improving the audio on the ICOM IC-27

Make an otherwise fine radio even better

The ICOM IC-27 FM transceiver remains, in my opinion, one of the most compact and versatile mobile units available. The performance of my IC-27A was very impressive, with just one noticeable exception: the received audio was poor. Low volume and poor treble response made many noise-free signals all but unintelligible. Many other Amateurs have experienced this problem as well. Fortunately, there's a very simple remedy.

My first inclination was to experiment with an external speaker. The speaker included in the IC-27 is very small. Often such small speakers are inefficient and have poor response. Yet experimentation with a much larger speaker with a more powerful magnet showed absolutely no discernible difference. The only way one could hope to increase the volume would be by mounting an external speaker closer to the user's ear. This would alleviate only part of the problem, however, since the tonal response would remain virtually unchanged. The latter fact was an important clue, because it led me to believe that the audio amplifier inside the IC-27 was the culprit.

strategy

The solution to this problem is simple. Just change one resistor and one capacitor in the audio section. There's absolutely no reason to fear the operation; it's very simple. ICOM has provided excellent schematics with the radio, the board layout is not terribly compact in this area, and all cables running to the important circuit board are provided with well labeled pulloff connectors in an unambiguous configuration. Aside from forgetting to replace a connector, it's virtually impossible to connect anything incorrectly.

As shown in the ICOM schematic, not reproduced here, Q5 and Q6 form an active filter which seems to effectively filter the intelligibility of many Amateurs' speech. Capacitor C67 was found to form the bottleneck; a smaller value is required here. This greatly improves the treble response.

The audio output power was increased by changing the feedback resistor, R31, of the audio power amp, IC6 (on the ICOM schematic). It was discovered that a larger value would give greater gain while still not introducing objectionable distortion.

modification procedure

Begin the modification by turning the transceiver over (speaker side up), and removing the four screws holding the bottom cover. Carefully place the bottom cover to the side (as shown in fig. 1) sd as not to strain the speaker wires. The connector for the speaker wires can then be pulled at the circuit board.

The objective is to remove the circuit board in order to gain access to the parts which must be changed. Carefully remove connectors J2, J6, JI0, and J12. With your fingers, remove the cover plate of the synthesizer section lit's the large shiny box on the righthand side). Remove the four screws in the corners of the large circuit board. Remove J1 (the entire circuit board at the rear of the unit). Remove the two heat sink screws on IC5 and IC6. Remove the hex spacer formerly under the rear circuit board.

At this point, the entire circuit board can be lifted out. Carefully pull it out and locate the audio section toward the front on the left side, as viewed from the component side. Locate C67. This is a 0.01 µF capacitor near IC6. Using a solder wick or a solder sucker, remove C67 and replace it with a tiny 0.001 µF capacitor. A small ceramic part will do nicely. Then locate R31. This is a 150-ohm, 1/8 watt resistor near J6. Replace this with a 270-ohm resistor. If possible, use a 1/8 watt part so that it will fit easily in the circuit board. Clean your solder connections and carefully check for solder bridges.

This completes the modifications. Replace the circuit board and reverse the disassembly procedure, being careful to replace all removed connectors.

results

After performing these modifications, I was much happier with the IC-27. Audio volume and clarity are much better than with the original unit, even with the original speaker. In fact, I seriously doubt the need for an external speaker. The internal speaker provides plenty of power,even in my very noisy automobile. I'm sure other Amateurs will agree that this modification makes an otherwise superb radio even better.