Icom IC746 - Unstable FM TX PLL
I hope this might help others who encounter the same problem with the IC746 - it could crop up quite often as they age.
I overheard a friend saying his IC746 was off frequency on FM transmit. It is quite a complex rig driven by lots of CPU trickery so of course the first course of action was to save all memories and perform a factory reset. That didn't cure it. I began to think hardware. It turned out to be quite interesting.
FM TX on the IC746 has a 9MHz synthesiser all of it's own with an LC tuned circuit plus shunt varicap diode where FM audio is injected. It seems over time that these components can drift and it loses lock. On the rig in question FM TX was off frequency by about 6kHz and it was hunting.
The PLL lock Voltage was around 0.6V when it should be 2.0V according to the alignment instructions. Adjusting C903 as per the manual appears to cured it. (No need to reset the deviation adjustments)
I have attached a few diagrams for anyone who might come this way in future.
Last edited by Jon_G4MDC; 9th Sep 2022 at 6:04 pm.
|