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

 
 

These observations are simply those of an enthusiast and not an expert ! The information may be of interest to a collector who simply wants to get one running with one or two slave clocks. Any comments, corrections or additional info would be welcome and I might include it here.

There is much information on the internet about the 36 but here is a link to a particularly detailed site:
http://gpoclocksystems.byethost22.com/pages/PDF/Installation.pdf

The three most commonly available electrical master clocks in the UK are the Synchronome, The PO 36 and the Gents C7. In general the list is in order of the likely cost. The PO36 is interesting in that it use the Hipp toggle and is easy to understand and adjust. It is also still relatively affordable.


The PO 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 and have included it below. There is no one way to connect it up but here are some practical ideas.

The diagram below is that of the later MK/5 clock but they are all similar. I've coloured the wires to add clarity.

The first thing to notice is that the pendulum drive circuit is electrically completely separate 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 supply. In the exchange the available supply would have been about 48 or 50 volts with the positive side grounded. It is not neccessay to use such a high voltage voltage in a domestic installation where only one or two slave clocks are connected.

It is also not neccessary to make use of the 6 seconds count wheel (the lefthand one) or the seconds contacts at the top of the pendulum rod. Moving the operating arms inwards and letting the 6 seconds pushing pawl hang down, will relieve the pendulum of some intereference.

My recently aquired Mk4, with (synchroniser intact) will run for 35 seconds between drive impulses, run this way.

If you intend powering the clock from a small lead-acid battery on permanent float charge it will continue to run through power cuts. A small 6 volt or a 12 volt lead acid battery will normally give be about 6.7 volts or 13.8 volts while on charge. Any small "wall wart" type power supply with a series resistance setting the float current to 10 or 20 Ma should do.

If you are not bothered by the possibility of a mains failure, then no battery is needed. A small "wall wart" power supply that can deliver 500 Ma (0.5 amps) will be fine. The most commonly available units are USB chargers for phones, tablets etc and they deliver 5 volts DC.

The "pendulum drive" requires only 4 volts. With a 5 volt PSU a 2 ohm resistor connected in series with the feed to the "pendulum drive" connections should be OK.

However, two of my 36's came with 110 ohm resistors still in the case showing that the exchange battery supplied the impulsing current in PO service. There may be an advantage in using a higher voltage through a series resistance because a fixed voltage coming through a resistance approxamates to a constant current source. This might make for a smoother impulse.. One thing I'm sure of is that if your pendulum rod makes ANY NOISE when being impulsed ( a sort of "boy-ing" sound) the current is too high. Some old printer power supplies of 16v or 32v may be ideal power sources but of course the battery back up idea becomes more difficult.Perhaps 12 volts is a good compromise ?

To find the value of series resistance take the excess voltage and divide by 0.4.
Example: For a 12 volt supply: 12volts -4volts = 8volts. Then 8/0.4 = 20 ohms.

Connecting up.

First, bring in the battery supply to the terminals marked BATT - VE and + VE.

Next link the "pendulum drive" -VE to the "batt" -VE terminals.

Then, link the BATT +VE to the "pendulum drive" +VE terminal VIA. whatever resistance is neccessary for the supply voltage you are using. *Note that there are no internal clock connections to the Bat +ve terminal - it is simply a convenient terminal for whatever else you plan to do


It is not normally recommended to connect a slave dial dial direct to the 30 second contacts butI believe many owners have done so.

To use a relay take a feed of the +VE supply from BATT +VE to one terminal of 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 feature of the key switch will be by-passed.

To include the key switch bring the relay's return wire to CLOCKS terminal. (or the slave dial return if not using a relay )
Almost any small relay with a "normally open" contact set will do, but just as with the slave dial(s), you may need a series resistance if the coil voltage is lower than the suppy you've choosen.

If as I have done, the 1 sec contacts are made inoperative, the ADVANCE feature of the key switch will not be available. The "retard" position will however prevent 30 sec pulses reaching the slave dial...

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 side circuit.

R* A resistance will be required in series with the clock dial movement to ensure the correct current from the DC supply or battery you have used. The value will depend on the actual dial used (normally about 200Ma) and the voltage of the supply.

I hope this will prove useful to someone....last updated 18/10/2017

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