Connecting, connections to, the PO No.36 master clock in a domestic environment.


These observations are simply those of an enthusiast. Any comments, corrections or additional info would be welcome and may be included here.

The online wiring diagrams of the No 36 clock can be a bit daunting to the non-electrician. I had to make some sort of sense of them in order to put mine to work recently. There is no correct way to do it but this is the way I chose.

The diagram below is that of the later MK4/5 clock. Important wires are coloured to add clarity.

The first thing to notice is that the pendulum drive circuit is electrically seperate from the rest of the circuitry.

As I've already said there is more than one way to connect up the PO36 for operation from a single battery. In the exchange there would have been a 4 volt supply for the pendulum and everything else would have worked at 48 volts with the positive grounded. This is not neccessary in a domestic setting.

If you intend powering the clock from a small 6 volt lead-acid battery on permanent float charge, the voltage it provides will normally be about 6.7 volts. Very little current will be required and so a small mains powered supply (wall wart ) would suit but a battery will keep the clock going during power cuts.

First, bring in the battery supply to the terminals marked BATT - VE and + VE. Note that there is no other connection to the the +VE terminal at this stage.

First I linked the "pendulum drive" -VE to the "batt" -VE terminals. This sets the scene for "low side" switching which means that a +VE voltage is connected to one terminal of a "load" (the pilot dial for example) and the switching occurs in the -VE circuit.

Next, link the BATT +VE to the "pendulum drive" +VE terminal via four standard silicon diodes in series. 1N4001 or almost any power diode will suffice. A voltage drop of about 2.4 volts will occur across the diodes leaving 4 volts on the pendulum drive terminals. Three diodes would be ok if it was needed to increase the time between pendulum drive impulses a little.

More or fewer diodes, or some other arrangement may be needed if the power source you use has a different voltage. The whole clock can probably be run from 4 volts though, in which case no diodes would be needed.

It is not normally recommended to connect a slave dial dial direct to the 30 second contacts butI believe many owners have done so. However I used a relay as I had a suitable 6 volt relay to hand.

To use a relay take a feed of the +VE supply from BATT +VE to one terminal on the relay coil. If the other terminal is brought back to the terminal marked "EXCH EQPT" the relay will operate briefly every 30 seconds as required but the fast/normal/retard key switch will be by-passed.

To include the key switch bring the relay's return wire to CLOCKS terminal. (or the dial return if not using a relay )

However, If as I have done, the 1 sec contacts are made inoperative, the ADVANCE feature of the key switch will not be available. . Disabling those features will result in reduced interference with the pendulums natural swing. I also do not use the 6 sec count wheel.

The 1 sec "ADVANCE" -VE line switching could be implemented with a simple 555 timer circuit if the 1 sec contacts are disabled.

The relay's Normally Open relay contacts can be wired to operate the clock dial either in the -VE or + VE supply. The circuit above shows the switch in the + VE wire.

R* A resistance may be required in series with the clock dial movement to ensure the correct current from the 6Volt battery. The value will depend on the actual dial used.

I hope this will prove useful to someone....R