Fuses
If you’ve ever spent time browsing through an electronic distributor’s catalog you will know that there are a wide variety of fuses available in the real world. We will limit this discussion to those fuses found in the gaming industry. Emphasis will be on those found readily available to the Slot Tech, and lightly cover those the bench tech gets exposed to.
3AG body styles
These are the older style. They are about ¼” in diameter and 1 ¼” long, cylindrical, and are a glass or ceramic case with metal caps on the ends. Most of these we run into are all rated at 250 Volts. Most of these are clear glass cases and you can see the element with no problem. 3AG fuses rated for higher currents and higher temperature applications may have a white ceramic case. Voltage rating isn’t too descriptive. It is the current rating that counts, and whether it is a Fast Blow or Slow Blow.
A fuse is a one-time device. It consists of a section of wire selected so that a certain current passing through the wire melts the wire and opens the circuit. Fuses visually consisting of a simple straight wire are designed to blow out fast. Fast Blow fuses are used to protect logic circuits where an increase of current indicates a malfunction of some kind.
Circuits like fluorescent lights and AC motors normally draw an excessive current when they are first starting up, then drop down to a lower level in normal operation. To protect these kind of circuits we need a fuse that will withstand an overload for a limited time. To accomplish this we build a contraption into the fuse to dissipate the heat in the wire for a limited time. We want the wire inside the fuse to withstand two times the normal current for a period of a few seconds. These are called Slow Blow fuses.
The part number usually indicates the type of fuse it is (Fast Blow or Slow Blow) and the current rating.
“AGC” in the part number indicates
that this is a Fast Blow Fuse. “AGC1” is a one Amp rating. “AGC2” is a two Amp
rating, and so on.
“ABC” is a Fast Fuse in a white ceramic body.
“MDL” indicates a Slow Blow type. “MDL1” for one Amp. “MDL2” for two Amp, and so on. “MDA” is a Slow Blow fuse in a white ceramic body.
Other manufacturers use other part numbering systems. What primarily counts is whether it is a Fast Blow or Slow Blow and the current rating. Current rating is usually stamped on one of the end caps.
5 mm x 20 mm body style
Slightly newer in body style is the smaller cylindrical shaped fuses. They also come in clear glass or white ceramic, and as fast blow or slow blow. Typically rated at 250 V. The same things are true. The things that primarily count are Amp rating and Slow Blow or Fast Blow.
“GDA” is a 5mm x 20mm case, ceramic body, fast blow type.
“GDB” or “GMA” is a 5mm x 20 mm case, glass body, fast blow type.
“GDC” or “GMC” is a 5mm x 20mm case, glass body, slow blow type.
The picture to the left gives you a good idea of the relative size of these two types of fuses. They can’t be mistaken for one another.
Variations
Most of the fuses we run into are socketed. Some (Williams Power Distribution Unit, for example) may be soldered to the board by leads coming out of the end caps.
Failures
Fuses don’t often blow out on their own. A blown fuse is an indication of some other problem. All games should have a log where all work done on the game is kept track of. When you replace a blown fuse check the log and see if this problem has a history. If the fuse was previously replaced you still have a problem to find.
You can
look at a blown fuse and tell if it was overstressed greatly, or just slowly.
If there is a major short in the circuit the fuse will blow violently leaving a
mess inside the fuse. If the stress is marginal over a long period of time only
a small section of the fuse may be blown open. On Slow Blow fuses the open
may not even be obvious on a quick observation. Fuse testers (a battery with a
light) or a meter is the best way to test Slow Blow fuses.
Normally you should always replace a bad fuse with the type suggested by the manufacturer. If this game has been worked on by somebody before you, don’t assume the fuse that blew out a second time was the right kind.
Troubleshooting
If you have a game that is blowing fuses, start troubleshooting by identifying what devices are on the circuit the fuse is designed to protect. Remove as many of these devices as possible and install a new fuse. If the fuse holds up reinstall these devices one at a time until you can get the problem down to one assembly that is causing the fuse to blow out. Always power down before installing or removing assemblies. Yes, sometimes you can get away with installing or removing assemblies without powering down. Our customers come here to gamble, not us. Be mindful of the fact that the game you are playing with will likely generated hundreds of thousands of dollars of revenue a year. That is likely to be more than you are making. Don’t down games by taking chances.
Once you get
it down to an assembly the assembly can be brought to the bench for
troubleshooting. The same process is followed. Identify the components in the
circuit that the fuse is designed to protect. Check all likely components
first. Electrolytic capacitors and power transistors are a good place to start.
Once bad parts are identified and replaced we are ready to try the assembly
again. First attempts should be done under as safe a condition as possible. If
the assembly runs off of DC, run it from a current limited power source and
monitor voltage and current while powering it up. If it runs off of AC line
voltage, plug it into a circuit fused at just above what normal operating
current should be. I have a standard power distribution box that I have
replaced the circuit breaker with a fuse holder. I keep a wide selection of
fuses on hand and select a fuse appropriate to the device I am testing. If you
plug a suspected bad assembly straight into a live line you make available
kilowatts of power that can be dissipated before you can unplug it. 120 volts
with a 20 Amp circuit breaker makes 2400 watts that can cause smoke and flames
before shutting down. Don’t plug suspected bad assemblies into a main
outlet.
Most of my test fixtures have voltage regulators with current limiting built in on their power supplies. The current limit is set to just above what normal conditions should be for that device.
If you have removed all possible assemblies and are still blowing fuses, you have a serious wiring problem. If the problem is not an obvious burned or pinched wire, start disconnecting all wiring in that circuit until you get the problem down to a specific cable.
Other types of fuses
I said we would quickly mention some other types of fuses. These are typically found on the circuit boards as fuses to protect the board itself or some specific circuit.
The
PC-Tron fuse is popularly found on the small boards added to the IGT DC
Ballasts for fluorescent lights. When these Ballasts first came out they were
prone to fail and burn up wiring and power supplies. This fuse assembly was
designed as a temporary measure until the new design came out.
The newer designs use an internal fuse, called a Poly Fuse (Polymeric). These poly fuses are designed so that a certain current level would cause them to overheat and pop into a high resistance state. When they cooled back down they would pop back into a low resistance state. These are functionally a self-resetting circuit breaker that is very slow in operation. They only pop into the high resistance state after being over stressed over a period of minutes.
When troubleshooting circuits that use poly fuses be mindful of the fact that the assembly may fail at normal temperatures, but appear to work fine after they have been removed from the game and left for you to troubleshoot. When testing DC Ballast assemblies that use poly fuses let the assembly reach normal operating temperature and monitor current before calling them good.
Polyfuses come in a variety of body styles. Leaded versions may look somewhat like a ceramic capacitor. Surface mount devices also come in various case styles.
Some circuits may use a low value resistor as a fuse. When in doubt consult the schematic.