Simple modification of the HM-36 Microphone from ICOM

The Icom HM-36 microphone is a simple electret mic. But many owners notice that some microphones sounds bad. What reason?

There are two different types of ICOM microphone HM-36 (exclude chinese fake):

First type (old model) sounds well, but other type (new model since late 2007/begin 2008) sounds pretty bad. The difference between two types you can find very simple. Old type microphone has three wires in cable – Black, Red and shield (grounded wire). New type has only two wires. If you have mic of second type, perhaps you needed to modify it, because after modification you will feel that higher and lower frequencies becomes better.

You need follow next steps:

1. Disassemble the housing of new type microphone
2. Find on the PCB Potentiometer R1 (10 kOhm) and Capacitor C2 (10 uF).
3. Remove them with soldering iron
4. Replace capacitor C1 with other nominal. (Better will be nominal 0.01 uF)

For better quality C1 should be tantalum capacitor (Although type is no too critical, and also it can be ceramic).

That`s it. Now you can assemble your mic and test it. I hope you have got better frequency characteristic than before.

Internal view

Schematic of HM-36 (before and after)

N.B. In some cases instead 2 kOhm potentiometer you can find 22 kOhm. Please keep in mind this.