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ICOM IC-A20 MK2 headset interface


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I am trying to get a headset to work correctly with an ICOM IC-A20 Mk2 handheld.


This ICOM uses a different interface to the later models. The PTT is combined with the mic such that if the mic input is grounded through a relatively low resistance the transmitter is keyed. I think this is intended to allow a dynamic mic to key the Tx.


However with an electret mic it is necessary to fake this PTT function. I have found info on the web giving a method, and I have looked at the circuit diagram for the Icom (also found on the Web) and can see how it should work. There is a +5v supply available as well as the mic on the double circuit jack so this can be used to energise the electret mic with a simple resistance and capacitance filter to feed the audio to the mic input and a resistor to ground to key the PTT. All very well.


However I get feedback on transmit coming , I think, from the RF signal getting back into the mic. I seems OK when the aerial is well away from the unit but I think it is still marginal.


Has anyone had experience with this setup? I am using an Altronics headset.







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From memory the A20 runs with an electret mic or possibly amplified electret - not the GA mic standard. They run OK with the audio from an intercom coupled via a capacitor and around 5 Kohms resistor for the ptt grounding.


You may need to do some work on the Altronics headset there has been a number over here struck trouble with squeals such as RF getting into the headset and I havent had one on the operating table yet to determine the cause or whether it is limited to that brand headsets or problems with particular radios.





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  • 2 weeks later...

Fix for electret mic


The problem that you cite is similar with the Icom ICA6/24 series when used with the optional connection cable. The wiring as you advise correctly is different on that radio as its 4 wire. The ICA20 is similar to the PMR Icoms F3 etc.


To prevent feedback and the simplest easiest way is to use decoupling cap for the 5volt supply from the radio, with a simple circuit (diode and cap), plus a current resistor, hitch up a 1,5volt aa battery in the headcup to power the mic element. This works brilliant and you can leave the battery on all the time as the drain rate is marginal. Maybe use a energizer Li Ion will last an age.





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Thanks for those replies.


At the moment it is working OK so long as the antenna is well away from the headset. On my plane this is the case. It is hopeless if the aerial on the hand held is used. I am powering the electret mic from the +5 supply and capacitive coupling into the mic input. A resistor to ground of about 27K seems to be enough to trigger the transmitter.


If I get problems again I will try the suggestion from Zulu1





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I too, am trying to use an A6 with a headset, and I'm having all sorts of problems. First the transmit feedback is caused (so Icom tell me) by the type of head set used/ My new Altronics is unusable.The old David Clark set that I have is fine. Plus I have transmit problems. It appears that I cannot use a headset without using a PTT button. I tried yesterday using the inbuilt PTT button/headset combination and found after I had landed that I was not transmitting. Although the little tx signal was coming up on the screen. (reception was terrific) . I wish I had not gone down this path. It's spoiling my flying not knowing if I'm transmitting or not! I may chuck all this in a drawer and get an inbuilt set!!..Geoff:crying:



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ICA6/24 series


Geoff, the ICA6/24 series are not user friendly and in most cases do not work with the headset adaptor supplied. When they were released they were based on the F30 GS PMR series (I think thats the model). They are perfect as a hand held. They require 11 volts in to power the tx function when used with the remote socket on the bottom and the board cracks quite easily if not using a angle plug.


Dont give up and try the following first...


To get them to work with a remote headset, before you trash the set up. Ensure that the ST function is set to about 1. To do that set the tx frequency to a "unknown one" so you wont upset anyone. Press and hold the PTT and whilst holding down, adjust the small knob on the top (not the volume) and you will view "st" turn to about 1. This is the mic side tone to the headset and default is 6. Use the supplied rf bead on the charger and clip around the headset cable close to the radio.


Next stage before you spend money is to find someone with the CL24 software and the Icom dongle. Turn the rf down to 0ne watt on function 27. If you have no rf meter about 57-60 as a guide line. This in most cases works.


Failing that (always works) you can make a small adaptor that decouples the mic supply from the radio supply and feed the mic from a seperate battery supply, about a 1,5 AA will work and use a 100nf cap to decouple the radio output supply. Good idea to use a current limiting resistor to the mic also. Say about 1kohms or so.


If using with a external supply, use a minimum of a LM350 regulator as they draw a peak of 1,8 amps whilst transmitting. set the reg to about 11,2 volts and use a rf bead and screen cable if possible. Close as possible to the radio as they have been known to collect rf in the feed cable. Try to source a wire wound line choke also (best place is the large sound shops..(UK Halfords sells great ones) and at least a 2,200uf (minimum 16volts pref 25 volts) smooth cap or more. Make sure the ANL is on the radio.


Or plan B go buy a radio that will work !! Virtually anything else...the designer did a great job to the outside but the rest makes hard work.


Good Luck





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Useful web site for ICOM mods


For the techies.


This web site seems to have a lot of useful info on the range of ICOM radios of all types.




In particular this section has information about the mic interface for the IC-A20


"ICOM IC-A20 alkaline headset-adapter lost-in-the-soup English language"


Service manuals in PDF for the A20 and A20Mk2 are here:




Hope this is useful to others.





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