Today I had an electronic gate brought to me that stopped working. Here’s the thirty minute way to figure and fix…
- The “customer” came in and asked me if I could fix a circuit board. I said, “I can definitely try, but I can’t make any promises.”
- I asked him what happened the day it quit working. He said. “I hooked the power up backwards, and the fuse blew. I put a new fuse in, and it blew, too. I imagine something else is blown.”
- Based on his testimony, I assume that something has caused a short across the positive and ground. I tested the circuit without power attached, and there was zero ohms between red and black.
- I began my search across diodes as my first resort checking both forward and backward bias. Backward bias should not beep with the DMM. I came to the last two diodes and noticed that one had a huge crack running from one corner to the other. I checked it, and the DMM went coo coo. There’s my problem.
Now, the fix would have been a week in the waiting. The diodes were SMTs. I checked the datasheet for the diodes, and I happened to have some through-hole diodes from the Shack in my parts bin. They matched the specs of the SMT diodes with one exception. The reverse voltage for my replacement was 200 higher than the original, but this shouldn’t be a problem since it’s only a 24 volt system. I removed the diode, and I installed the replacement. I checked the resistance across the power rails, and it was 200 ohms. Power was reapplied, and the unit worked like new.
Moral of the story: Do an investigation to try and determine what they, the customer, might have done to cause the fault. If your friend/customer tells the truth, fixing it should be a breeze. All together I spent 30 minutes on this repair, and that includes reading a datasheet and driving home twice; once for my DMM and once for the part and other tools.