AMD HD6750 bad 78m05

[DeniS182]

Hi all,
I got this graphics card as a gift because is faulty. I have removed the cooler and saw bad IC 78m05g - 5V voltage regulator, with hole in package. After desoldering this regulator I measure short circuit between gnd and output. I tried also desolder one decoupling smd ceramic capacitor who is soldered between gnd-out but short circuit is still.. this regulator produce 5v for HDMI output and possibly for others circuits. Why they not use 5V from PSU instead of using 5v regulator? Do you know what could cause a short circuit? I upload photos soon. Thanks for answer and sorry for my bad english

[mariushm]

Re: AMD HD6750 bad 78m05

The pci express slot only has 12v and 3.3v - it's easier to use a chip and two capacitors to make 5v out of 12v, instead of using chip, inductor, diode, resistors, capacitors etc to make a boost regulator and make 5v out of 3.3v

Check that picofuse .. right above the regulator, with a p on it .. and the cap after it.

[DeniS182]

Re: AMD HD6750 bad 78m05

Thank you for the explanation.
I found a bug. R450 is bad, after desolder this resistor I dont measure short circuit between gnd-out. But I dont know value of this resistor. Have someone this graphics card? Or where I can find the schematic?

[dj_ricoh]

Re: AMD HD6750 bad 78m05

look i don`t wanna cut your hopes but first you need to take the bga chip out!
that`s like the first thing you do , then you check on the board.

[dj_ricoh]

Re: AMD HD6750 bad 78m05

think like this!
if there is a bridge on the balls underneath everything you do its wrong because it will show short until you reach the chip.

[DeniS182]

Re: AMD HD6750 bad 78m05

I do not have the tools for release BGA. I do it for fun and when it not work nothing happens..

R405 is connected from output of 78m05 to gnd.What is its function?

[stj]

Re: AMD HD6750 bad 78m05

fixed load so the voltage stays stable if the stuff the regulator drives turns off.

try lifting any chip NOT bga to see if the short goes away.

[DeniS182]

Re: AMD HD6750 bad 78m05

I tried to desolder some capacitors, resistors which are connected between the output and ground. After desolder this resistor R405 short circuit disappeared. I only don't know a value of this resistor. What value is there usually or do you have any idea how to determine this value?

[stj]

Re: AMD HD6750 bad 78m05

dont you have a meter that shows resistance?

[DeniS182]

Re: AMD HD6750 bad 78m05

Yes, I have...but this resistor cause a short circuit. Is damaged - on diode tester is beeping and resistance is 0.4ohm. I need to know its original value before damage.

[stj]

Re: AMD HD6750 bad 78m05

then it's a zero-ohm resistor used as a jumper.
nothing wrong with it.

the good news - you just isolated the fault to a smaller part of the board.
so - which side of the resistor-pads still shows a short to ground?

[mariushm]

Re: AMD HD6750 bad 78m05

If that's a faulty resistor and not just a jumper..

If you look in the datasheets, it says in charts Load regulation from 5mA to 500mA. So the regulator like to have at least 5mA of power consumption to be absolutely stable.

V = I x R => 5v = 0.005A x R => R = 5 / 0.005 = 1000 ohm

Power dissipated in resistor P = I x I x R = 0.005 x 0.005 x 1000 = 0.025 w - even a 0603 can handle 0.1w and you probably have there a 0805.

[lti]

Re: AMD HD6750 bad 78m05

It looks like an 0402 to me, and it probably is a jumper.

[momaka]

Re: AMD HD6750 bad 78m05

IIRC, 5V is used for VGA or analog connection through DVI. Not sure about HDMI. May also be used for port detection logic, so check all of those SOT-23 transistors near the ports. Also check your ports for any shorted wires. I once had a PC motherboard with a bad VGA connector that caused the PSU to trip due to a short circuit in the VGA connector. The issue could also be a shorted ceramic capacitor, but I'm a little doubtful of that.

[mariushm]

Re: AMD HD6750 bad 78m05

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