1A2 Diode Ringing

The ringer in 1A2 phones can be 'programmed' to ring on different lines by using diodes to prevent reverse electrical paths to ring unwanted phones. The reason for this is there's only one ring generator and one bell in each phone to manage ringing for multiple lines. Diodes allow programming each phone to ring only for specific lines.

Typically diodes are punched down either on boards specifically designed for this purpose, or punched directly onto 66 blocks. 1N4005's seem to work fine for this purpose.

Things to keep in mind:

• Although ringing is an AC signal, in 1A2 it's best to think of ringing as polarity sensitive.
  Especially in the presence of a diode network. The bells in 1A2 phones seem to be polarity sensitive, which is to say they only ring on one side of the ring generator's AC wave, (whether the diodes are present or not).

  There's a red + black wire on the bells for a reason; red being "positive" and black being "negative". So when arranging the diodes and checking the wiring, be sure to take this polarity into account. Signals R1/RR/S-Y/Pin#20 are "positive", and signals B1/RT/Y-S/Pin#45 are negative.

• For any diode ringing to work, you have to bypass the capacitor in ALL 1a2 phones.
  Normally the capacitor is wired in series with the bell by way of the "K" and "A" terminals on the network hybrid in the phone. Put simply, the default wiring is:

                  R1[20] --- S-Y --->(A)   (K)----> BELL(RED)
                  B1[45] --- Y-S ------------------ BELL(BLK)
               

  The (K) and (A) are screws on the network hybrid, and internally there's a capacitor between them.

  So for diode ringing, as Ed on Sundance Communications put it, (paraphrasing) you need to move bell's wire that normally is connected to (K) to join the wire on the (A) screw, the end result being:

  
                  R1[20] --- S-Y --->(A)----------> BELL(RED)
                  B1[45] --- Y-S ------------------ BELL(BLK)
               

  This leaves the (K) screw unconnected, and two wires connected to the (A) screw. (It doesn't actually matter /which/ screw has both wires, K or A, as long as both wires are on one screw, and the other screw is unconnected)

The following diagram shows how diode controlled ringing is typically implemented, with all phones shown with the K -> A wiring change. The idea here is to remove a diode to disable ringing for a particular line/extension:



The important bits to take away from that diagram being:

• The placement of the diodes between the Ring Generator and the phone
• The diode symbols all "point" toward the phone's "red wire"
• The bell's wire skipping over the "K" terminal on the network, joining the S-Y wire on the A screw
• The common wiring for the *lines* being on the *positive* side of the diodes (left side of diodes in this diagram)
• The common wiring for the *phones* being on the *negative* side of the diodes (right side of diodes in this diagram)
• The R1/RR/pin20/S-Y wire must go to the *red* wire on the phone's bell
• The B1/RT/pin45/Y-S wire must go to the *black* wire on the phone's bell

To ring any line/phone combo, a diode is needed for *each* line/phone combo, to prevent unwanted reverse electrical paths. To have a particular phone /not/ ring for a particular line, simply leave out the diode for that line/phone combo, leaving an open circuit in its place.

This has been working for me for the various eight 1a2 phones I've tested with, as I've had to modify all my phones for the A/K capacitor bypass and ensure red/black bell wires for programmed ringing to work.

Making the "K->A wiring change" was apparently standard Bell System operating procedure for any 1A2 installations where diode ring programming was employed.

Problems

• If the bell wire colors are reversed, the bell may hum or barely ring, if it makes any noise at all.
• If the diodes are reversed, the bells will also not ring correctly, as the wrong side of the AC wave will be passed to the polarity-sensitive bells, giving similar non-ringing behavior. So it's important to get the wire colors and polarity correct, even though you'd think ringing would be an AC circuit not sensitive to polarity (not the case!).
• If the capacitor isn't bypassed on all phones (the A/K wire change), the bells won't ring at all.
• Although RB/RG can be switched, once you get past the diodes, R1/RR (the S-Y wire) must go to the red wire on the bells, and the B1/RT (Y-S) wire must go to the black wire on the bells.