iPhone 7 Plus 4G signal difference on repaired

Machine model: iPhone 7 Plus

Malfunction: 4G signal of mobile phone

Maintenance process: At the moment of card insertion test, the 4G signal is normal, but after one minute, only one cell can be displayed, and it can be used for normal calls and Internet access. When it is placed in the corner of the room, it becomes a 2G network, and the 2G signal is normal.

When the motherboard was removed, it was found that IF power amplifier was completely passive, and Baseband CPU and Baseband power supply were also pried. It seems that the corresponding chip was replaced last time, and the fault was not solved.

iPhone 7 Plus 4G signal difference on repaired Figure 1.
After cleaning the motherboard, open Xinzhizao Soft drawing to measure two VCC of TDD power amplifier, PP _ QPOET _ VCC _ PA has 1.5V ,MLB_PA_VBATT also has 3.9V, and eliminate VCC problem.

Have you considered that the power supply of Baseband has been changed? Is there a problem with 19.2M Crystal? No results after replacement. The home has moved IF, and the size IF has been changed again, by the way, and the fault remains.

iPhone 7 Plus 4G signal difference on repaired figure 2.
According to the experience, it is judged that the 4G transmission line is normal when talking and surfing the Internet in 4G mode, and it is more likely that the crash point will appear in the 4G signal receiving part of the motherboard. After eliminating the problem of IF chip, the line between IF and IF is found to be normal. After eliminating the above problems, it is decided to remove the big IF to measure power amplifier to diode value.

iPhone 7 Plus 4G signal difference on repaired Figure 3.

shorted to ground was found when measuring the 33 pin of IF, and the normal diode value should be 700 or so. Looking at the drawings, we found that this pin bit is the A10 pin to pin CPU, which is the output line for receiving signals from IF to pin.

After finding the fault point, remove Baseband CPU and find that Baseband CPU A10 pin and left A11 pin bit Short bridge,Baseband CPU are reinstalled in Baseband. After the boot, the 4G signal is full without diving, and the fault is completely solved!