Galaxy Pluto FM transmit issue

[Christophe Barbe]

Hi all,
I just want to discuss with you this issue and most of all get your opinion, maybe you already have had that issue on a similar chassis.
Fact is that I have a fully working radio (all modes, RX and TX) with the exception of FM TX which works, but the modulation seems raspy aka not crystal clear at the end of sentences I would say.
Ideas could be a lack of grounding at the mic section, but then why is AM and SSB flawless?
Also, maybe some caps around the PLL.
Other than that the radio is fully original, no caps were changed nor any mod performed on it.
FYI, I tried to act on VR5 (FM dev) but no changes, a bigger dev will make the issue bigger, a low dev will make it less noticeable, but then modulation is not strong enough.
Thanks for your inputs.
73 de HA7WX / 14AT733 / KP733

 

[midnight special]

microphone? i have seen some mikes that make fm sound bad but work great on am and ssb,,,, what mike are you using?

 

[Christophe Barbe]

microphone? i have seen some mikes that make fm sound bad but work great on am and ssb,,,, what mike are you using?

Hi, thanks for your input, using a hand mike, not the original one, but it sounds fine (the trapezoid mike), and I have tried a base mic too, but no improvement.
It sounds fine in FM, just when finishing a word or sentence (difficult to explain), the audio sounds raspy (when talking too i would say, it's not the best either), well it's not clear as it is in AM or SSB or would be in FM on an other radio.
That's why i have left the audio chain of my thinking as this works fin in AM and SSB and has something to do where FM gets mixed to the audio and maybe around the PLL.
I have some caps 100uF and 220uF in its vicinity and could try to replace them, i just want to know if some of the guys here have had this kind of issue on a radio already in the past, or have an idea of what can alter FM audio other than the audio path.
So it may be not a ground issue either.
So weird as FM is the simplest mode of modulation...
Thank you anyway and sorry if some spelling is not perfect, not my mother language

 

[nomadradio]

Since the mike audio is clear on other modes, I'd look at parts used only for FM transmit.

It's a short list. D31 and D32 switch the mike audio to feed into the FM modulation trimpot VR5. Trimpots are susceptible to oxidation damage. The diodes are pretty much the only other thing I can think might fail and affect it.

This radio simply feeds mike audio into the tuning voltage of the PLL. A dirt-simple modulation circuit. Not many things to go wrong with it.

73

 

[Christophe Barbe]

Great, thank you for your answer, i will have a look at them too.
I have the schematic and will look diodes up and figure out the list of susceptible components.
Hopefully there is no oxidation damage on the trimpots (rig seems to have no trace of oxidation anywhere) but good to know.
I am just learning from your answer as my tech knowledge is limited, but eager to learn.
When i find the solution (hopefully until the WE), i'll update here so that sb could benefit of it, should a similar problem happen in the future.
73, Chris