If you're familiar with the Army surplus switchboard type BD-72, I need your help.
I know someone who wants to connect one of these switchboards to the PSTN; i.e., to have a dial tone on one of the lines instead of a ringdown circuit.
Please tell me if you can do it, or know someone who has already done it. You may contact me offline if you prefer.
Thanks in advance.
Bill
That's a local battery switchboard, it will require extensive modification to use on a common battery system.
It has drops on the switchboard, so when people crank their phones, it energizes the drops and makes it clear to the operator that someone wants service. Then the operator plugs a passive headset into the jack for that line and the local battery in the telephone provides talk power for the operator to ask what service the subscriber wants. The operator plugs his passive headset into the jack for the other subscriber and cranks the magneto to ring the second phone. Then when the other party answers, he connects both.
One of those two subscribers may be another switchboard connected to other subscribers, possibly through a relay-and-transformer arrangement to graft the field telephone network to a commercial local battery exchange.
If the fellow wants to use the switchboard with common battery phones on the PSTN, he's going to have to figure out three things: 1. how to provide -48V talk power for the operator to talk to the subscriber phone 2. how to provide ringing voltage for the subscriber phone to signal the operator 3. how to provide signalling so the operator can signal the PSTN to dial the call, and a facility to keep the line locked between the time the operator disconnects his headset from the PSTN to the time the operator plugs the subscriber into the PSTN.
This is going to mean pretty extensive reworking of the switchboard. A good introduction to how field telephone systems are meant to be operated can be seen in Army TM 11-330, and a good introduction to the principles of operation of common and local battery systems can be found in TM 11-498.
There is another TM called "Handbook for Wiremen" which is about deployment and configuration of large field telephone networks as well as how to document networks, but I can't remember the TM number offhand.
I wouldn't be surprised if some poor Frenchman did it after the war, sadly.
--scott
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