Advanced Telephones & Electronics

This is a list of experiments for those that are “advanced”, or at least think they are!

Will be revised or added on to as time marches on

Series A Activities

One – Measure the line current on your telephone line. Place the leads of your VOM across tip and ring and see what the short circuit current is, then read my section on high line current if the current is over 50 MA.

Two – Measure the AC ringing voltage of your phone line. Put an 0.47 MFD 200 VDC capacitor in series with the VOM and set the VOM on 200 VAC or more.  What is your ringing voltage? Try it without any phones connected, then will all phones connected, perhaps add another phone to measure it under load. I have shown an analog VOM (Volt Ohm Milliamp Meter), You can use a digital or this  analog one. Cost varies from $5 to $15 for a low cost one.

Three – Using a standard 500 or 2500 telephone set, unscrew the T1 transmitter. Have a friend call you, hold a conversation by yelling into the receiver on the handset. Then reverse roles and have your friend do the yelling. You will need to be outside or somewhere that you do not hear the yelling, except into the phone. You should also take two U1 Receiver elements (they have screws on the back and are easy to use) and attach about 100 feet or so of two conductor wire, with one person outside and one person inside, with closed doors, talk into the receivers and you will be surprised how well they will work. Also have the person outside cup their hands to form a funnel (or use something like a traffic cone with the receiver at the small end) and listen to the street and outside noises. These also make good voice microphones, since the frequency response is only in the voice range.

Four – Dial a standard 500 or 2500 telephone by using the hook switch. Remember each time you press it, that is a digit, press it three times and you have dialed a 3. Keep in mind you must enter 10 digits in 1 second as the speed of a rotary dial is 10 pulses per second. This will take some practice, wait too long or go too slow and who knows where you will end up.

Five – Build the 1 mile of cable from HERE. Add this in series with your telephone, notice any difference? Repeat activity One on measuring line current, what changed? Depending on your distance from the central office, keep adding a mile of cable and see when you get so far away from the central office, what happens? Get too far and you will not get dial tone. High line current can cause a problem click HERE to read information on that. Look at http://sandman.com to find information line current regulators.  The picture shown is a simple device to reduce high line current.

Six – If you have two 500 or 2500 telephone sets, exchange one of the ringer gongs, make one set go ding-ding and the other set go dong-dong when it rings.

Seven – Booster battery was sometimes used on long telephone lines, with a repeater and additional battery, lines could be very long. Using a 9 volt batter or two 9 volt batteries for 18 volts, add this battery in series with a 500 or 2500 telephone. The telephone line voltage will be either 48+9 or 48+18. The 500 and 2500 telephones have an automatic self compensating network for line current that is from low to high. Do you notice any difference when you use the additional battery power?   The Picture shows a commercially available product for doing this. Check out http://www.vikingelectronics.com they have great products.

Eight – If you have “sacrificial” telephone, perhaps one that is in really bad condition and it has a pc network (not the old potted one in goo, unless you do not mind a mess) remove the line varistor. You will need to study the schematic drawing for this, but usually it has a 200 Ohm resistor in series with it connected directly across the telephone line. Removing this line varistor removes the automatic self compensating portion of the network. Repeat experiment Seven, you should notice a significant increase in volume.

Nine – On a 500 telephone set, remove the set of contacts that short the receiver when you dial, note the loud clicks in the receiver (hold it away from your ear it is loud). Remove the varistor from the back of the U1 and again listen to the clicks, they will be much louder.

Ten – On a 500 telephone set adjust the C4A ringer for a very light spring tension. Remove the dial pulse filter, look at terminal F one side is connected to the 200 Ohm resistor on the varistor. Disconnect one end of this resistor. Now dial, the dial pulses will have a sharp spike the bell should tinkle, if not reverse the connections on the telephone line.