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Thread: Mic Pin-out Icom IC 735

  1. #1

    Default Mic Pin-out Icom IC 735

    Hi friends. New to this forum and a BIG question! I recently got a Icom IC 735 in working condition, but looks terrible. I feel to disinfect my hands after touching the mic! Not the original, but a Icom HM-56A. Let me not describe how disgusting the mic looks!

    I downloaded the PDF manual but struggle to figure the pin-out. In short, I want to place a regular standard mic without up or down buttons on it... Similar to a regular CB mic. One of 2 ways... keep the old disgusting cord, or just change the 8 pin plug to the new standard mic cord.

    As I understand... Pin 1, Mic input, 2 open, 3 open, 4 open, 5 PTT, 6 PTT ground connected to 7 Mic Ground and Pin 8 ....?

    Normally looking at the PTT switch 6 pins, Top left, Yellow, Middle, Mic insert red, top right open...
    Opposite those bottom row.................Bottom left, Red, Bottom middle Screen+ insert black and bottom right Blue.

    Having said this... On the Icom plug, I assume... NOT SURE, pin 1 Yellow, 2 open, 3 open, 4 open, 5 Red, pin 6 and 7 bridge with screen and pin 8 Blue.

    My final question... I suppose the standard mic insert would be good enough audio for the 735?
    Please help Thanks! de Johan, ZS one JDT

  2. #2

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    Looking at the diagram it would appear that a standard spst momentary switch between pin 5 and 6 (6 being ground) would activate TX. The mic utilizes pins 1 and 7 (7 being shield, 1 being audio). It also looks like pin 1 (that carries the microphone audio) is DC biased to power the stock microphone. If you use a dynamic microphone (typical of CB mics), you will need a .47uF capacitor in line with pin 1 (positive side of the capacitor facing pin 1 or it will eventually fail) to block the DC voltage that otherwise powers the stock electret mic. The mic jack input impedance is 600 ohm, try to match that with the replacement mic you choose.

    The type of microphone you substitute will determine how you wire it. Do you intend to use a dynamic mic or an electret mic?
    Last edited by brandon lind; Mon 20th Jan 2020 at 21:48.

  3. #3

    Join Date
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    JDT,

    I have a IC-735. The intended microphone is the HM-12 or SM-8, but I use the SM-2 desk microphone or sometimes my D-104. Both of which are amplified.

    The wiring of the microphone for the IC-735 depends on what mike you choose. The wiring of the microphone connector is detailed in the manual.

    Pin 1 - Mic Input
    Pin 2 - +8V Output
    Pin 3 - Freq Up/Down
    Pin 4 - NC
    Pin 5 - PTT
    Pin 6 - GND (PTT)
    Pin 7 - GND (Mic)
    Pin 8 - AF Output

    If you don't need the "Freq Up/Down", leave that pin empty. The "AF Output" could be used with a speaker microphone.

    As Brandon pointed out, if you use a Electret microphone, or a amplified microphone, you will need to isolate the IC-735's "Mic Input" from power. This is pretty simple to do with a capacitor. The capacitor value will affect the low frequencies, so a 1 to 3 uF should work fine. Try not to depend on wire colors. These can change, depending on the maker. Do not assume. Know where all the wires go.
    Martin, K7MEM
    http://www.k7mem.com
    Ash Fork, AZ - 60 miles from the Grand Canyon on Rt-66. Elevation 5,300 ft.

  4. #4

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    K7MEM, good catch on the capacitor value! I grabbed the value from the circuit diagram of the HM12 in the service manual which is clearly not correct. At 200Hz, that's about 1700ohms reactance, far too many ohms! You are 100% correct on 1 to 3uF!

  5. #5

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    Quote Originally Posted by brandon lind View Post
    K7MEM, good catch on the capacitor value! I grabbed the value from the circuit diagram of the HM12 in the service manual which is clearly not correct. At 200Hz, that's about 1700ohms reactance, far too many ohms! You are 100% correct on 1 to 3uF!
    I'll try and post a photo.... hope this works.


    Maybe just a question on the voltage on the 1 uF cap. The mic I got with the 735 is from a 2 meter Icom rig with DTMF code buttons on the rear. Horrible mic!

    I tried posting before this one... but it didn't show. Sorry, I have difficulty navigating the forum. I'm still not sure wiring a regular mic. Maybe someone can guide me to some other site for help. Thanks.

  6. #6

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    Quote Originally Posted by JDT View Post
    Maybe just a question on the voltage on the 1 uF cap. T
    16v should be fine

  7. #7

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    Quote Originally Posted by brandon lind View Post
    16v should be fine
    Thank you sir!
    Old saying... I don't know why... but just do it cause you have to.
    My English is poor, so I googled terminology. What is a dynamic and electret mike. The new mike I wired on my 735 is a dynamic mic.
    I found some other write-up on fixing a standard mic to a Icom 720. The pin out on the 720 is not the same as the 735. In a moment you see... read why...
    He wrote..." Connect the audio wire to pin 1. Connect the TX to pin 6 and the RX to pin 5 and pin 7 screen."
    The standard mic for the 735, uses the exact same pins, but pin 5 and 6 must be swapped around... as if you wire as above, the radio go into transmit the moment the mic get plugged in.

    This is how to wire... I can't post a pic... so, use your imagination.

    The colors I'm using is for visual reference and as it was mentioned before... colors are different by manufacturer.

