B 1304
Issue 1, Sept 1970

Unit, Clock Pulse No. 1

1. General
Unit, Clock Pulse No. 1 is used in conjunction with a master clock (Clock No. 36) to facilitate the distribution of clock pulses in all accommodation other than telephone exchanges. This Unit supersedes Clock Unit, GMT35. Installation details will be given in a revised issue of B 3301.

2. Description
The Unit, which is shown in Drawing PE/T1-1, contains one Strip, Clock Pulse No. 1 and has a space to allow for the provision of one further Pulse Strip. The Units are designed to work on 24V or 50V, and are wired in accordance with Drawing PE/T1-2. The case is aluminium, finished in polychromatic silver/grey stove enamel.

3. Pulse Strip
The Strip, Clock Pulse No. 1 is a solid state DC amplifier mounted on a printed circuit board. When an input pulse is received from the master clock, the output is a constant current 250 ± 25mA, provided that the total resistance of the wiring and clock coils does not exceed 90 ohms on a 24V system, or 190 ohms on a 50V system. This means that each loop will normally consist of up to 9 or 19 clocks, depending on the voltage. Because the Strip has a constant current output, however, it is not necessary to add balance resistors if less than the maximum number of clocks is used in a loop. The Strip should not suffer any damage if the output is short circuited. The fuse mounted on each Strip will only rupture if there is an internal fault on the board, in which case the Strip should be replaced. (See also B 5305).

4. Large Installations
When the number of pulse clocks to be installed exceeds the capacity of one unit, additional Units, Clock Pulse, No. 1 should be wired in parallel as shown in Drawing PE/T1-2. Additional Strips, Clock Pulse, No. 1 may be requisitioned to double the capacity of each Pulse Unit, and a few may be held as spares.

B 5305
Issue 1, Sept 1970

Unit, Clock Pulse No. 1 - Maintenance

1. Routine Maintenance
This unit requires no routine maintenance. If it is suspected that a unit is not functioning correctly, the fault procedure should be carried out in the order shown.

2. Fault Procedure
If the following tests show. that a Strip, Clock Pulse, No. 1 is faulty, it should be replaced with a new pulse Strip. Defective Strips should be scrapped.

  1. Check the fuse, if fitted, on the Strip, Clock Pulse. If the fuse has blown, the Strip is faulty.
  2. Measure the voltage across the busbars to ensure that power is reaching the Strip, Clock Pulse. The voltage should be approximately battery voltage. If it is not, the fault lies in the wiring, not in the pulse Unit.
  3. Disconnect the clock circuit and measure its resistance to ensure that it is within the maximum permissible figure - 190 ohms for a 50 volt system, 90 ohms for a 24 volt system.
  4. If (a), (b) and (c) are satisfactory, reconnect the wires with an ammeter added in the clock circuit in order to measure the clock circuit current. The drive pulse wire should be disconnected, and in its place a jumper wire should be connected from the -ve busbar to the input pulse terminal on the Strip. This causes the clock circuit to be continuously energised, so that the current can be measured accurately. If this current is not within the limits 250mA ± 25mA, the Strip is defective.

3. Replacement of Pulse Strip
The busbars in the Unit, Clock Pulse No. 1 may be ‘live’ when a faulty Strip, Clock Pulse, No. 1 is being replaced, or when an extra Strip is being fitted in the spare position in the pulse Unit. In such circumstances, care should be taken to ensure that, when positioning the Strip, the busbars are not short-circuited by the printed wiring on the back of the Strip.


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Last revised: January 03, 2022