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Small PCB From Power Supply Keeps Blowing 3A Fuse

Discussion in 'Troubleshooting and Repair' started by Sean G, Dec 3, 2014.

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  1. Sean G

    Sean G

    Aug 22, 2014
    Hi There,

    I need to say first that my electronic experience is very minimal!

    I have a PCB from an arcade machine power supply, the whole Power Supply had around 4 PCBs, but I believe this one takes 120V and outputs 5V (I measured the 120V, and the wire terminal has a small 5V sticker on it, so I assumed it's +5 -5 and two ground wires).

    Here is a video of the board (it came out better than the pictures did, view in HD mode)

    When I got the machine, the fuse was already blown and it was soldered onto the board. I added a fuse case and a new fuse and it blew, so I suspect something else is wrong! I searched online for a pcb diagram but I can't find anything like it at all!
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Sean and welcome to Electronics Point :)

    That's a great video you posted there. I can make out a lot of detail.

    This is a switching power supply, and switching power supplies are well-known for failing in a way that blows the input fuse. The failure is often caused by a power surge, or by dry joints.

    The first items to suspect are the bridge rectifier (in the red oval) and the switching device (behind the clip identified by the red arrow).

    bridge and switching device.jpg

    It's best to take the bridge out of the board. Then you can measure it separately, and you can also measure the switching device without removing it, and the bridge won't affect your measurements.

    Do you have a multimeter? And do you know how to use it?

    Can you remove the spring clip, lift the insulating plastic, and post the markings on the device that's buried in there?

    The components in that part of the circuit are live while the power supply is plugged in, and you need to be very careful when working on it live. You seem like a sensible person, and I'm happy to give you guidance because I think you will be able to work safely on it. That is, if you want to continue working on it.

    The big black cylindrical component at the bottom of the picture is the main electrolytic. This can store enough charge to give you a nasty shock. It's unlikely to be charged now, but before each time you work on the board, you should discharge it. You can use a piece of insulated wire across the two pins underneath, but this will make a bang and a spark if it's charged. Alternatively you can make a resistor discharging device that will discharge it safely.

    How do you feel about proceeding?
  3. duke37


    Jan 9, 2011
    Has the main electrolytic got a domed top. If so it is probably dead. You could take it out and see if the fuse blows.
  4. Sean G

    Sean G

    Aug 22, 2014
    Thank you for your comments! Really helpful and you put a lot of effort into it, greatly appreciated!

    I did forget to mention (and i'm sorry) that I already switched out the large capacitor as it did have a domed top, but that didn't fix the problem.

    The markings of the chip under the clip and plastic state:
    [M]9N5 (The N has a line underneath it)
    maybe this guy?

    I can use a multi-meter, but the main problem is always knowing which setting to use for each component. I'll usually Google each component, learn about it, and work it out from there. Usually it's not very obvious without some extra understanding though

    In the meantime, I'll desolder the bridge and work out how to test the switching device under the clip and plastic :)
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Right, the data sheet for the MIP0227SY is at

    What's the brand and model of your multimeter? Can you post a photo of it?

    You can check the bridge rectifier and the MIP0227SY using a resistance (Ω) range. If the meter has manual range selection, choose the highest resistance range - often 2000k, 2M, or 20M. Otherwise just set it to the resistance position.

    At this setting, your meter will measure resistance. In this case we're looking for either a short circuit or an open circuit. Touch the probes together to see what the meter shows for a short circuit - it will show 0, or a very low number. This means 0Ω which is a short circuit. Separate the probes to see what the meter shows for an open circuit. It will be an indication like "0L" or ". 1" - something that's not a proper number.

    Check the bridge rectifier by measuring resistance between the two AC terminals (probably the middle two, and often marked with squiggles, like tildes). If it's fried it will probably measure short circuit (0Ω or close to it). If it's OK, it will read open circuit.

    You can also measure the MIP0227SY's resistance between the middle pin and the right side pin (looking at the front of the package, with the leads pointing downwards). Try the probes both ways round. If you measure a short circuit, there's a short there, and it's likely to be the MIP0227SY's fault. If there's no problem there, you'll get a reading that starts at some number, and gets gradually higher until the meter displays open circuit. Each time you reverse the leads you'll get a reading, but it will eventually read open circuit.
  6. Sean G

    Sean G

    Aug 22, 2014

    Sorry it's been a while since I updated this thread. I have had to move house and handle everything that comes with that, so I've managed to finally get some free time between that and work to look into this again :) Sorry if it's a little late!

    I removed the bridge rectifier and tested it with the 2M setting on the multimeter. The reading began at roughly 3.00 and started to count towards an open circuit. I stopped testing when the multimeter hit 6.53 and took a picture. I placed the probe on the two tildes which were labelled on the chip itself.

    I then tested the MIP0227SY transistor while it was still in the board. The pictures shows the right two pins I used to test it.
    2M Setting on multimeter. The range went from 0.00 and slowed more around 3.49. When I flipped the probes, it gave the reading as negative and went up to 0 then back up the positive numbers again.

    I also took a video of the MIP0227SY test just in case I may have missed something!

    I was thinking if I couldn't fix this, maybe there would be a way to use a PC Power Supply to handle the job of this board? I think I mentioned in the original thread that it takes a normal US household voltage and outputs +5v -5v (Just what a small sticker said on the output wires) but I wasn't 100% sure!
    Last edited: Jan 19, 2015
  7. davenn

    davenn Moderator

    Sep 5, 2009
    Hi Sean

    your first 2 images didn't show up
  8. Sean G

    Sean G

    Aug 22, 2014
    Thank you, Google Drive privacy issues! Fixed now
    davenn likes this.
  9. Sean G

    Sean G

    Aug 22, 2014

    Sorry for delay in the update.

    I never managed to fix the power supply board, I have no idea what the issue was, I replaced a lot of components on that board and the fuse kept blowing!

    I did confirm that the board has a 120v input and outputs 5v, so I used a standard PC power supply and jumped the on-button pins so it auto-starts, and just connected the 5v line from the molex cable to the end of this complete power supply.

    So hurray it works! I'm not sure if anybody will ever have this same issue, but if they do, this was my very bad workaround.

    Thanks for the help
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