GALAXY GT720 graphics card boots black screen

Video card model: GALAXY gt720 tiger general 1g ddr5

Fault phenomenon: graphics card boot screen is black or frozen

Maintenance process:

GALAXY GT720 graphics card boots black screen Figure 1.

A video car gt720 graphics card, the fault is the graphics card boot screen is black or frozen.To repair the display card, first check whether the display card has Short circuit, and use multimeter to test several groups of circuits to ground diode value:1
1.GPU_VCC to ground diode value is 5 Ω, normal
2.DDR_VCC to ground diode value is 60 Ω, normal
3.PCI_VCC to ground diode value 462 Ω is normal

4.5V to ground diode value 1 1.2m Ω normal
The ground diode value of the above groups was basically normal.

GALAXY GT720 graphics card boots black screen Figure 2.

Several groups of VCC values were tested
1. Gpu-vcc 0V normal 1V or so
2.DDR_VCC 0V normal 1.5V or so(DDR3 display memory is 1.8V, ddr5 is 1.5V)
3.PCI_VCC 0V normal 1.0V or so
4.5V VCC 5V normal 5V
All the above VCC have been tested. Except 5V, other VCC has no output.

GALAXY GT720 graphics card boots black screen Figure 3.

When testing VCC, it must be tested according to VCC sequence flow. Generally,VCC sequence of N card is 5V_VCC 供电→GPU_V core VCC→ DDR_VCC memory VCC→ PCI_VCC 供电

First repair GPU VCC, the graphics card GPU VCC is using rt8120a chip, first look at the chip working conditions.Chip data:

GALAXY GT720 graphics card boots black screen Figure 4.

5 pin VCC VCC tested to be 12V normal.

GALAXY GT720 graphics card boots black screen Figure 5.
1 pin is the independent boost terminal, 11.8 is basically normal, because the circuit is not working, the normal will be higher than VCC VCC.

GALAXY GT720 graphics card boots black screen Figure 6.
7 pin en pin is the core,VCC is opened pin, the test shows that 0V is abnormal, normal is above 1V.

GALAXY GT720 graphics card boots black screen Figure 7.
There are two problems with pintriodeThe n-tube is converted by VCC to open VCC, which is provided by 3.3V PCI bus VCC. The test results of both triodes are good. The signal is also connected with GPU chip.VCC of T2 C pole is 0V, B pole is 0.6V, e-pole is GND,MOSFET is fully connected, T2 b-pole is connected with T1 c-pole, t1-b-pole is normal, there should be 0.6, but there is no VCC.

GALAXY GT720 graphics card boots black screen Figure 8.
The B pole of T1 comes from R1 and R2 partial voltage, and there is no VCC at the left end of R2. The pin is divided into two channels, one of which is connected with PCI 3.3V through a 1K resistance. After testing, the resistance diode value is normal, and the other is connected with GPU. It seems that GPU pin pulls down 3.3V VCC, but diode value does not have Short circuit.Disconnect the C pole of T2, and VCC of 7 pin is normal,But not yet out of the 1V core VCC.

Directly replace the IC, test GPU VCC, DDR VCC, PCI VCC, several groups of VCC are normal, test, screen display normal, troubleshooting, maintenance is over.