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 useThe 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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