Adonis 308 Mic pulsing on 20m

[gw6pxw]

Hi all,
At our local radio club we have an Icom 7400 and run an Ameritron AL-811 linear.
All kit is new and is earthed, the patch lead from the Icom to the linear has ferrites fitted to stop any RF feedback.
We have an two Adonis 308 desk mics, which work perfectly on all bands except 20m. On this band there is a loud 1 second interval pulsing in time with the led on the mic, and the audio is rubbish. Both mics have the same problem.
If we change to the Icom fist mic, this pulsing noise disappears and the audio is excellent.
Any ideas or suggestions would be most welcome.
Thanks in advance.
Geoff.

 

[Captain Kilowatt]

The Adonis 308 manual says to avoid high SWR and to make sure everything is grounded good to prevent RF feedback. Have you tried ferrites on the mic cable? Try running a separate ground wire from the mic case . Are you using the external power supply for the mic? If so then install ferrites or bypass capacitors on the power leads. There are notorious for introducing RF into devices. Since it only happens on 20m I would start looking for a resonant length of cable somewhere in the shack as well.

 

[gw6pxw]

Hi there, thanks for reply. ATU gives radio 1:1 SWR, Have tried ferrite ring with mic lead wound on it 4 times, have tried using mic with its internal batteries and also from the Icom mic socket supply. We thought about a resonant length wire somewhere but all the leads are short. Maybe have to clip ferrites on everything. Will also try earthing mic case.
Best wishes for 2010,
Geoff

 

[bob85]

what type of mic cable are you using geoff ?,
the factory adonis cables have very poor shielding i never use them,
if you cant find a cure try making a new cable using the 9way fully foil shielded wire from maplin, use the bare drain wire trapped under the cable clamp at each end to ground the mic body to the radio, it may not cure the problem but it makes the adonis much more capable of operating in a bad rf environment, the same trick works with kenwood mc60's which also have crappy factory cables,
dont know what the pulsing sound is on a 308,
the only thing i have heared like that is the timeout timer in the adonis 707.

 

[gw6pxw]

Thanks Bob, yes the mic cable is the factory one that came to suit the Icom. Will try making up a shielded one and also make up a choke balun to fit at the antenna feedpoint, just in case any rf coming back into shack.
Best for 2010,
Geoff

 

[gw6pxw]

Thanks guys for the replies, the suggestions were great. Turns out the SWR was high on 20m on the G5RV. Choke balun at the feedpoint has cured the problem, and we will look at making up shielded leads for the mic.
Best wishes for 2010, look forward to working you all during the coming year.
73, Geoff

 

[bob85]