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12543Re: [midatlanticretro] Testing a TIP120 transistor

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  • Dan Roganti
    May 9, 2009
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      Joe Giliberti wrote:
      One of my pinball machines (that was built it 1977 and uses a Motorola 6800-based computer for scoring, solenoids, etc, which (I think) makes this post on topic) broke down earlier today. I have reason to suspect a bad solenoid driver transistor, which are TIP120s. There're about 20 of them on the board, and I don't want to replace them all. Is there a means of testing them? I have a DMM and scope available.


      Sounds like a vintage Bally or Williams pinball machine -- which game ? -- I restored a Bally Power Play (with Bobby Orr) for someone recently--it looks mint. You have to be more specific on the "broke down" statement. There's a lot of things that can go wrong in there. There a few steps to troubleshoot the solenoids, depending on what kind of problem you have -- it also helps to have the manual.

      Have you run the solenoid test yet ? You want to see what solenoids are still kicking during this test. Yo have to make sure that none of the solenoids didn't have a meltdown whenever a driver transistor shorts out. You would've smelled something by now if it had.

      You can then search which driver transistor corresponds to that solenoid which fails using an alligator jumper lead-- this is a simple check which will bypass the driver transistor -- connect one end to ground and then tap the other end on the metal tab(Collector pin) of each driver transistor to make the solenoid kick. This checks to see if the coil, connector, emf diode(on coil), and wire harness are all good. Once you find the driver transistor that kicks the solenoid that you're trying to fix, then you can get out your dmm to measure this.

      You can see the pinout for this in the datasheet here:
      http://www2.applegate.org/~ragooman/datasheets/Transistors/TIP120.pdf

      Basically, for a NPN transistor(this is done while in-circuit--power off), you put the pos.lead on the Base and measure the resistance on each the Collector and Emitter, you should get a high mega-ohm reading for a good transistor, then another measurement is done between the emitter and collector with the neg,lead on the emitter - you should get a low to mid  kilo-ohm reading for a good transistor Anything lower on any pin is a sign of damage. Sometimes, a dmm can have a transistor checker on them, but they tend to have a lot of limitations and don't fully checkout the component. I use the ohm scale mostly, because it also helps to find leaky transistors - it takes practice.

      hope this helps alittle,

      =Dan
      -- 
      [ = http://www2.applegate.org/~ragooman/   ]

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