Western Electric 151C desk stand

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The 151C desk stand is a variation of the 151-type candlestick telephone. It was used with a 634-BC or 684-BC desk set box. This combination was used only on two-party lines with message rate service, which is indicated by the suffixes "C" and "BC", respectively.

For the message rate service type, the central office automatically sensed which of the two parties on the line originated a telephone call for billing by detecting the presence or absence of a resistance of 1000 ohms from the line to ground.  The TIP party had the resistance, while the RING station did not. For the TIP party the 151C telephone was used, while the RING party used a 151AL.

Like the more commonly familiar 151AL, the 151C telephone set is an anti-sidetone instrument. It was never newly manufactured in the factories, but was an upgrade during equipment refurbishing. Its sidetone predecessor was the 51C with a 554C subscriber set, and was available starting in ca. 1931 with the introduction of the improved telephone circuit for canceling sidetone.

This telephone type is essentially the candlestick-equivalent of the contemporary 203-type telephone, which used one of the desk set handset mountings B6 or D6.  The circuit is also almost identical to the 653-BC wall telephone for the same service type. The only notable difference is that in the 653-BC the dial shunted the entire audio circuit including the primary winding of the induction coil, while in the 151C, it only shunted the transmitter and the receiver circuit and kept the primary winding in the local loop during dialing. Thus, the 653-BC was more tolerant against signal path variations, such as high impedance and line capacitances.

With the arrival of the 300-series telephones, the 151C, the 203 telephone, and the 653-BC were displaced by the 304 desk set telephone (ca. 1938) and the 354 wall set (ca. 1947).

The 151C desk stand is connected with its subscriber set with a six-conductor (D6) mounting cord. The dial shown in the diagrams is a 2A dial, which has only four terminals, Y, BK, BB, and W, implementing the dial pulse switch between Y and BK, and a break-make switch of BB between BK, and W.

The circuit is drawn based on BSP C63.374 Issue 3 (6-28-45), from which the wiring diagram (Fig.3) is also presented, somewhat annotated in color.

Diagrams

• Circuit diagram drawn based on C63.374 (1945)
• Wiring diagram from C63.374 Issue 3, page 3.

Modification for modern use
The 151C/684BC combo can be modified to work on modern lines with bridged ringing:

• Move the black ringer wire from GND to L1.
• Move the red ringer wire from YY to RR.

This modification puts the ringer and the 1µF (1 M.F.) condenser in series and straight across the telephone line on L1 and L2 whether the set is on-hook or off-hook.
The impedance of the ringer is fairly low, with a DC resistance of 1000 ohm. It represents a REN load of up to 2.5.

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The 684BC (or 684-BC) subscriber is similar to the 684A subset, but has extra screw terminals labeled YY and RR, which replace the terminal K, which is used on the 684A for connecting the ringer to the ringing condenser.  In the 684BC, the ringer is disconnected from the condenser during a telephone call, and the ringer is switched between Earth ground and the TIP lead of the telephone line to identify the TIP station to the central office.

Pictured is a 684BC subscriber set of 1937:
Clearly visible is the type designation and the resistances of the two ringer coils, providing a combined DC resistance of 1000 ohms.  The ringer type is No. 78AA with No. 36A and 37A gongs. The two capacitors (2 µF and 1 µF) of the set are contained in the No. 194A condenser unit.  The induction coil in this set is a No. 101A anti-sidetone induction coil. Early sets, before ca. 1935, typically used a No. 146 induction coil.

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Most of the time when finding an old candlestick, it does not come with its requisite desk set box (subscriber set). Moreover, the 634-BC and 684-BC subscriber sets for the 151C desk stand are not as readily found anymore as the 634/684A.

Only a few collectors and some museums are interested anymore today to use a 151C for its original service type in message accounting on two-party lines, which requires the two line connections and the Earth-ground, in addition to a private switching system that provides the service.

It is easy to use a 151C candlestick with modern telephone service, however.  For this, a 634A or 684A can be used, but it is also possible to use the much later 685A.

Shown is the diagram when used with a 634A or 684A subset.

To make the conversion, essentially no internal changes in either the 151C desk stand, nor in the 634A desk set box are needed.  The 151C has a six-conductor mounting cord which has the standard green, red, yellow, and black wires of any 202/211 type telephone set.  They connect to the subset in the standard manner as all telephones that use the 634A or 684A.   The additional two wires, one double-red and one double-yellow coded conductor, are not needed, and should have their spade connectors insulated and stored inside the subscriber set.  The black ringer wire is connected to L1 instead of GND.
This is the only required modification.

As an alternative to insulating and storing the YY and RR conductors, they may both be connected to the L2 terminal in addition to the other connections on L2. This shorts the second line switch (W-B), which is really not needed.

As a result, it is obvious that one can also use a standard four-conductor (D4) mounting cord, in case the instrument had its mounting cord cut off and a replacement is needed.

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