I will try to post here faults from various sources and how the problems were mitigated.
The post will be periodically updated.
Last update: October 16, 2022
4. Failure at Power On / Intermittent Power On
" When I press the G90's power button it turns on, and then turns off right
after. All this started to happen after I turned off the equipment by the
power supply, instead of having turned off the G90 first through its power
The microphone is fed with 9V from the first LDO from the 13.4V.
The Front Panel is fed with 5V and 3V3 from an inside LDO.
It might be a good starting point to check te second LDO 7805 on the lower PCB inside the Main Unit.
You must feed the first LDO with 13.4V through a 500 Ohm from a lab power supply limitd to 200 mA and check the presence of 5V at the output of the LDO.
Before trying to find a solution for the problems, it's a good practice to try to understand where, how and why the fault is ocurring.
A short version of what is happening when you push Power:
a - A short signal is applied to the POWER SWITCH MOSFET;
b - The Main CPU is "talking" to the Front Panel CPU
c - If Main CPU receive valid comms from Front Panel CPU and check is everything is OK, then go step d.
c' - If there is no valid comms from Front Panel CPU, "COMM LOST" is shown, go to e'
d - The Main CPU send a signal to the same POWER SWITCH MOSFET to keep in energised.
e - Radio boot, radio ON stays ON when Power button is released
e' - Radio shut down
The first noise you hear is from 9V being applied to the Audio amplifier.
After that, because the Front Panel has no 5V it cannot communicate with the Main Unit CPU which cannot keep POWER SWITCH ON.
Probably will be a good option to check if the silicone rubber is in the right place inside the Front Panel, I remember someone on Facebook had the same problem, solved with this. The single differente to that case is the microphone backlight in your case.
3. Various sources. Lightning discharge near antenna.
This problem is fairly common and affects the receiving front end of the radio.
The fault is located on the Small Signal Board (the PCB that is exposed when the upper panel of the Main Unit is removed) and the fault can be repaired without removing it from the radio as the PCB is one side populated with components.
First, the RF is coming from the antena through a Rx/Tx relay (PA RF Board) to that small connector.
Let's explain what is near the RX connector. Here is the relevant part of the schematic:
On the left there are the two ESD protection components, D4 and D40. These are not regular diodes but specific protection semiconductors.
The original components are GBLC08I-LF-T7 but might be very well replaced with PESD2V8R1BSF. In order the protection provided to be effective, the replacement components must be connected in anti-paralell mode across the D4-D40!
If Rx is still not working, we must check the proper working of the Attenuator circuit, which is made with U31 and U32.
Those two circuits are similar to SPDT relays but they are semiconductors. In vast majority of cases I saw the issue, the problem is here also, at leat U31 is burned but U32 is still working.
U31 and U32 are located under the metallic shield.
The shield is not soldered and can be lifted with a sharp tweezer. Pay attention when you are removing it, be gently!
When you put it back, pay attention to the alignement of retaining clips on the PCB!
Pin 5 on U31 is on the middle of the LEFT side.
Pin 5 on U32 is on the middle of the RIGHT side.
You can test the RF then by putting a 22-100 nF capacitor from the pin 5 of U31 to the pin 5 of the U32 (or before and after the separation capacitors).
Here it is how:
U31 and U32 Original part:
Here is a picture sent by one of fellow hams with this kind of problem...
2. From Facebook Xiegu G90 Users, Ronan Cantwell