the studio (hereinafter referred to as talent) and a caller participating by telephone. The phone patch therefore is to be used as an interface between the talent, the radio station audio and the caller. A handset will likely be used by a call screener, but once they decide to take the call, then the patch will be switched in and needs to hold up the phone line, since the screener will hang up at that point to reduce noise and minimize changes in level. The patch will drive a headphone amplifier (or an audio distribution amp which will drive individual headphone amplifiers) as well as the main mixer board in the studio. The patch will accept audio (and drive the phone line) from either a station monitor so the caller can be advised of the air program or from a microphone preamp which will provide mic to line level to drive the patch as well as the main mixer board. These options are selected by a switch which also disconnects the caller (phone patch output) from the main mixer board when the air signal is monitored. Because processing is done by a computer program running under Windows, neither the talent nor the caller will be listening to air audio because the digital delay is very difficult for most people to cope with. (There will probably be a low level input from the microphone preamp to the headphone amplifier(s) to provide some side tone.) So, that's the plan.

For dacsman: The holding current is probably all I really need; thank you very much. I couldn't find that spec anywhere. The central office battery is probably not relevant; the phone line is hauled by a cable modem and so the source of line current is in the local modem. However, I presume its current requirements conform to the Bell System standard; I need only measure its voltage and plan to land in the middle of the current range.

For Dale, a Heathkit HD-15 is exactly what I am using. It is on the bench right now, where I have just finished converting it (I hope) to balanced line operation. I would be interested in your patch for spares should it turn out that I have irreparably damaged something. PM to follow.

For Richard: 1.6 K with 48 volts is 30 mA. That's 3/5 of the maximum loop current at 48 volts... although the cable modem is more likely to supply 24 volts or so. Therefore, the resistance seems a bit low to me but it does have the advantage of not being frequency sensitive. I was looking for data on a hold coil because that's the way Ma Bell used to do it in the good old days, but that's not necessarily the best way for my application; I will have to do some experimentation. Having some beginning values does help to limit the number of experiments I have to run. Thanks.

For everyone else, thank you again for your input; all of it is appreciated.

Regards,

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Jim T.
KB6GM


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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 4:34 pm 
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Jim,

FWIR stations had a 6 second delay on all conversation when the "outside" line was in use. Then it was an endless tape loop on a "special" tape recorder. I would imagine it is some sort of solid-state device to create the delay these days. The announcer, would have his hand on the "button" and create dead air if the caller went bonkers...

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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 8:09 pm 
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Joined: Jan Fri 02, 2009 11:32 pm
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Location: Equinunk PA 18417
Jthorusen wrote:
For Richard: 1.6 K with 48 volts is 30 mA. That's 3/5 of the maximum loop current at 48 volts... although the cable modem is more likely to supply 24 volts or so. Therefore, the resistance seems a bit low to me but it does have the advantage of not being frequency sensitive. I was looking for data on a hold coil because that's the way Ma Bell used to do it in the good old days, but that's not necessarily the best way for my application; I will have to do some experimentation. Having some beginning values does help to limit the number of experiments I have to run. Thanks.

Jim, you'll discover that when a phone is "off hook", the POTS line voltage drops way less than 48-53 volts that is typical of an inactive line, usually between 10 and 15 volts. The exact voltage the line drops to depends on line resistance, receiver and source characteristics. Those specs are probably searchable, I don't recall the specifics. If your patch can "hold" the line, then no extension phone is needed to answer or maintain a call(except maybe dial out to place a call). 1.6k is a good value.

Dale's offer to help with a Heathkit "hybrid" patch looks fantastic! That will be much more easy and probably better-performing than if designing from scratch. It will take care of the very large signal level difference between what you receive from a caller and the signal level you must drive into the phone line in order for the caller to hear the station. The caller's voice will usually be 20 or more db lower, again dependent on several variables. The old phone sets use a transformer that is wound to cancel out most of the outgoing (microphone) signal into the set's earpiece, thus the level difference is not noticed. That transformer's line side also allows for the dc "hold current" to flow through it, thus no other resistance is used. A well -designed phone patch for on-air radio use will have an output that is primarily the caller's voice only, and will accept an input that is driven into the phone line for the remote caller to hear. Signal cancelling of that input must be done by the hybrid patch in order for the output to be the caller only. Excellent cancellation is complicated by phase shift of your outgoing audio by the phone line. You also need to allow for the very significant "ring" voltages as well as lightening protection.

Again, I'd suggest you look into a used phone hybrid rather than design one from scratch. Almost no one has POTS lines anymore anyway. If using an internet phone system, which I'd imagine is more likely, just tap into the phone's receiver.

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-Richard


Last edited by richfair on Sep Sun 26, 2021 8:14 pm, edited 1 time in total.

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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 8:11 pm 
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Dangit, I see you already have the Heathkit. So most of what I posted is not relevant to you, sorry. :oops:

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 Post subject: Re: Telephone "hold coil"
PostPosted: Sep Sun 26, 2021 9:39 pm 
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Joined: Jun Fri 01, 2012 8:05 am
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Location: North Carolina
Way back when ma Bell had all sorts or restrictions that everyone ignored we had two phone lines in our house. One primarily for my dad who occasionally got called after hours for work and the other for general house use. My brother and I quickly figured out a way to use both lines in several creative ways. We created the "conference box". This was WAY before conference calls were a thing.

A standard minibox contained a ring light for each line (Ne51). Line selection was done by a double pole double throw switch to select between lines. Hold (germane to this discussion) was accomplished with a 1000 ohm 2 watt resistor switched onto whichever line needed holding by an spst switch. The all important "conference" function was accomplished by 2 10Mfd electrolytics with the positive pointing in opposite directions. It would pass AC talk to the other line but not put the line on hold. this was switched in and out by SPDT switch.

This setup would work very nicely for hold and for phone patch. We used it for years. Sometimes we would call two girls and as long as they answered pretty much at the same time each would think the other had called. We gained some valuable intelligence that way more than once.


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