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Bondwell B310 display problems / Repair

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First post, by matze79

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Hi

have a little Problem with a Device like this: https://www.youtube.com/watch?v=7JS3K324Q5w

here a a Picture of my issue,

The attachment IMG_20151031_111309.jpg is no longer available

here are some more:
http://www.dosforum.de/viewtopic.php?f=1&t=9960

i have this little Bondwell B310 Laptop with 40Mb Harddisk and Harris 286 12Mhz CPU.
It had a defective Capacitor in the negative voltage for the LCD Contrast.

The attachment capacitor_leak.jpg is no longer available
The attachment capacitor_leak1.jpg is no longer available

but after i replaced the capacitor, the new one got hot and died too.. (polarity right)

On + of C4 i read a Voltage of -12.5V.

Has someone encountered a similar Problem and knows what i have todo ?
looks like some sort of charge pump, which generates the negative voltage.
The trace runs to a coil and to the diodes

Thanks for all Help

Reply 1 of 2, by Imperious

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I think You are in luck, as someone posted a vid on Youtube of a repair of one of these just Yesterday. Go here https://www.youtube.com/watch?v=5QdprppIXPY
At 4.18 You can clearly see that the negative pin of C4 is closest to what seems to be the + symbol. Most likely the reason there is a square where the pin goes
instead of a circle for the other capacitors. I would say You likely were bumsteered by the + symbol into putting the capacitor in the wrong way round.

In negative voltage rail circuits the positive pin of the capacitor goes on the Ground and the negative pin goes on the negative rail, -12.5v in this case.

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Reply 2 of 2, by matze79

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Thanks, so it whas my fault.. 😀
Now its working Perfect!

i just suck the solder off with my desoldering station and the capacitor felt off.
And expected + to be + of the capacitor.

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