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Repair flag GS on PFD and msg on GNS430 G/S has failed

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.

Shoreham EGKA, United Kingdom

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?

Last Edited by engineer at 04 May 10:30
OSF, Russian Federation
12 Posts
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