Two Shorted Diodes But Four Replaced
A super market owner across the street came running to my shop and asked for a 1.5 amp fuse. He explained that the electronic scale that they have at the shop went dead. He said he will replace the fuse and let me know. Soon enough I found out that the fuse he installed gone open again which confirms my fear that the power supply in that device was shorted and needed repair.
The scale is too big to fit in one shot so this is the display of the scale and the next photo is the rest of the scale.
It was not the best clean electronic scale that I ever worked on; it was dirty and needed a lot of cleaning. The repair is my job and the cleaning is the owner part.
So let us repair the power supply inside this device
It is a 24 volt power supply with 3 amps rating. I was not thinking of repairing it when I first opened the power supply. I thought of ordering a new power supply. The owner did not want to wait for the new part to come so he told me to go ahead and get it fixed.
If you look closely to the fuse, you would see or think it was intact but in fact when testing with the multi meter, the fuse was open. Since the fuse was open, the logic and common sense is to check first of all the bridge rectifier.
As expected, one of the diodes was reading a dead short and the other diode was giving me reading both ways. Four of the diodes of the bridge rectifier were replaced.
I saw this burnt mark on the metal holding the power supply board but discovered later on after testing the Mosfet that this burnt mark was old and other technician replaced the shorted Mosfet with a new one.
After checking all the parts on that power supply board, I determined to use the light bulb trick to find out if there is still shorted part that I may have missed while testing.
It was confirmed that the power supply board is free of any shorted component and this is the time to put the fuse back and get the machine working again
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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Jestine Yong works as an Electronics Repair Lecturer in Kuala Lumpur Malaysia and his training website is http://www.NoahtechElectronicsTraining.com. You can check out his repair blog at http://www.JestineYong.com
8 comments on “Two Shorted Diodes But Four Replaced”
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I can understand that the super market owner was in a hurry to get it fixed right away. But seeing that it needed some cleaning, and knowing that it was defect before, I’m afraid that the electronic scale will have to be replaced eventually.
Albert: Yes I understand but when the customer want it fixed against my recommendations, all it was left for me to do is to get is fixed. Yesterday I was in that super market and the scale is still up and running.have a blessed day.
Excellent! A quick professional solution, indeed! Hope you cleaned the heat zinc of the burnt marks, which the other technician did not! (I hope you agree that cleaning inside any device is our job!)
Parasuraman: yes, I always clean and make things look better but most of the time I do not take a photo of it due to heavy repair load at work. Have a blessed day
Not only should cleaning on the inside is our job, cleaning on the outside should be a task undertaken by us as well once we have worked on the machine. Returning a sparkling clean item to a customer advertises us as true professionals, not only taking of fixing things, but also taking care of the customer’s product. The customer will be highly pleased about care we have provided and will reward us for pur efforts.
I agree. Part of excellent service is returning the product to the customer in better condition than received. Not all customers will appreciate the effort, but a lot really do. First impression counts.
Andre: totally agree.
Devon: I usually clean on inside and outside but since the owner did not want to clean it on the outside because he said he wants to do it his way I let it out of my shop on this form.
here is a joke for you: I was working on a dirty VCR so I got it fixed and cleaned it on the inside and outside and when the owner came to collect his device, He insisted that this device was not his because it looked different. it was so dirty that his owner did not recognize it when it was clean. have a blessed day