Hold Coil

[jthorusen]

Greetings to the Forum:

I am looking for specifications for a "hold coil".   These coils were used in multi-line telephone systems to "hold the line" while the subscriber switched to another line.   They had an appropriate resistance to prevent the call from being disconnected but had sufficient inductance such that audio could be recovered across them for monitoring purposes.

I am attempting to build an old-fashioned "Phone Patch" for radio interface and I need a coil to hold the line up while the patch hybrid (which is capacitively coupled) can pass audio in both directions without appreciable loss.

Any data as to inductance and coil resistance (or series resistance if employed) would be greatly appreciated.

Thanks,
Jim T.
KB6GM

[countryman]

I'd suggest to try coils from relays (110V types?) or windings of small mains transformers.

[Babybearjs]

you're probibly going to look st some old BSP's to see what was used. browse the TCI library and see what you can find. looking at a #202 KTU schematic may help....

[dsk]

Usually the coils has a DC resistance of approx 200-400 ohms, and a high impedance to not take to much of the voice signals. I have used  relays with a good result.

dsk

[rdelius]

At the radio stations I worked at, we used the transformer primary to hold the line while doing remotes . You can use a series capacitor with a switch in paralell .turning on the switch will seize the line after you dial or answer.

[dsk]

The most common phone with an holding coil must be the US field telephone EE-8.

Holding Coil

100 ohms

600 milliHenry

dsk