    The mic is face down, PTT pin to the right. On the 6 pin PTT contacts... left is yellow (it's own little screen wire inside) and middle is the red from the insert and right is open.
    Bottom, left is red, middle screen and mic insert black and right brown.

    On the yellow wire is the 16V, 1uF cap, neatly tucked in a spot and the stripes on the cap faces the radio.

    The 8 pin plug is as follows...
    Pin 1 yellow (make sure inner sc don't touch)
    Pin 5 is red...
    Pin 6 is brown...
    Pin 7 is screen.... and all the other pins open.

    To summarize...
    Connect the audio wire to pin 1. Screen go to pin 7. Connect the TX to pin 5 and the RX to pin 6.
    Looking at the pin out in the manual... no wonder its confusing as pin 5 and 6 is described as PTT and PTT ground that makes no sense.
    Thanks

  8. #8

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    The problem with using wire colors and terms like that is that it creates the possibility for us to mislead you by accident. For example, audio could mean audio out or audio in, the radio's mic jack supports both. RX could mean a variety of things based on microphone. Some radios require an RX pin to be grounded for the receiver to function ,whereas some radios consider RX at the mic jack to be audio out for a speaker built into the mic. Either way, I doubt it means ground for the TX (PTT) switch. The way you to describe the situation leaves too much ambiguity.

    The difference between microphones is as follows. A dynamic mic is one that most closely resembles a speaker ~ it has a diaphragm, pickup coil and magnet... a speaker in reverse. A condenser microphone is essentially a large surface area capacitor with an external voltage applied between the plates. When pressure waves from sounds in the air hit one of the plates, the capacitance changes superimposing an ac voltage on the DC plate voltage. An electret microphone is simply a condenser microphone with a piece of statically charged material which acts as the DC voltage. In these microphones, the signal level is typically very low requiring the use of a high impedance jfet amplifier. The need to power this amplifier circuit (sometimes built into the electret mic housing, sometimes not) is why these mics still require a DC voltage to operate. Your radio's mic audio-in puts out a DC voltage to power such microphones and the AC audio is simply superimposed on this DC bias and separated inside the radio. It is this DC voltage that will destroy a dynamic mic or oppose the electrets natural charge, thus the capacitor.

    PTT and TX are the same thing from a microphone perspective. Without looking at the manual for the other mic (i could later today, heading to town soon) the RX term could mean a few things.

    If you take a look at the schematic diagram for the HM12 you will see on the left side a onponent labeled ECM. This is the electret microphone of the stock mic which has 3 terminals on it. the top terminal is for the supply voltage that powers the JFET amplifier within. The middle terminal is audio out from the mic. The bottom terminal is ground for both the audio and the amplifier. You will also see a few components next to ECM, 2 capacitors and a resistor. How all this works: A small voltage leaves the radio on pin 1 and heads to the mic circuit. Here, the DC goes up through the resistor (which limits current to the JFET or it would be toast). This goes into the mic powering the amplifier. The amplified audio comes out the middle pin and goes through the capacitor back on to the same wire into pin 1 of the radio. The .47 cap in the diagram (that value is a bit suspicious to me thanks to K7MEM pointing it out) is to block the DC from going into the mic and charging the condenser mic (which is already charged by the static charge on the electret material). The 10uF cap is to filter the DC so the audio from the mic doesnt get into the JFET supply voltage as that would also be a bad thing. You can ignore pin 2,3,4 and 8. Pin 5 is PTT (TX) and that needs to be connected to pin 6 to activate TX. You can figure out which wires on your new mic are for TX by holding the PTT button and seeing which 2 wires go conductive while holding the PTT. Then release the PTT and see which of those two shows absolutely no conductivity to any other wire, this one is the TX wire. The other is ground ~ possibly for the mic as well, it really depends on the mic

    feel free to email the pictures to me
    [email protected]
    Last edited by brandon lind; Wed 22nd Jan 2020 at 16:22.

  9. #9

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    I altered the picture you sent me in a photo editor to show the proper wiring and emailed it back. I tried to post the picture here but I am out of storage space on this forum (hoping the admins can help with that). Just be sure to have the positive side of the capacitor facing pin 1 (the cap - stripe facing the mic) or the capacitor will very rapidly turn into a bad version of a resistor and damage the dynamic mic's coil.
    Last edited by brandon lind; Thu 23rd Jan 2020 at 15:26.

  10. #10

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    Quote Originally Posted by brandon lind View Post
    I altered the picture you sent me in a photo editor to show the proper wiring and emailed it back. I tried to post the picture here but I am out of storage space on this forum (hoping the admins can help with that). Just be sure to have the positive side of the capacitor facing pin 1 (the cap - stripe facing the mic) or the capacitor will very rapidly turn into a bad version of a resistor and damage the dynamic mic's coil.
    Thank you Brandon!
    1. As per email, I did clean black outer around the yellow wire to expose the yellow on the mic socket side.
    2. I will do the cap now again, I had it the wrong way around!
    3. Can the forum moderators assist to get that photo up! That is what this is all about!

    See, even up to the end even using and testing the mic... I still did something wrong! Never be too sure of yourself!
    With kindness and gratitude to Brandon! de Johan

  11. #11

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    Try to post picture again...
    [IMG]AF1QipOCi-V2Sh7uKB7awY1XgqjJlWye28jXOzqywDb8.htm[/IMG]
    Let see if this works!

  12. #12

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    The mic does work now, correct?

    DSCN9774(1).JPG
    Last edited by brandon lind; Thu 23rd Jan 2020 at 15:34.

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