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Shorted FETs In Dental Device Repair

By on July 16, 2021
dental equipment repair

This device has visited my shop a while ago with water damage issue. Today the same machine has come back dead but not sure what the cause; let us find out.

uniko mariotti dental equipment repair

When powering up the device there was no sign of life whatsoever and that was enough reason for me to open up the machine and check it out.

how to fix mariotti dental machine

While the machine connected to power, I traced the 220 source to the main transformer and I was getting that.

I also checked the main AC power going out of the transformer and I was getting 10 AC volts to the red and black wires as you can see in the photo.

mariotti dental circuit board fix

When I checked the power DC power going to the main capacitors I was getting nothing so the logical thing to trace backward to find the problem and sure enough I found it.

The fault was the two Mosfets that feed power to the main capacitors and when I checked the power there sure enough there was no voltage reading either. Part number: P40NF10L (I already shared the data sheet in the previous article).


Both were shorted out and needed replacement. I still have the same exact parts from the first repair that I did on this device.

how to fix mariotti machine

how to repair mariotti machine

I replaced the two shorted Mosfets, checked for any cold soldering but found none did some cleaning and I powered on. As it is seen in the photo, the device is up and running. The customer was happy to get his machine working again and I was too.

Mission Accomplished.

This article was prepared for you by Waleed Rishmawi, one of our ‘Master Authors’ and currently working in the Bethlehem area of Palestine repairing electrical and electronic equipment.

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Note: You can check his previous article on Shorted 2SK2837 FET In Welding Machine



  1. Albert van Bemmelen

    July 16, 2021 at 12:26 pm

    Let's hope that the dental firm doesn't make a habit of returning previously already repaired devices by causing new waterdamage!

    • Waleed Rishmawi

      July 16, 2021 at 4:28 pm

      Albert: I would love that because I will get paid for their carelessness. have a blessed day

  2. Parasuraman Subramanian

    July 16, 2021 at 3:41 pm

    Well done! Thank that guy who works as an agent to get you work (LOL)!

  3. Anwar Y Shiekh

    July 16, 2021 at 11:44 pm

    How does one make a living repairing? It is a sadly dying art.

    • Waleed Rishmawi

      July 17, 2021 at 11:56 am

      Anwar: my income is just fine. It is all about advertising and not just sitting in the shop waiting for customers. We used to have many repair shops but now we only have three left and soon the other two will close because they are not doing their job right. I am still going strong because my repairs not just TV sets but I expanded to different kind of fields like iPhones, welding machines, cameras and a lot more. Have a blessed day.


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