Yaesu FT290. No or miniscule modulation on both FM and SSB.

[Wendymott]

Hi Peeps. Somewhere along the line, my FT290 has developed a "virtually" no mod fault . Receive is ok. The PLL is locked, the FM Mod from the FM Modulator is ok, but on its way up to the Q1011, it gets lost. However I note G2 voltage on Q1011 is ok on RX but goes AWOL on TX. Now anyone who has one of these boxes knows it really is impossible to locate components and pins, and i am fortunate to have two scrappers, but even that is not helping. Has anyone had this fault or similar please.

[Jon_G4MDC]

I'm a bit puzzled Wendy. Isn't this the FM mod section of an FT290?

10.81 MHz Oscillator with direct FM at Q2005

If the FM modulation is low the fault will be here rather than at Q1010/1011. You do get normal RF output I suppose?

I had another thought. Do you notice that there is an input wire for tone squelch connecting to the bottom end of choke L01? I don't know if you have a CTCSS board, original or retro fitted, but that would be a good way to shunt your FM mod to ground and give the symptoms you are reporting. Good luck.

Just re-read the title. So it's not just FM but SSB as well?

[Wendymott]

Hi Jon. I do that all the time You're maybe onto something as I removed the front panel at some stage and I may have inadvertently nicked a wire. I shall have a look.

[Jon_G4MDC]

I think you have probably killed the microphone input cable. It divides and feeds the SBB mic amp via C01 in that snippet. It's out of picture above.

[Jon_G4MDC]

grr ^ SSB!

[Wendymott]

Jon... Used more than one Mike... Please read above.... Modulation OK at FM Modulator. Its after modulator.

[Terry_VK5TM]

Check Q1010, that is the common start point of both SSB & FM throught the IF - likely the dual gate FET went phutt!

[Jon_G4MDC]

How does a mixer strip off FM modulation?
Won't two different mics to the same open or short circuit mic socket/cable have the same result = no mod?

[Terry_VK5TM]

Originally Posted by Jon_G4MDC

How does a mixer strip off FM modulation?

It doesn't, in the FT290, FM mode modulates the 10.8MHz oscillator and that is fed through the mixer, up the rest of the IF chain and out the spout.


In SSB mode, the oscillator is unmodulated and used as the LO for the mixer and so SSB through the rest of the IF and out the spout.


They all come together at Q1010, the start point of the IF.

[Wendymott]

Hi Terry.. The strange thing is that in RX Mode the sensitivity is very good.. and G2 voltage is there.. In TX mode G2 voltage is 0. I will leave it a few days to gather my enthusiasm.