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Western Electric 400K subscriber set with LB AST 202H hand telephone

Started by unbeldi, July 18, 2014, 06:56:41 PM

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The Western Electric 202H handset telephone with a 400K wooden desk set box was an anti-sidetone local-battery telephone for manual (non-dial) service.

The 202H desk set was either a B1 (circular base) or a D1 (oval base) handset mounting with an apparatus blank, an E1 (or F1, later) handset, and was connected to the subset with a D4 mounting cord.

Presented here are diagrams and a brief circuit discussion.

The circuit diagram explains the electrical principles of the telephone. The wiring diagram is from a Bell System Practice (exact reference lost) and was used to draw the circuit.

The schematic only shows the audio operation of the subset. As the wiring diagram shows, the generator as well as the ringer are simply directly connected across the line terminals L1 and L2, and are therefore omitted here from the circuit diagram. The generator has a switch in its contact assembly that connects the generator only when operated.

The induction coil in the 400K desk set box was a No. 113 coil originally, but many sets found may have been upgraded with the 104A closed-core version, which is functionally equivalent.

The No. 113 induction coil has four windings, one of which (terminals A–C, or S2–S3) is a non-inductive winding that is used as a resistance for loop length balancing in the anti-sidetone compensation.

The primary winding of inductions coils in telephone sets is usually designated as the winding that is contained in the transmitter circuit. In this local-battery circuit it is the BL–SL (or P–P) winding and has the lowest DC-resistance of only 1.4 ohms. The resistance is low to provide a sufficiently high transmitter (TX) current  in the talk circuit, powered by the local battery cells (typically 3V or 4.5V) This is the right-most part of the diagram. Hookswitch HS2 turns on the talk current when the phone is taken off-hook.

In contrast to most common-battery telephones, the local-battery set places the receiver (RX), and not the transmitter, in the local loop circuit in series with one (L1–RBK or S–S1) of the secondary windings of the induction coil. This is seen in the left-most part of the diagram.

The third winding of the induction coil (RBK–C, or S1–S2) functions for sidetone compensation. Its phase relationship is opposite to the secondary. The fourth winding (A–C, or S2–S3) is an approx. 300 ohm resistance to permit balancing the impedance of long local loops in conjunction with the 0.5 µF (M.F.) condenser.  The set should have a short wire strap on terminal A (or C). It was used to shunt, from A to C, this non-inductive fourth winding of the coil to permit a limited amount of adjustment. In later versions of the 104A part, the function of this fourth winding was implemented with an external carbon film resistor soldered between terminals A and C.

In principle no metallic interconnection is needed in this circuit between the transmitter and receiver parts. They operate independently. However, it is convenient to interconnect them at the BK terminal inside the subset, because this permits using a handset with only a three-conductor cord, instead of four conductors.

The standard handset until ca. 1937 was the E1 handset, but unit may have been upgraded with the F1 handset. Both used an H3 cord, but WECo also produced the E2 and F2 handsets which were equipped with a four-conductor cord. This was needed when the telephone set was powered by a shared battery supply, located centrally in an enterprise or large residence, for several telephones. The connection between the RX and TX circuits was severed in this case, such as in the 307–E, –F, –G, and –H sets, which have a diagram based on the 400-type as shown here, with modifications to accommodate common-battery signaling.

I also included in the last figure a comparison of the most prevalent LB induction coils, the sidetone version (No.13), and the two anti-sidetone models (113 and 104) with measurements of coil resistance.