I have a circuit board that doesn't boot up. After probing the VCC pin of the BOOT ROM, I found out it wasn't getting enough power on 3.3V, but only got 0.850V. I traced that pin back to the component responsible for providing 3.3V which is marked in the picture below:

enter image description here

I have 2 questions:

  1. What is the name of that component so I can replace it?
  2. You can see that component gets 3.5V as an input, and it outputs 0.850V. Is there any chance that some other component is the cause of this output, or its 100% certain that this component is responsible for this low voltage?

In the image below, the 3rd pin from the left of the white connector, is getting 3.5V straight from the power supply block, and then, that 3rd pin is connected directly to that component as an input giving it 3.5V as well.

enter image description here


After probing around, I came to the conclusion that that component should have the same voltage at both of its sides. So it would be +3.5V at both sides instead of 3.5V at one side and 3.3V at the other.

  • 1
    \$\begingroup\$ Vague question. What you think is a ROM is actually a microcontroller. Problem can actually be anything, a bad power supply, a bad cap, a bad mosfet, anything. The transistor, mosfet or LDO on the left side looks pretty bad. Looks like it start to develop a hole. Did you short it? Anyway, do not try to investigate a problem you cannot solve because you don't know the components and their characteristics. Here is a guide of Big Clive to start with to identify components and to learn the basics. youtube.com/watch?v=6Maq5IyHSuc \$\endgroup\$
    – Codebeat
    Aug 25 at 11:15
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    \$\begingroup\$ It's far more relevant to figure out which part the big 3+1 pin square IC named IC601 is. Because that's the voltage regulator, which is responsible for giving out the correct voltage. The part you are on about could either be a TVS protection diode, a fast switching diode as part of a switching regulator circuit, or a zener used for polarity protection or similar. \$\endgroup\$
    – Lundin
    Aug 25 at 11:57
  • \$\begingroup\$ @Lundin The IC601 top left pin, gets a voltage of 7.18V from the power supply block directly, the middle left pin has no voltage, the lower left pin gets a 5V, the right wide pin is ground, but this component isn't connected to the ROM. Btw, that 7.18V should be +8V, does this much drop makes a difference? \$\endgroup\$
    – John Sall
    Aug 25 at 12:06
  • \$\begingroup\$ Have any luck fixing the transmission ECU? \$\endgroup\$
    – Kartman
    Aug 25 at 13:15
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    \$\begingroup\$ The designation "PS..." suggests it is a fuse. Measure its resistance. If more than a few ohms it's probably blown. But why did it blow? \$\endgroup\$ Aug 25 at 17:04

I've seen SMD fuses like this before. This one looks to be a UNHH20600L.

Rated for 2.3A, 50V, according to this site.

enter image description here

The part seems to be from Panasonic Spec. sheet.


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