A simple but effective tool for troubleshooting EM pinball machines is a bulb tester or test light. It was described in an article by Russ Jensen in Coin Slot Magazine in the early 1980s but it was a common tool for debugging electrical circuits long before then. Here's a page from the manual of a 1953 United Tahiti bingo game for example:
In its simplest form it is just a light bulb with two probes that can indicate whether there is voltage between two test points. While a multimeter, oscilloscope or other test equipment can certainly provide more information a bulb tester is often easier to use while providing all the information you need in many situations.
When troubleshooting a faulty circuit in an EM pinball machine the question is rarely whether sufficient voltage is reaching a misbehaving device, but whether any voltage it reaching it. Typically a failing device receives no voltage at all due to a dirty or maladjusted switch, or is mechanically or magnetically bound. In either case the actual voltage level is less important than the presence or absence of voltage so a light bulb is all that is really needed.
Also note that relay coils and solenoids in pinball machines often operate with short voltage pulses. Meters have a response time that can make it difficult to determine an accurate voltage value for a pulse and to display it long enough for you to comprehend it. The flash from a bulb on the other hand is a pretty clear indication that a pulse came through the bulb.
While there are certainly commercial solutions available it's pretty simple to cobble together a tester of your own, from spare parts if you're lucky. There are a couple of design considerations to keep in mind whether buying or building your own tester.
A simple 25 volt bulb tester is shown here using a pair automotive bulbs wired in series.
Automotive bulbs are rated at about 12 volts DC. A pair wired in series like these can handle 24 volts DC and works well as a 25 volt AC bulb tester. At 25V AC automotive bulbs may be slightly over their rated voltage but many are actually rated at slightly more than 12 volts DC. Your results may vary based on the bulb you use.
The case is just an empty plastic clam shell box often used to sell fasteners in big box stores.
The simple design above works well for Gottlieb and later Williams pinball machines that use 25 volts AC but cannot be used in Bally or early Williams games that use 50 volts AC. You could build two different testers for the different voltages or you could combine them into one.
This tester uses two strings of four #44 or #47 bulbs wired in series. #44 and #47 bulbs are commonly used for illumination in EM pinball machines and are rated for about 6.3 volts AC. A group of four wired in series can easily handle a 25 volt AC circuit. Adding a second string of four in series provides a set of bulbs that can run at 50 volts AC.
The toggle switch between the rows of bulbs switches the second row in or out of the circuit making the bulb tester suitable for both 25V and 50V AC circuits.
The banana plugs and jacks make the test leads removable. Not required but handy. The case was 3D printed to accommodate the assembled tester.
If you do much EM pinball repair work you may already have most of these parts on hand. Used sockets should work just fine and used bulbs may be plentiful since many collectors replace all the bulbs as part of a renovation.
You can use your bulb tester in almost any situation where you need to understand whether power is getting to part of the circuit. For example if a target on the playfield isn't awarding 50 points when it should you could check to see if the 10 point relay activates 5 times. The videos below show how you could do that in a 25 volt Gottlieb game. Note that the 10 point relay has been removed from its rack for clarity.
This might seem like a contrived example since you can see that the relay is working even without the bulb tester. But the bulb tester would be more useful if it were moved to the front of the game so you could see the results while activating targets or relays away from the back box. It would also clearly tell you whether or not the relay is getting pulses if the relay were mechanically bound and not moving for example.
Here is a similar test done on the 1000 point relay in a 50 volt Bally game:
Note that the tester uses all eight bulbs to accommodate the 50 volts.