Icom HM-77a Mic Fix

This Mic is used on the IC-2340, IC-281, and I believe it can be used on the IC-706. It is a DTMF mic and is handy in that it has several DTMF "speed dial" memories, as well as UP/DWN buttons to control the rig. It is no longer made by Icom, and as far as I know, Icom never released a schematic of it.

This mic developed a couple of common problems, most notably it would key up the rig all on its own. The other problem is that DTMF audio level would drop and become too low to be effective. There is a workaround on the web for the keying problem that has you cut the green wire coming from the main board (with the LED and DTMF pad contacts) that goes to the mic cord, and connecting the mic cord end of the green wire to the red wire coming off of the PTT switch (taping off the board end of the green wire). This bypasses the PTT line coming from the DTMF board so that the mic will not key the rig spontaneously. However, you lose automatic keying when you press a DTMF button.

My mic developed the phantom keying problem some years ago, so I did the above workaround. But recently, it lost DTMF altogether. Having been a commercial 2-way and public safety radio tech for many years, I wasn't ready to throw it in the trash. So I decided to dig around a bit.

I removed the 3 screws holding the case together. Then I carefully removed the rubber DTMF key pad from the main board. Next to the LED, there is a small phillips screw that holds the main board to the case. After removing the screw, I carefully flipped the board over (you may have to disconnect a couple of the cable plugs on the board).

There I saw them. There are five early version surface mount electrolytic capacitors mounted on the board, three are 1 Mfd @ 50 VDC, two are 22 Mfd @ 6VDC. They are small silver and black "cans". I have had to replace a lot of these in commercial 2-ways of the late 80's to 90's vintage. It seems these early versions wouldn't last more than a few years. They usually leaked their electrolytic and went open. Sure enough, at least one solder tab on three of them was discolored.

I replaced them with small epoxy tantalum caps. OBSERVE THE POLARITY - The Black Mark on the original can denotes the negative lead. If you are comfortable and have the equipment for surface mount work, then go for the newer SM caps. I used Jameco #33662 for the 1 Mfd@35 VDC, and Jameco #545852 for the 22 Mfd@6 VDC, epoxy tantalum capacitors. The maximum voltage in the mic circuitry is 8 volts, so any capacitor with a rating a few volts higher than that will work. With a small soldering pencil and a large magnifying glass, I was able to install them.

WARNING - If you are not familiar/comfortable with surface mount soldering, DO NOT ATTEMPT THIS FIX YOURSELF.

I have restored my mic to full operation. Hope this works for you.

73, Bill, W8LGX