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Old 9th Sep 2022, 4:56 pm   #1
Jon_G4MDC
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Default 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.
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Last edited by Jon_G4MDC; 9th Sep 2022 at 5:04 pm.
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Old 9th Sep 2022, 8:16 pm   #2
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Default Re: Icom IC746 - Unstable FM TX PLL

It's quite a common issue with PLLs in amateur radio rigs. Many people assume that if the right frequency comes out, then all must be OK.

In reality this gives you no information about whether the adjustments are still good and that the loop is in lock with decent margins. It could be close to dropping out.

Every several years, it's worth sticking a voltmeter (high Z) or scope on the tuning line of each loop and spinning the tuning across its range to check the voltage is within the correct limits. It's an excellent health check, and no other test reliably tells you how safe their lock is.

David
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Old 12th Sep 2022, 3:08 pm   #3
G3VKM_Roger
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Default Re: Icom IC746 - Unstable FM TX PLL

The fault Jon describes sounds exactly the same as my IC-746 suffered about 4 years ago and in that instance it was jumping 9kHz on 144 MHz FM when the PTT was pressed. A minor tweak of the associated capacitor sorted it out.

73

Roger
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Old 12th Sep 2022, 5:28 pm   #4
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Default Re: Icom IC746 - Unstable FM TX PLL

There is also an issue in many Icoms of their brown anti-microphony guck corroding the metal out of their trimmer capacitors.

David
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