Strange burning hole on my STF...

[Marsupilami]

Hello Yoda masters,

On this motherboard I have a problem with the floppy connector.
As soon as I power it up, the pad circled in yellow makes flames (and you can imagine the smell...).
I don't understand how this is possible because it's empty, it doesn't seem to be shorted on the other side either...
A continuity test shows that there is no shortcut with the 2 ground pads just under.

B.T.W. is the "joe master" capacitor from factory ? It seems odd...

Do you have an idea? How could i fix it ? There are only 2 layers on an STF, right?

Thanks.

20230223_212030.jpg

20230223_212117.jpg

[derkom]

A continuity test shows that there is no shortcut with the 2 ground pads just under.

Rather than just a yes/no, what is the actual measured conductivity between this pad and ground? Might be good for someone to compare this to a non-flaming board. You might have a high enough resistance that a digital meter doesn't consider it a short, but it's precisely this resistance that would create the heat, without being enough of a short to shut down the PSU.

[rubber_jonnie]

I'd agree with @derkom

If you checked for continuity and got nothing, then is could be a high resistance, which my own multimeter wont class as a short in continuity mode.

Check using resistance mode to see if there is some sort of high imedance path between the two points.

Also, if it has previously burned for some reason, then there may be carbon present which will also affect conductivity.

To answer your other question, yes, the board is only 2 layers, top and bottom.

[Badwolf]

Is that the 12V line?

BW

[rubber_jonnie]

Is that the 12V line?

BW

Going by some other mainboard pictures from the forum, it certainly looks like it is 12v.

[stephen_usher]

In which case I'm wondering if there's a short on the 12V line further down the line and that via is just the highest resistance part of the circuit and hence is overheating and acting like a fuse.

[Badwolf]

Is that the 12V line?

Going by some other mainboard pictures from the forum, it certainly looks like it is 12v.

That's good news in that it probably hasn't fried anything important, then. The clearance to the ground plane on the bottom side looks very suspect, though.

I'd be getting in there an Ohm-meter, as described above but then giving the clear space between the pad and the ground plane a good scratching with an awl, a dart or even a multimeter probe, to make sure no charring or tin whisker is across there.

I think the wires are normally soldered into those holes so it's possible some partial-bridge has been introduced during the desoldering process. A good scrub under magnification is in order, I'd reckon.

BW

[Marsupilami]

This is what I measure in Ω.
The 2 center pads are grounded.

20230223_212030-3.jpg

I add a photo of the MB.

20230223_212044.jpg

[frank.lukas]

Measure is ok ...

Zwischenablage.jpg