Engineering Radio

This does not have much to do with Broadcast Engineering.

Recently, I saw a video about an engineer in Vermont who was fixing and placing single slot pay phones at welcome centers around the state. These are functioning as courtesy phones for travelers to use, free of charge. That is an interesting project.

My father worked for New York Telephone Company in various capacities from 1954 until he retired in 1988. We had a number of old telephone sets around the house which were removed from homes, old farms, businesses, and the like. One such telephone was a three slot pay phone from The Hitchcock estate in Millbrook, NY. This was a very unique item, as it dated from the Timothy Leary period of residence. It had white, blue, pink, and purple swirls painted all over it. At some point, G. Gordon Liddy, who was the then assistant District Attorney for Dutchess County, rented a Cessna and flew over the house to obtain a search warrant. After Leary was arrested, the group of LSD experimenters living in the estate house soon dissipated. My father went in and removed the pay phone. He said that those folks were living in squalor, “like filthy animals.” In fairness to my father, his Korean War experience while serving in the Marine Corps may have slightly colored his opinions about such things.

We called it the “hippie phone.” I believe it was a Western Electric model. After I moved out of the house I grew up in, all of those things disappeared.

I would have liked to have kept that pay phone.

All of that got me thinking about pay phones. Could there possibly be a similar pay phone for sale on eBay? Yes, there could. But they all seemed terribly expensive. Except for one reasonably priced unit, which I purchased. This is an Automatic Electric pay station, which is almost identical looking to the Western Electric unit from the same era.

When it arrived, I discovered the reason why it was less expensive; it is missing all of the internal parts. I was a little irritated by this, but soon discovered that it was typical. The phone company often removed the coin collecting relay and vault door as soon as the phone was taken out of service. The hook switch, induction coil, condenser, terminal board, and wiring harness were all removed, probably to be used to restore another phone.

Fortunately the coin slot and coin validating mechanism are still in place. If you drop a dime you will hear a nice “ting ting.” A nickle will result in a single “ting,” a quarter will make a satisfying “bong” sound from the gong.

I found a manual on line for the Automatic Electric pay phones which had a wiring diagram with the wire colors.

Unfortunately, there are no part numbers. I had a difficult time trying to figure out the correct parts to search for. I finally stumbled upon a Youtube video where someone was tearing down one of these units. That revealed the OEM part numbers for the coil (D284476A) and condenser (D68850A). With that, I was able to find the original manufacture’s assembly which included the hook switch, induction coil, wiring harness and terminal strip for a reasonable price on The Old Phone Shop website. I also ordered the condenser and a ringer (these phone had no internal ringers as manufactured). There is an option to buy an entire modern network board, but I wanted to keep this as original as possible.

The wire harness from the rotary dial was also missing. I used some wire from Belden 9423 cable to make those connections, keeping the wire colors shown in the schematic.

Next, I purchase a Pulse Dial to Touch Tone Converter. On POTS lines, most CO switches still recognize pulse dialing. However, POTS lines are expensive and I do not want to pay for one at my house. The answer is an Analog Telephone Adapter (ATA) with this converter. I purchased a Cisco/Linksys 2102 unlocked ATA on eBay for $16.00. Lowest cost DID number I could find is about $8 – 9 per month. There are directions on how to DIY a pulse dial to touch tone converter, but I simply purchased one on Amazon.

A bit about Plain Old Telephone Service (POTS) lines. It is a circuit switched data system. These systems are very simple and consist of a two wire circuit between the Company Office (CO) and subscriber Point of Presence (POP) known as a local loop or subscriber loop. The circuit is made by closing the Hook Switch. When the circuit is open, there is nominally -48 VDC across the Tip and Ring. When the Hook Switch closes (goes off hook), that drops to about 7-9 VDC. When the circuit closes, the CO switch detects a current flow and puts a Dial Tone consisting of two tones, 350 Hz plus 440 Hz on the loop. The subscriber can then dial a number and be connected to another party through the switch and what ever local or long distance carriers are needed to complete the call. On the other end, the CO switch will put a 20 Hz 90 VAC ring voltage down the line to ring the receiving subscriber’s phone. If the distant station is already off the hook, a busy signal is sent to the call originator, consisting of 480 and 640 Hz tones.

Internal to every POTS phone is a network consisting of an inductive coil and a capacitor or two and a resistor or two. The purpose of this is for line equalization and to create a two wire to four wire hybrid. The four wire hybrid is what feeds audio to the ear piece and receives audio from the handset microphone. It cancels echo from the distant station and provides a small amount to “side tone” or audio from the microphone so the user can hear themselves when they talk. The audio pass band is from about 300 Hz to 3,300 Hz.

In addition to that, some type of dial mechanism is required; either pulse or DTMF (Dual Tone Multiple Frequency). Pulse dial interrupts the closed Hook Switch in rapid succession (39 ms on, 61 ms off) to send a number to the switch at the CO.

DTMF consists of the following tone pairs:

Number	Tones (Hz)	Number	Tones (Hz)	Number	Tones (Hz)
1	697, 1209	2	697, 1336	3	697, 1447
4	770, 1209	5	770, 1336	6	770, 1447
7	852, 1209	8	852, 1336	9	852, 1447
*	941, 1209	0	941, 1336	#	941, 1447

Provided the local loop stays intact, the POT system is extremely reliable.

The Analog Telephone Adapter (ATA) converts a POTS from a circuit switched system to a packet switched system, then uses the public internet to complete a call. ATAs do not accept pulse dialing, hence the Pulse Dial to Touch Tone Converter. Where ATAs often fail is the ring voltage and tip/ring polarity.

Back to the task at hand: When the parts arrived, they all matched the mounting holes in the back plate of the phone. That is a good sign. I took the rotary dial off and put the new wires on. I also took the time to de-gunk and clean the mechanical parts of the rotary dial, then re-oiled it. It works as well as the day it was manufactured. The old mechanical phone system parts were built to last a long, long time.

The Hook Switch, Terminal Board, harness and condenser all fit into the phone almost perfectly. The condenser was hitting against the coin validator which didn’t seem quite right. I moved the condenser down to where the coin relay was located so it didn’t get scratched up.

I also bought a small ringer, which I installed where the coin relay was. I purchased a replacement vault door and lock. This is a plastic unit, not a OEM part, however, it does the job and I am not worried about anybody prying the vault door off and stealing all the change. Putting it all together, it looks great! Now I need to find a used phone booth to put it in. There are a few of those on eBay as well, but very pricey.

I took the phone to the office and plugged it into a POTS line and called my cellphone. The old carbon element microphone might need to be replaced, other than that, everything works great. It is now worth considerably more than I paid for it, which is also nice.