HM-36 Modification to make it work with an IC-7200

I recently acquired an IC-7200, but quickly found that it’s TX audio was HORRIBLE! Friends were telling me the audio was "Muddy", "Distorted" and had no "Highs".. I was hardly recognizable..... I tried all of the simple internet fixes (blocking cap and resistor changes) to the HM-36, but nothing helped. Different microphones/headsets were also tried, along with numerous electret/condenser elements with no significant improvement.

Transformer coupling my signal generator to the radio produced a beautiful audio passband response, so why did the microphone sound so BAD? Surely ICOM would NEVER include an incompatible Mic with a new radio...Right?

My first attempt at correcting the problem was to isolate the Mic Element from the IC-7200 using a miniature 600:600 ohm audio coupling transformer. This did not make much difference, but moving the secondary connection from the 600 ohm tap to the transformer center tap (150 ohms) did the trick. What an amazing difference!

Here's the "FIX":

Open the HM-36 and remove R1 & R3 and C1 & C2, but leave R2 (1k ohm) (Next to the "DN" push button switch).. A miniature 600CT:600CT audio coupling transformer was pressed into the small depression near the top of the front half of the mic shell. (Xfmr Primary wires to the mic element). A 100ufd 10V tantalum cap was used to bypass the low side of the transformer to MIc Low (Ground), and is located between the transformer's connections. This capacitor prevents the +8V, superimposed on the Mic Hi lead, from being grounded. The +8V passes thru both windings of the transformer, to bias the electret element. Total cost is about $6 bucks. Here's the schematic of the modified HM-36.

Here's a couple of closeup pictures of the interior of the HM-36 showing the PCB connection points of the three wires that connect to the audio coupling transformer. The Xfmr simply presses into the empty space where an older mic element was located. The 100uf tantalum capacitor sits between the transformer's connections. The Yellow wire connects the electret element (Mic-Hi) to the primary side of the Xfmr (600 Ohms); the Red wire connects the center tap of the transformer secondary (150 ohms) to the mic cable which connects to the IC-7200. The Black wire connects to Mic-Low (ground).

HM-36 PC Board

Transformer Mounting Location

This last image, taken from my IC-7700's Bandscope display, shows the flat audio response of the IC-7200 with the modified MH-36.

IC-7200 Audio Bandpass

If you have an IC-7200 radio with the provided Chinese HM-36, you are most likely experiencing really crummy TX audio, unless your friends are lying to you. J It all seems to boil down to the fact the IC-7200’s microphone impedance is much lower than the HM-36. There a several mods out there that claim to correct the TX audio problem, but they did not work for me. This DID! If you call Icom, they will probably not acknowledge any complaints or problems with the IC-7200 and MH-36. Friends cannot believe the difference this Mod has made.
Good Luck.
Frank – K7SFN