By Russ Jensen

Russ Jensen begins Part 7 of his Pinball Troubleshooting series this month and will conclude Part 7 in the October edition of The Coin Slot.

In Part 6 of this series the highly technical subjects of switching circuits and Ohm's Law circuit theory were discussed, and the application of circuit theory to a basic game circuit was described. We are now ready to describe some typical circuit configurations used in games, and how they operate.


Often in games it is required to have a series of playfield targets (bumpers, targets, rebounds, etc.) that each perform two functions when struck by a ball in play. Typically, one function would be to add points to the score ( a momentary function). The second function would often be to light a light, such as one of a series of numbers or letters on the backglass or in lighted bumpers (a sustained function). To implement this type of dual function game designers often resorted to the use of what were typically called "series" or intermediate relays.

Figure 5A illustrates a typical circuit in which three bumpers (although any number could be used), labeled #1, #2, and #3, are shown. Each bumper has associated with it a bumper relay that would be on a relay bank, as described in the previous article on relays. An additional relay (labeled series relay) is of the simple type and is wired in "series" with a common connection to one side of every one of the bumper relays.

When any one of the bumpers is struck by a ball in play its switch is closed and a circuit is completed so current can flow through its corresponding bumper relay and through the series relay. The bumper relay is thus tripped, its switches providing a sustained function, since it will not be reset until the bank reset coil on the relay bank is subsequently energized by some other game function (such as the start of a new game or the completion of a series of events). The series relay will also be energized, (but only as long as the bumper switch remains closed) its switch(es) thus providing a momentary function, typically the scoring of points.

Two things should be noted about this type of circuit configuration. First, that subsequent hits of the same bumper (before the relay bank is subsequently reset) will cause current to flow in the same manner, however, only the operation of the series relay is significant since the bumper relay has already been tripped (current will, however, still flow through its coil although no mechanical action will result). Secondly, because the bumper relay coils and the series relay coils are operated in "series" with each other, these coils are designed to operate on lower voltages and cannot be interchanged with other coils in a game, which are designed to operate directly from the full coil voltage.

Finally, it should be pointed out that these circuits are designed to operate with only one bumper switch closed at any one time. If more that one is closed at the same time more current will flow through the series relay than it was designed for, and this could result in damage to that coil.


Probably the most common of all circuit configurations used in games is the so called "hold on" circuit used with relays. This idea has been used since the mid 1930s and is fundamental to game circuits. It must be thoroughly understood by anyone troubleshooting games. This type of circuit is basically used for one of two purposes, assurance or timing.

In the case of the assurance function, a relay whose operation is to cause an event to occur is "held on" until it receives feedback to indicate that the event has properly occurred. An example of this will be given shortly. In the case of the timing function,a relay is energized by one event and "held on" until a second event occurs. The relay's switches are thus activated for the period of time between these two events. An example of this will be shown later when typical complex game circuitry, to produce an entire game function, is described.

Figure 5B illustrates a typical example of a relay "hold on" circuit. It is one that provides the assurance function, yet its circuitry has the characteristics typical of all "hold on" circuits. The circuit in the example is the one commonly used in pingames having pop or thumper bumpers, which kick the ball away from them when struck. Before discussing how this particular application works, however, this illustration will be used to point out some typical characteristics of the relay "hold on" circuit.

You will note the relay coil, labeled bumper relay, has its top side connected (by a wire labeled jumper) to one of its own normally open switches (labeled bumper relay SW #1). This is the telltale sign of all "hold on" circuits; that is, that one side of a relay's coil is connected to one of its own normally open switches. I call this switch the relay's "hold on" switch. A relay wired in this configuration can usually be easily spotted by noting a short wire (jumper) connected from one of the relay's coil terminals to one of its switch terminals (usually the switch in the stack closest to the coil itself). Occasionally this connection may be made via a cable harness and is not as easy to spot.

It should next be noted that this "hold on" switch is wired in "series" with a normally closed switch (labeled pop bumper E.O.S in the example) to the other side of the coil power line. This is also typical of "hold on" circuits, in that the "hold on" switch is connected to coil power via a normally closed switch (or combination of switches that provide a normally closed function). This switch(es) I call the "drop out" switch. The entire circuit, consisting of the "hold on" switch and the "drop out switch, is referred to as the relay's" hold on" circuit

(NOTE: In some cases another normally open switch (or combination of switches) may be wired in "series" with the "hold on" and "drop off" switches. This would be used to open the "hold on" circuit under some condition when the game situation required the "hold on" function to be disabled).

Finally, note the other switch connected to the relay coil (labeled bumper switch in the examie). This is representative of the relay's energizing circuit, which is the switch (or combination of switches) that initially energizes the relay after which the "hold on" circuit keeps the relay energized for some period of time after the energizing circuit is opened. A discussion of how the particular circuit in the example in Figure 5B works should serve to illustrate how these circuits interact to produce the desired result.

