Author Topic: Automatic Electric Type 34A3A Monophone ('easy lift variant')  (Read 1300 times)


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Automatic Electric Type 34A3A Monophone ('easy lift variant')
« on: August 18, 2015, 10:50:05 AM »
Automatic Electric produced a version of the Type 34A3 desk telephone that they advertised as the easy lift variant, type 34A3A. This is supported by only a few sources, but is amply supported by the record of found examples.  The type was advertised in telephony periodicals ca. 1938, e.g. , as reported here and pictured below, and also was mentioned in a remark in a 1939 article in Automatic Electric Review, which introduced the new Type 40. It mentioned that the older models were still going to be available.

Found specimens of this variant have a modified cradle area with a space in the rear that permits insertion of two fingers to lift the telephone set.  These sets had the ordering numbers L250A0 and L253A0 stamped on the bottom plate of the dial versions. Manual versions with dial blank were labeled B0.

AE had to literally create the space for this finger hold; therefor they had to lay the ringer flat on the base plate.  Previously it was mounted at an angle towards the rear of the phone completely filling the space underneath the hookswitch.  The condensers, one or two, were place on the sides of the ringer, topped by a four-terminal screw connector plate.

The L-250 was for metallic (bridged) ringing service using a two-conductor mounting cord, while the L-253 variant was for grounded ringing with a three-conductor cord and a separate ringing condenser.

Presented here are circuit and wiring diagrams for these telephones as equipped with a dial.
The circuit diagram in the first exhibit here is drawn according to AE Form number D-53677 Issue 1. The insert in the upper right corner are the changes from Form D-53676 Issue 1 for the modifications for grounded ringing. Both forms are shown in the second and third exhibit, respectively.

The circuit of this phone is identical to the circuit of the standard Type 34A3, but it has additional connection points on the two terminal strips next to the ringer. The standard version only has screw terminals on top of the end blocks of the induction coil. The circuit presented here includes the extra connection points as numbered circles on the traces.

The circuit diagram is drawn in the on-hook state, so that only the ringer is connected to the telephone line which is connected to the L1 and L2 terminal points (in pink).  This is in contrast to the original wiring labels, which presented the switches in the off-hook position, in operating mode.

Circuit diagrams can be drawn in many layouts; this circuit diagram emphasizes the signaling and transmission circuit in a straight line down the center from line connection L1 to L2. To the left is the ringing bridge, and to the right is the handset and reception circuitry.

The hookswitches are labeled by HS followed by a serial number. The hookswitch has essentially two separate switches, a three-terminal switch, and a simple two-terminal switch, both close when operated (off-hook).  HS1a and HS1b are the components of three-terminal switch, which results in connection of all three contacts, and places the transmitter (TX) into the local loop when the dial is not operated.  When dialing, off-normal switch ON1 closes and shunts the transmitter so that the dial pulses from switch DP are sent out to the loop. DP opens the appropriate number of times for each digit dialed.  The pulses traverse the primary winding (P) of the induction coil, which has a DC resistance of 20 Ω.  When dialing, the receiver is also shunted by dial switch ON2, so that the user cannot hear the pulses.

The hookswitch (HS2) also connects the receiver to the secondary winding (S, 30 Ω) of the induction coil. The primary and secondary windings function like a traditional booster circuit.

The anti-sidetone feature of the circuit is implemented by a tertiary winding (T) added to the induction coil. It is wound from wire with inherently higher resistivity, resulting in a total DC resistance of 220 Ω. This resistance functions as the balancing network and is drawn explicitly in my diagram as a resistor (--\/\/\--, above the T winding).

In the version of this telephone type for metallic, line-bridged ringing, the condenser of the circuit is shared between the ringing circuit and the audio circuit. It has a capacitance of 1 F.  The grounded ringing variant, L-253-A0, uses a separate 1 F condenser for the ringing circuit. This isolates the audio circuit from ground.

This type of telephone could be equipped with either a straight-line ringer (as indicated in the diagrams), or with a frequency-selective ringer, in which case capacitor C1 would be chosen appropriately for each frequency of ringing current.

Discussions of observations of these AE34 types may be found on this forum:
« Last Edit: September 16, 2015, 10:59:16 AM by unbeldi »