Chime coils arrive

Gottlieb A-5195 coils and beersealThe parts I need to complete the chime unit arrived yesterday – all the way from Poughkeepsie in the USA. Strangely, they were wrapped in recent pages from the London Financial Times! Paid the $34.84 invoice by sending cash – which cost £17.76.

As you can see, the coils have removable sleeves in which the armature slides, so in the days when these machines were sited in arcades, cafes and pubs it would have been cheap and simple to replace them as they wore out. A home pinball machine has such light usage that these coil sleeves are never going to need replacing!

The coils are each fitted with a diode across the solder lugs which must be removed for fitting in an electromechanical (e/m) machine. I imagine that an electronic machine needs diodes to protect the solid state switches controlling the coils. Protect them from what? Well, when the current to a coil is switched off its magnetic flux, which is linked by the coil turns, collapses. This induces a transient of many hundreds of volts and this, plus the coil supply voltage, appears across the switch. A diode conducts in one direction only, allowing the transient to be shunted back through the coil and the current and magnetic flux to slowly and harmlessly decay.

I suspect that an e/m machine actually needs coils which de-energise rapidly, but in any case they are not sensitive to voltage spikes in the same way as electronic machines are.

The black strip you can see in the picture is a short length of self-adhesive ‘beerseal’ which is normally fitted under the lockdown bar at the front of a machine to prevent liquids spilt on the glass from getting inside the body. Its secondary use is as a cushion for the three strikers in the chime unit when they drop down by gravity after hitting the chime bars.

It should be simple to bolt the coils into place and attach the unit with four screws inside the body of my Big Brave. Then I simply have to solder the wires to the coil lugs – and my pinball machine will play music once more!

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