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It seems unlikely that anyone else has come across this minor problem but worth a try!

Instead of the standard Kenwood mic I sometimes use a basic (cheap!) single earpiece / with electret mic on my TS850S. All is OK unless I try to use speech compression then unless I keep the Mic gain well down I get RF feedback even when running into a dummy load.

I've tried a combination of inductor in series 1n C's to ground, ferrite rings, keep the RX audio to zero etc. all to no avail. The results are the same on all bands 80 - 10m. I? use a similar headset on my H/B HF transceivers without any problems.

Any suggestions? Maybe buy one of these absurdly expensive headsets that are advertised in the Radio Mags.? I wonder if they suffer with the same problem?

73...

Ken

G3LVP

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Here’s a YouTube video of a ham testing a cheap electret mic on a Kenwood. He’s testing and setting the mic parameters using his Kenwood. Try this. If it doesn’t work, then check the mic pin-out. If correct then try using the mic on another Kenwood. If that works then try another cheap mic on your Kenwood. If that fails then you may have to do a diagnostic on the audio input of your radio.?

I learned several years ago that the Kenwoods prefer a dynamic element as opposed to an electret-conderser type element. The electrets work great on the vhf and uhf mobile rigs, but touchy on the hf units. From Heil Microphones: "Condenser microphones are typically found in the studio. The element is made up of a thin film coated with a conductive material such as silver or gold which is suspended over a polarized (powered) backplate. This condenser diaphragm is part of an electrical circuit that changes voltage with movement and this voltage becomes the output of the microphone. Sensitivity, wide frequency response and transient response were always the performance advantages of condenser microphones over ribbons, dynamics and all other types of microphones.

Unlike dynamics or ribbons, condensers require a power source to operate. This power is typically delivered via an external “power supply” or something called “phantom power”. Phantom power is a dc voltage fed back through the microphone cable from the console or mic preamplifier to the microphone to run its internal electronics. The term “phantom” was coined because there is no additional wiring or connections to carry the dc power from the console to the microphone. Most all consoles or mic preamps have a built-in 48 volt phantom supply that can be sent to the condenser mic up the mic cable.

The phantom power situation is also something you must be careful of. These microphones are more susceptible to RF and electrical interference since you have dc voltage being fed on the exact same two conductors as the audio from the microphone to the console. Because there is no heavy voice coil attached to the diaphragm."




Heil recommends a dynamic element for the 870S, and that configuration is what I used with my 850S.

Thanks Steve & Dave,
Some useful ideas, I'd already spent some time looking at the cct diagrams of the IF unit to see excatly what happens when the? compressor (RF Clipper) was switched in, I need to have another look but it seems that as to be expected the mic gain is increased to drive the clipper which might explain why the mic is more sensitive to RF. What I might try is providing the mic bias from a battery & not form the 8V line on the Mic socket.
I didn't mention that when running 250W on 2m from a transverter there's no problem with RF.. The easy solution as Dave suggested is to find a headset with a (small) dynamic mic.

73...

Ken

G3LVP

2 meters at 250 watts is not the same as HF at 100 watts; 100 watts on 75 meters would get into my telephones and set off my security system.