PINBALL TROUBLE- SHOOTING (PART 5)
By Russ Jensen
This month we will discuss the third and final type of component which makes up the basic electrical circuit, the 'switch' (often referred to as 'contacts' or 'points'). The switch provides the control of the operation of the circuit by turning on and off the flow of electric current from the 'power source' to the 'load.' Switch malfunctions probably account for 80 to 90 percent of the electrical problems occurring in games. Most of these troubles are caused by dirty or misadjusted switch contacts. Correction of these types of problems will be discussed on next month's issue.
Early electric games contained a small number of switches (some had only one) but as the complexity of electrical circuits in games increased the number of switches began to increase rapidly. Games of the sixties and seventies contained well over 100 sets of switch contacts each of which was capable of causing a problem
Basically a switch consists of two pieces of metal, each with an electric wire attached to it When these two pieces are touched together an electric current can flow between them, completing the electrical circuit to which the two wires are attached. The switches used in games generally consist of metal strips (called 'blades') with a solder terminal at one end (for connection of external circuit wiring) and small metal 'contacts' (often refered to as ‘points') embedded in the blade near the other end. Two of these blades generally make up a 'switch' although there are some exceptions which will be discussed shortly. The blades are separated from each other, at the terminal end by insulating 'spacers' (usually made of bakelite). Two or more of these switches may be mounted together, using additional spacers between them, to form a 'switch stack' An example of such a 'stack' (as used on a relay) is shown in Figure 2 A.
One of the blades of each switch is slightly longer than its 'mate' and is the one which is 'actuated (moved) to cause the switch to operate. In most relay applications (and sometimes on score motors) the tips of these actuator blades protrude through slots in a non-metalic ‘actuator' attached to the relay armature. When the relay is energized this actuator is moved causing the actuator blades to move with it (thus operating their respective switches). This is illustrated in Figures 2A (relay unenergized) and 2B (relay energized).
In other switch applications involving a 'stack' of switches, one of the actuator blades is moved by the device operating the switch (e.g. score motor 'cam follower, 'playfield' rollover wire,' etc). Attached to the opposite side of this first actuator is a round fiber 'spacer" which pushes against the next actuator blade in the stack causing it to move as well. Two or more switches can thus be stacked in this manner such that moving the first actuator blade will move all others in the 'stack' thus 'operating' all switches in that stack
Each switch has two positions or 'states.' The 'normal state' is when the device actuating the switch (relay, cam follower, playfield bumper, etc) is in its 'normal or 'at rest' condition. The 'operated state' is when the actuating device has been activated (e.g. relay energized, cam follower moved by a cam, bumper struck by a ball in play, etc.). It is important to understand these states when working with switch circuits as most of the terminology involved with switching circuits is connected with these concepts.
The two most common switch configurations each involve two contacting blades and are referred to as ‘normally closed' and 'normally open.' Referring to Figure 2A, the uppermost switch (of the three switch -stack' shown) is a 'normally closed' switch. The relay is shown in its unenergized state, therefore its associated switches are in their 'normal state.' The upper switch can be seen to be 'closed' because its two contact points are touching (allowing current to flow between them.) This I have illustrated in the drawing by placing small dots to the left of all switches which are closed. When the relay is actuated (as shown in Figure 2B) the switch is in its 'operated state' and the upper contacts are 'open' and thus no current can flow between them. This type of switch is called 'normally closed' because its contacts are 'closed" (touching each other) when the switch is in its normal state.
Just the opposite is true of the middle set of contacts shown in Figures 2A and 2B. They are 'normally open' since, as can easily be seen, they are 'open' when the switch is in its normal state' (relay unenergized) and 'closed' when it is in the 'operated state (relay energized). Thus for this type of switch, current can only flow when the switch is operated as opposed to the 'normally closed' switch where current flows when the switch is not operated.
The other common switch configuration used in games involves three switch blades and is referred to as a 'Single-Pole-Double Throw' switch. This is most often abbreviated as 'SPDT’ and often also called a 'Form C' switch. Incidentally, 'normally open' switches are also referred to as 'NO' or 'Form A' and 'normally closed' switches as 'NC' or 'Form B.' The SPDT switch is actually a 'normally closed' switch and a 'normally open' switch sharing a common switch blade. It can easily be seen by referring to the lower switch (the one with three blades) in Figures 2A and 2B, that the center blade is the 'common blade,' and the upper blade forms a ‘normally closed' switch with this common blade, and the lower blade forms a 'normally open' switch with it. The dots placed to the left of the solder terminal end of the switches in the figures illustrate that when this switch is in its 'normal state' (figure 2A) current flows between the 'common blade' and the 'normally closed' blade of the switch. When the switch is in its 'operated state' (Figure 2B) that circuit is opened (no current flows) but current now flows between the 'common blade' and the 'normally open' blade. This illustrate the action of SPDT switches and should be thoroughly understood.
Another type of switch sometimes found in games is what I call the 'Normally Open-Normally Open' (or 'NONO') switch. It has three blades, the contacts on all of which are normally open. When the switch is actuated the contacts on all three blades touch thus electrically connecting all three circuits connected to them. This configuration was used on relays and some playfield switches on older (mostly pre-war) games and occasionally on playfield targets on later machines.
The symbols used on schematic diagrams for the various types of switches discussed above were shown in Figure 1, which was with Part 1 of this series of articles. Schematics often used abbreviated terminology such as NO, SPDT, etc. mentioned earlier. Often the abbreviations 'OWE' (open when energized) and 'CWE' (closed when energized) were used next to switch symbols This was normally used when a switch was operated by a solenoid coil, 'energized' referring to the condition where that coil had current applied to it The terms 'OWI' (open when in) and "CWF (closed when in) were used to refer to switches operated by the movable 'shuffle panel.' found on older games, which moves 'in' when the coin chute was pushed in at the start of a new game.
At this point, it should be pointed out that although I have used switches on a relay (as shown in Figures 2A and 2B) to illustrate switch operation, the exact same ideas apply to switches operated by any other device (such as score motor cams, playfield scoring devices, stepping switches, etc.) when you consider the 'normar and 'operated' states of these devices to be equivalent to the 'unenergized' and 'energized' states of the relay respectively.
Switches in games provide three basic functions. The switches which are operated by playfield scoring devices (bumpers, rollovers, etc.) act as 'sensors;' they sense the occurance of some playfield event (such as a ball striking a bumper) and pass this information on to the game's internal circuitry. Switches on relays provide 'control functions; they pass on information regarding one event to control another. The third basic switch function is that of' feedback.' The 'end-of-stroke' switches connected with some solenoids are a good example of this as they provide information to indicate that an action (the pulling in of the solenoids plunger) has been properly accomplished. Other examples of this function would be the 'zero switches' on stepping switches and 'score reels' which indicate that these devices have been successfully 'reset' to zero.
The applications of switches on 'score motors' were covered in last month's article. Switches on relays were also discussed in a previous article. The functions of these switches vary, but usually provide 'control' of other circuitry as mentioned above. The discussion of an example of typical game circuitry in a future article will provide a better understanding of the functions performed by relay and score motor switches.
This discussion of switches will be continued in the June issue covering stepping switches, playfield switches and switch maintenance.
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