This how-to document explains the steps on how to modify an Icom HM-36 microphone. Note: this modification is applicable only for HM-36 unit which has a Made in China tag in the back of the microphone unit. Figure 1 shows internal unit of a HM-36 microphone. There are four components that will be removed for the purpose of this modification. There components are two resistors (labelled R1 & R3) and two electrolytic capacitors labelled C1 and C2) as shown in Figure 1. Remove all of the components mentioned above using de-soldering suction unit.
Figure 1. Original circuitry of an Icom HM-36 microphone.
Figure 2 shows HM-36 PCB unit after the removal of components R1, R3, C1 and C2.
Replace C2 with a 1.5K resistor. Replace R3 with a 10K resistor. As shown in Figure 3.
Insert a small signal NPN transistor (i.e., 2SC 828) to the available holes as shown in Figure 4 (left).
Note that for 2SC828 insertion. The emitter pin is at one hole of the R1, the Base pin is at negative hole of C1 and the Collector pin is at positive hole of C1. Pin assignments of 2SC 828 is shown in Figure 5.
Figure 5. Terminals position of 2SC828 transisor. The Collector (C) and Base (B) terminals are shown in the picture (above). The Emitter (E) is not shown.
Next step to is to remove and reduce the thickness of the sponge placed in the condenser microphone hole. Take out the condenser microphone carefully at remove the grey sponge and cut it into half, as shown in Figure 6. Half part of the sponge is not used. This is reduce the thickness of the original sponge.
Figure 6. Removed gray sponge and cut into half.
Insert one half of the thinner sponge into the condenser microphone hole and installed the condenser microphone unit back to its original position.
Re-assembled the modified HM-36 microphone and it is ready to be tested.