You found the schematic for the GNS430 RF section?? Awesome!
I can’t read a lot of it but I would start by changing the input MOSFETs, one at a time.
Peter wrote:
You found the schematic for the GNS430 RF section
Yes, a diagram of the COM part.
But there are many similar nodes in it, such as PLL, VCO and LD signal to the controller and the output of their UART controller.
Also, the LM317 power stabilizer and 3EM(VHF/UHF Transistor) in the VCO generator.
For some reason, a digital potentiometer is used for feedback, like the bendix / king KX165A, but on my glide path board I did not even find a digital potentiometer microcircuit – perhaps they still implemented it in the communication board and control it by uart.
Measured the input and output parameters of the PLL:
From good board – OSCin:12Mhz and Fin: 290 MHz
From bad board – OSCin:17Mhz and Fin: 290 MHz
The fin frequencies OSCin are different, because the crystal oscillators are one at 11.975 MHz and the other at 16.800 MHz.
Has anyone worked with synthesizers for RF receivers?
For example LMX2306?
Can’t figure out how to calculate the lock bit (Fo / LD) – digital lock detection.
From the Bendix / King KX165A documentation states:
In a normal synthesizer locked state, the LD of U3011 (pin 7) will be high.
Low going pulses will be seen at the LD output if the synthesizer is unlocked.
will be high – means any voltage value other than zero?
The problem is resolved.
In order to “revive” the topic – I will not write an answer … yet.
Ask questions.
What was it? 
Peter wrote:
What was it?
EEPROM – dump problem.
Can you be more specific?
The eeprom chip had a littered dump.
A good dump was loaded and everything worked
Presumably the EEPROM was some data read by the processor on that board. How did you find the real content? It is quite tricky to program a serial EEPROM soldered to a PCB. I recall doing that on an early Thinkpad (which back then cost £5000) to unlock it.
I have experience in laptop repair, and I often flashed the BIOS chip there and have skills in this.
In the case of the glide path receiver board, I considered that it(EEPROM) was based on the bios principle.
In general, it is not clear why the eeprom was made for the miscontroller, but after checking the data on the UART bus (after reading the 24MHz bus analyzer), a suspicion of a defective eeprom flash drive crept in.
Subtracted the dump from the microcircuit, wrote down the correct dump and the board started working.
It is difficult not to solder and solder it out, but to diagnose what exactly is in the eeprom business and in the firmware.