When a ball in play strikes the bumper, the bumper switch is momentarily closed. This comletes the circuit, applying current to the bumper relay coil thus initially energizing the relay. The energizing of the relay then causes its two switches (bumper relay SW #1 and bumper relay SW #2) to close. Switch ^1 now provides current to the relay coil via the normally closed "drop out" switch (pop bumper E.O.S.) even if the bumper switch subsequently opens. The relay is now "held on."

The closing of bumper relay SW #2 applies current to the pop bumper coil, the energizing of which causes the metal ball kicker ring surrounding the bumper body to be pulled downward, repelling the ball from the bumper area The movement of this ring also causes the pop bumper E.O.S. (end of stroke) switch to open when the ring has been pulled all the way down, providing feedback indicating the pop bumper action has successfully occurred. The opening of this "drop out" switch removes power from the relay's "hold on" circuit and the bumper relay coil is de-energized. Both its associated switches then open, thus de-energizing the pop bumper coil as well. Everything is then back to its original condition until another ball strikes the bumper. (NOTE: The bumper relay typically has additional switches in its stack, which energize the other bumper functions (scoring of points, etc.).)

The circuit just described used the "hold on" circuitcto provide the assurance function previously mentioned. One characteristic of this type of "hold on" circuit is that if it is not operating (due to a defective"hold on" or "drop out" switch, etc.) it is sometimes difficult to notice, since the function it controls will still occur, but often not completely, In the above example, for instance, the pop bumper would still operate, but sometimes the ball kicker ring might not be pulled all the way down resulting in a weaker kick of the ball.

It is often useful in trouble-shooting a game to test a "hold on" circuit to be sure it is going to operate. Again using the example of Figure 5B, it can be seen that manually closing the "hold on" switch (bumper relay SW #1) should result in energizing its coil even without the closing of the energizing switch (bumper switch) provided that the "hold on" circuit is functioning properly. There are two easy ways to per- form this test. The first is to short the two blades of the relay's "hold on" switch (remember that is can usually be spotted by observing which switch has one of its contacts wired to one side of its coil) with a metallic object, such as a screwdriver blade or metal tip of a mechanical pencil. When this is done the relay should immediately energize. This tests all of the "hold on" circuit except the switch you have just shorted since you have bypassed its points with your shorting device.

The second method, which tests the complete circuit, is to manually move the relay armature slowly toward the coil. As soon as the "hold on" switch closes you should feel the magnetic pull of the coin on the armature as the relay energizes. If this is not felt, the "hold on" circuit is not operating. When the relay energizes let go of the armature, the relay should perform its function (operate the pop bumper in the caseof our example) and then "drop out." If it remains energized the "drop out" switch is not operating properly and should be checked. (NOTE: Sometimes in performing one of the above tests the relay will energize and remain so with nothing else happening. This means that the device that should be activated by the relay (such as the pop bumper in our example) is not operating at all, and thus never opening the "drop out" switch. If this occurs, that device, and the relay contact activating it, (bumper relay SW #2 in the example) should be checked.)

If either of the above tests are performed and indicate the "hold on" circuit is not operating at all, the "drop out" switch on the device activated by the relay (and any intervening quick disconnect connectors) should be checked until an open circuit is discovered.

The Zero Ohms Test, described in a previous article, is one way to isolate such a problem. Other useful test methods will be discussed in a future article.

(NOTE: The tests just described will not work if the "hold on" circuit contains a disabling switch(es) in "series" with it, as noted earlier. In these cases, that switch(es) must first be closed or shorted in some way prior to performing the tests.)

This concludes the specific discussion of the relay "hold on" circuit, however, other examples of such circuits (including one employing the timing function) will be illustrated when an example of circuitry performing a complex game function is discussed. The only major difference between "hold on" circuits providing the assurance function and the timing function is that the latter usually employ normally closed score motor switches as the "drop out" switch(es).

In October, Russ Jensen will continue his descriptions of the typical circuit configurations used in games and how they operate. He will discuss relays and their functions, and a complex game function to tie together what has been covered in the previous articles.


In the August Issue an error was made in Russ Jensen's article "Pinball Troubleshooting, Part 6 Continued." Some copy was inadvertently left out of the second paragraph under the heading "Voltage Drop," first column on page 46. This paragraph should have If, on the other hand, any or all of these components have unwanted resistance (denoted by the small "r's" in the boxes in Figure 4) they will produce a corresponding voltage drop (represented by the small "e's"). For example, if the rollover switch had a resistance "r(4)," it would have a corresponding voltage drop, "e(4)," across it, equal to the current, "I," multiplied by "r(4)." The same idea would hold true for any of the other circuit elements by multiplying their's" by "I" to get their respective voltage drops ("e's").

We apologize to Russ and our readers for this error and hope it has caused no inconvenience.

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