Uxbridge, ON
I am trying to get some specific guidance on how to correctly select fuses. Excuse me if this is newbie stuff, but I am new to pinball maintenance. What I have developed so far is based on the work I have been doing with four Bally and three Gottlieb machines from the mid to late 70's. Pointing out any errors would be appreciated and might save a few machines.
A fuse is designed to be the weakest link in a circuit. When the load on a circuit increases, the fuse is designed to blow before damage is done to a more important, possibly irreplaceable, component like an IC, transistor, transformer, solenoid or relay.
I always start by reading the documentation for the machine. I have come to rely on the schematic drawings as the primary source of info. Additional information in the user manual and other documents is used only to confirm details on the drawings. Typically the drawing will specify the required amps. Based on the circuit it is usually easy to figure out the voltage. Sometimes there will be a notation if a fuse should be slow blow or fast blow.
Selection of a replacement fuse should be based on,
1. Physical size and package - Choose fuses that are the same size and package. In all cases the fuses I have replaced are glass tube 1/4 x 1-1/4" (6.3 x 32 mm). These are AGC type fuses ("All Glass" with size C being 1/4 x 1-1/4"). I suspect newer or older machines might use different sizes or packages.
2. Amps - the replacement fuse should equal the specified amps. In a pinch a slightly lower rated fuse could be used, but the greater the difference the more likely it will blow. i.e. putting a 7 amp fuse in where a 7.5 amp is specified won't damage the machine, but the fuse is more likely to blow. Never use a fuse rated for higher amps. This will make other components in the circuit the weakest link and cause them to burn and not the fuse.
3. Voltage - the replacement fuse should be rated for a voltage equal to or greater than specified. i.e. using a fuse rated for 250V into a circuit carrying 6V is acceptable. But putting a fuse rated for 32V in a circuit carrying 60V is not good. I am not sure what would happen, but I guess the fuse would blow and the circuit would be safe.
4. Fast vs Slow - the replacement fuse should match what is specified. Fast blow (fast acting) fuses are used in purely resistive circuits with no or few load spikes like circuits where IC and other sensitive components need to be protected. Fast acting fuses are suppose to be marked with an "F". Slow blow (time delayed) fuses are used in capacitive circuits where surges occur upon power-on/off and power input/output like circuits with solenoids, relays and transformers. Time delayed fuses are suppose to be marked with a "T".
This is the limit of my knowledge.
What should I select when fast or slow blow is not specified? In several instances the drawings don't indicate fast or slow blow and fuses aren't marked with an "F" or "T". Is there such a thing a a "normal" blow fuse whose time delay falls somewhere between a fast and slow blow fuse? Or is this a users choice and I can select either? Or is it whatever I have on hand? Or should I choose a fast blow fuse to be safe and see what happens?
Some circuits it seems obvious they would benefit from a fast blow fuse, like a circuit including the MPU board. Other circuits containing a drop-target or flipper solenoids would probably benefit from a slow blow fuse. What about a circuit for general illumination? I would think load should be pretty constant, so I would guess a fast blow fuse should work well. What about controlled lighting? Several lights turning on and off would change the load, but the total load could be pretty small. Fast or slow blow? Or is load on any circuit in a pinball machine so varied that I should always select a slow blow fuse? Some group wisdom would be appreciated.
Seattle, WA
