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voltage issue on lpc core ?


disketto
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Hi all,

i am assembling a lpc17 core, to build a midibox sid. 

 

i am following the steps reported on the walkthrough web page.

In one point it says to check the voltage between pin 54 and 28 of the core board, with USB plugged. It says that i should have a 3.3V, but i have 0.0, no matter what.

 

Anywhere else i check, anyway, i get the measure of 5V (that i think comes from the USB plug).

 

What can i check to understand where i did wrong ?

 

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Uhm... the jumper is on J17, but on LF33 the reading is 0.10 between 1 and 2 and 0.0 between 2 and 3. The LF33 is positioned with the metallic part pointing outside of the board...

Can this LF33 be broken ? or there are other parts i could check ?

 

Thanks for the help :smile:

Edited by disketto
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Pin 1 is the Input of the regulator LF33. If you don't have 5V between Pin 1 and Pin 2 (Ground) you won't get anything out of the LF33. Maybe there is a short anywhere between the ground and the 5V lines? Could you measure the resistance between the Pins at J2? There should be no(!) continuity between the Pins Vs/GND and 5V.

 

Could you post some pictures of your PCB (both sides, hires prefered)? Maybe some other pair of eyes see something.

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Not sure if I understand what you have written. Do you get 5V at Pin1 of the LF33, or not? As ground I would use Vs at J2 or the metal parts of the USB or the 7805.

 

From the pictures I would propose to reheat the solder joints of the LF33, they look ugly. Also you should reheat all other solder joints which look like "bubbles", as this is an indication of cold solder joints. Then try to measure at the solder side and also at the upper side! It's also a good advice to clean the board (see my comments in ) as you can distinct a lot better between good and bad solder joints.

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Thanks for the advices ! 

i was looking for something to clean up the board from the stuff that comes from the tin solder.

By the way, yes that what i was doing, measuring from the metal part of the usb to the input pin of  LF33, and i got nothing. Could that mean that the line is broken in the between ? 

 

In example, i can see clearly that the line goes from usb to the capacitor C11, and indeed i have there 5V measuring from the metal part of usb, to the C11 first pin.

 

I can't see though how the LF33 should receive the 5V from the USB. What's in the between ? 

Edited by disketto
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I was putting that apart, when i just gave a quick look at the tracks while smoking a cig... and noticed that the two capactitors C11 and C13 are connected to the LF33... now.. since i knew i had the 5V on the C11, the culprit could be only the LF33.

So i decided to desolder that... and bad news... the holes were totally burnt.

 

That should have been happened when i desoldered without a sucker and pulling the 7805 i put there by error... 

Now.. i have the holes burnt.... i have 2 options:

 

- do it from scratch (it's a pain to desolder every component, and even if i had the patience, i would break a lot of pieces, i'm not an expert)

 

or... (and i hope it works)

- cable the LF33 with hops connected to both the C11 / C13 and C12 / C10

 

Do you think it could work ?

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