The Base Unit

Let's take a look at the guts of the Computox base unit in a bit more detail.  The view from the back of unit is pretty impressive, with its three racks of relays.

Drof Computox base unit viewed from the back

View between the relays and front panel

If we look between the relays and the front panel we can see the power supplies and wiring loom sitting on the base of the case. In this first image the relays are to the right, and we can see a transformer at the bottom left, the metal rectifier that converts AC to DC in the middle left and peeking out from behind this are a second transformer and a bell.  The foreground transformer doesn't seem to be marked, but as we shall see, this is believed to drive the relay logic.

It's not really visible in the image, but there is a large ventilation hole in the bottom of the case where the  loom runs laterally across the unit.  This is hidden by a thin cloth (the light area visible under the loom), which presumably acts as a dust filter.

In the second image, we are looking in the opposite direction, so the relays are on the left.  At the bottom we see the bell and second transformer.  This is made by Varley and claims to output two 2VAC supplies.  More on what these are used for in a bit.  Behind the Varley we see the rectifier again and the primary transformer.  Below the rectifier, the tangle of wires leaving the loom are connecting to what look to be a couple of power distribution bus bars.  We can also see the back of the controller connectors here.  In the one closest it's possible to make out where the wooden block that retains the X controller is missing.

View of the power supply

Although they are not visible in these images, there are two microswitches, one in each corner of the front panel behind the transformers.  These reach through holes in the bottom of the case and exit next to the front feet.  They can then be pressed from below by reaching under the bottom of the case.  It's not known what purpose they serve.

 

Power Input

If we follow the mains input from the socket on the right of the main panel it runs to the large rotary switch in the centre of the panel (below left), and from there back to some terminals below the power socket (below right).

Power switch

Power contacts behind panel

 

These terminals connect to fuses, hidden beneath the right hand panel on the front of the base unit. Handily, the purpose and rating of each of the four fuses is detailed on the back of the door that closes the panel.

Fuses behind the right panel

Fuse labels on the back of the panel door

 

From this it's pretty clear that the 2V Varley transformer is used to run the lamps directly, with one output running the X and O lamps respectively.  The relays then run at 24V from the primary transformer.  This is a little strange, as my understanding was that telephone switches and uniselectors required 48V to switch, but maybe I'm wrong on that.

Connectors behind the left hand panel

For completeness, the left hand panel on the front of the unit hides a couple of connectors.  Currently we're not certain what they are used for, our best guess is that they are test points for diagnostic purposes.

Display Tech

Getting access to the back of relay frame is a bit tricky, owing to the tidy nature of the wiring harness.  It looks to be easiest to try to get some slack from the white wiring loom running from the top of the rack to the display connector.  Looking down from above, we can see the connector and locating pin in the recess that the display plugs into.

Display connector and locating pin viewed from above

Having loosened the connector, in order to get it to pass down into the main body, we have to remove a small circuit board that contain the three bulbs that drive the "winner" symbol.  This is clamped between two pieces of wood by a nut and comes out easily.

Winner light bulb assembly

The display connector can now be carefully threaded down through the body and free.  Having removed the bulbs and wiring it's possible to see the light guides that create the X and O winner symbols from the back.  This assembly is held in place with a similar wooden clamp to the bulbs that illuminate it.  Light bleed between the two symbols is prevented by a metal shield with a square cross-section.

Winner light guides viewed from the back

Presumably, a similar assembly is found in each square of the game display.  The clear path to the light guide also means both symbols are clearly visible on the front of the unit when lit from the back.

Winner light guides viewed from the front panel

Access to the Back of the Relay Rack

The purpose of removing the display connector was to create slack in the wiring loom to get access to the back of the racks of relays.  With this done, we can now undo the six bolts that hold the relay racks to the base of the case (getting these back in is definitely going to be a pain), and gently tilt it.  This gives a much better view of the relay connections.  The white wiring most of which heads up to the display connector is immediately visible.

Rear view of the relay rack

Some Lego bricks make an excellent impromptu prop to hold the rack at a reasonable, but stable angle whilst working on it.  This means the relay rack does not have to be rocked back and forth between measurements, reducing stress on the wiring and preventing the loom being trapped under the substantial weight of the frame.

Relay rack propped open

Next, the real work of evaluating the Computox's logic starts.