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Rick Glidden (KA1AIL) on February 16, 2013
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By, ka1ail Rick Glidden

This is the fix that I came up with for the poorly made HM-56A Microphone .

Warning! you won't have the touch tone buttons anymore or the battery but you will have a good microphone with the up/down buttons. So if you can live without them, give this a try and start using you radio again.

First open up the microphone by removing the three Philips screws.

Then remove the screws from the processor board and the Microphone board.


Take these boards out and Clip the wires at all of the connector plugs on both boards and put them in the junk draw this frees up all wires 12 total.

Now at the top of the Microphone where the up/down buttons are, skin back and tin the ends of all three wires, Red, Black, and Green.


Then solder the green and Black wire together, now add a short length 1 1/2" of black wire to the end and solder them, then put a small piece of shrink over the joint. Now solder the free end to the grounding bond at the bottom. "see picture"


Now solder the remaining red wire from that group to the thin black wire coming from the microphone cord but first put a piece of shrink on it.

That completes the up/down switch.

NOTE the microphone cord has two black wires, the bigger one has the sleeve over it, the other is the actual smaller wire we want.

Now the PTT switch wire red gets soldered to the Green wire from the cord but remember to put the shrink on first and that completes the PTT.


Now for the microphone, either undersold and remove the element from the microphone board or use a new one, anyhow solder the short black wire from the element ground pad to the ground on the bottom of the microphone. "metal Bond"


Now clip the ends of the resistor to about 3/8 long and solder a 10k 1/4 watt resistor to the other pad, also solder the white wire from the cord to the same pad.

Now the other end of the resistor gets soldered to the red wire from the microphone cord and dont forget the shrink.

Now clip that fat black ground wire off on the cord with the tube over it and we are done soldering.

I glued the t-tone rubber back in for aesthetics!

Now stuff it all back together and put in the screws, DONE!

Now the microphone works and has the up/down function also.

73 Rick

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by AG2AA on February 16, 2013 Mail this to a friend!
I have one of these mics and i'm just wondering what problem is being addressed by this procedure, other than removing the DTMF?
RE: HM-56A  
by AA4PB on February 16, 2013 Mail this to a friend!
The original circuitry on these mikes is rather complex. It contained a battery powered processor than handled DTMF, mike audio, and PTT.

Icom has a service buletin that says it is common for the aging electrolytic capacitors to short and cause the PTT to remain keyed. The Icom solution is to replace all (4 I think) electrolytic capacitors. The problem is that they are all surface mount devices on the PC board so it's not an easy task for those who do not have the appropriate tools and experience.

Provided that you can do without the DTMF functions, the described procedure of removing the PC Board and hard-wiring the mike cartridge and control buttons directly to the radio is and easier solution.
RE: HM-56A  
by WB6DGN on February 17, 2013 Mail this to a friend!
"The problem is that they are all surface mount devices on the PC board so it's not an easy task for those who do not have the appropriate tools and experience."

Its not that hard to r & r a surface mount capacitor or resistor and it doesn't require special tools to do a passable job.
While I agree that a TT keypad is generally not necessary, I think I'd rather replace the caps. than have a modified mic. that WILL be a point of discussion if the radio is ever sold.
People had better get used to working with surface mount as it is certainly NOT going to go away. What is a guy going to do when a surface mount fuse blows? Send the radio out for repair for, probably, around $100 (with shipping)? For a $1.00 fuse? You know, fuses DO blow for no reason other than a bad fuse. I'd rather get my feet wet and get the job done.
by KE6SLS on February 18, 2013 Mail this to a friend!

I think it seems goofy to have a battery in the mic in the first place. Seems like a small mod could make it possible to use the radios dc instead. I don't have one of these dudes so I can't comment.

I would have done the caps--even if you don't have a magnafying glass, you could always ask your local shop to do it or even let you use their gear for a few minutes!

Anyway, thanks for the post--could be the cure for many hams.

RE: HM-56A  
by W0CBF on February 19, 2013 Mail this to a friend!
Another example of how radio makers over complicate everything including a microphone. Whatever happened to a microphone having just a mic element, push button and a coiled cable! By the way, when are vacuum tubes making a comeback! 73's WCBF
RE: HM-56A  
by AA4PB on February 19, 2013 Mail this to a friend!
"Whatever happened to a microphone having just a mic element, push button and a coiled cable!"

That's what the author converted it to :-)
by K1DA on February 20, 2013 Mail this to a friend!
For decades Icom has had +8 on a mike connector pin. What's happening now?
RE: HM-56A  
by AA4PB on February 20, 2013 Mail this to a friend!
The HM-56A went with the IC-229H 2M radio mfg starting in 1990 so it is not new. I think the radio did have +8V DC on the mike connector so I'm not sure why Icom decided to use a battery in the mike to power the circuit. Maybe they wanted to use the mike on some older radios that didn't have power available.

I expect they routed everything through the processor so that pushing a DTMF button would automatically mute the mike, key the PTT, and generate the DTMF tones.
by N6CAZ on March 15, 2013 Mail this to a friend!
I actually repaired one of these mics not too long ago. Icom has a service bulletin that covers replacing the four capacitors, and yes they are surface mount and crammed in on a very crowded circuit board.

I had the typical symptom - the rig would start transmitting as soon as the mic was plugged in.

Replacing the four capacitors - aluminum "solid" caps - seems simple enough, but I think where it gets tricky is when you attempt to lift the cap off the board, it is tacked down by glue and in the process a circuit trace or the solder pad under the cap can easily be lifted as well.

On the good side, the caps are relatively cheap from Icom and they do provide instructions on which ones to replace.

The reason I'd still replace the caps rather than gut the mic is that besides DTMF you also get remote rig functions off the mic as well. My guess is the replacement cap solution might also work for an HM-90a "wireless" mic they made too.
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