ICOM SM-2 Microphone

A friend of mine gave me a ICOM IC-735, when it was giving him trouble. He could operate the radio but was lacking in support capabilities, if there was problems. The radio included a ICOM SM-2 microphone. When new, there is a removable windscreen. However, by the time I came in to possession of it, the windscreen was missing. I think you can get a replacement, but I never bothered.

The ICOM SM-2 is a unidirectional desktop microphone. Specifically it is a pre-amplified 600 Ω Electret Condenser type, produced by ICOM during the 70's. It was designed to fit the IC-701, IC-551, and IC-211 radios. The SM-2 features a switch for PTT (push to talk) with lock feature.

When I was trying to find a good schematic diagram, I found this spec sheet. I also found this schematic diagram which appears to be a redraw from the spec sheet. The spec sheet has some hand modifications that are intended for use with a radio that does not have voltage on the audio for powering the Pre-Amp. An external power supply would be needed to power the Pre-Amp and the microphone itself. The small schematic insert, on the spec sheet shows a modification that can be done to your radio, to make it directly compatible with the microphone.

It seem that radios, like the IC-211, had 4-Pin connectors as opposed to the 8-Pin microphone connector on the IC-735. I guess that, somewhere along the line, ICOM also produced the SM-2 with a 8-Pin connector. In the response characteristics listed below, other sources indicate that the output impedance is around 600 Ω rather then 130 Ω as listed.

• 130 Ω +/-30% at 1000 Hz
• -56 dB +/-5dB at 1000 Hz, 0 dB = 1 µV
• Frequency Response: 150 Hz to 8,000 Hz

So, to figure out the actual schematic, I took the microphone base apart. Inside the base are effectively three pieces. The Volume Control, the PTT switch, and the Pre-Amp PCB. There are also two cables. One comes down the free-bending neck and another that goes to the radio, with a 8-Pin microphone connector. The schematic, below, is the result of that disassembly.

The schematic for the SM-2 is split into several sections. On the left is the "Microphone Head". This shows a bit of circuitry that is built into the head of the microphone. I didn't disassemble the head to prove it, but there is a 3-wire cable (shielded audio cable plus power) coming from the microphone. So I assume that it is correct. The 3-wire cable coming from the microphone connects to the Volume Control (VR1) and the Pre-Amp PCB. The Volume Control is mounted on the bottom plate.

Comparing the actual SM-2 PCB to the original schematics, I found that R1, R2, and R3 were different. The actual values should provide for a little higher output than the original values. That's a good thing since the original SM-2 was said to have low output.

The SM-2 gets it's pre-amp current from the MIC+ input wire (1-White) which has a +8 to +12 V potential. The voltage is tapped off at the junction of R5/C5 and is isolated from the pre-amp with C4.