ICOM IC-M402: Rehabilitating an older VHF Marine Band Radio

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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Location: Michigan, Lower Peninsula
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ICOM IC-M402: Rehabilitating an older VHF Marine Band Radio

Postby jimh » Wed Jan 17, 2018 5:10 am

ICOM IC-M402: Rehabilitating an older VHF Marine Band Radio

For over fifteen years I have owned an ICOM IC-M402 VHF Marine Band radio. I believe I purchased this radio in c.2001, and used it on at least two of my boats. In 2007 I purchased another VHF Marine Band radio, a newer model from another manufacturer that was rated for digital selective calling to Class-D specifications. The ICOM IC-M402 radio was retired from active use on the boat and became a bench radio, used mostly to listen to NOAA weather radio.

Recently my interest in the old M402 was renewed. I began to use it on the bench for some testing. I found the radio had become very unsatisfactory. It had several problems:

--display contrast was terrible; the LCD display could barely been seen;

--transmitted audio modulation was very bad; the modulation level was extremely low and its audio characteristic was excessively "tinny";

--the DSC features seemed impossible to use;

--the DSC section of the radio appeared to be ignoring any NMEA input;

--the receiver appeared to no longer be monitoring Channel-70 for DSC calls.

I was at the point of throwing the radio into the trash. Fortunately, I discovered that I could remedy all of these problems.

DISPLAY CONTRAST. The secret to restoring display contrast was discovered (accidentally) while reading the manual to search for some other information. Display contrast is only controlled by escaping from the normal control sequence during power-on. To branch into the special control section, hold the down the "16" button and turn the radio to ON from OFF. The radio then enters a SET MODE configuration. The last option in SET MODE is the LCD CONTRAST. See page 20 of the user manual for the procedure, as it is too complicated to describe here.

Once the display contrast improved, the overall operation of the radio became much easier. When the display was in low contrast, it was very hard to see any detail on the LCD, and only the very large channel numbers could be clearly seen. Now, with better contrast, much more information was visible on the display.

BAD TRANSMITTER AUDIO. The modulated audio on the transmitter was very bad. A Google search turned up a ten-year-old posting on THEHULLTRUTH that mentioned this problem occurring on many M402 radios, and that ICOM in c.2004 was offering a free repair as a remedy. I called ICOM to inquire about this, but their present day technical support had no recollection of this repair, and they indicated they had almost no records of any problems like this with the M402. It was clear ICOM was not going to be a source of a remedy. I would have to fix this problem myself. I disassembled the microphone and found the actual cause, which was very easy to fix. The result is an astonishing improvement in audio quality on the transmitter modulation.

The problem occurs as follows: the front half of the molded microphone housing has a circular cup molded on its inner face into which a rubber gasket is fitted. The electret condenser microphone on a circuit board inside the microphone housing is positioned to fit into this rubber gasket. The rubber gasket is tubular, and does not restrict any passage of sound to the microphone. A very small hole in the molded housing opens a path for air to reach the microphone element. This small hole is supposed to be located at the center of the circular cup molded area, but on this particular microphone (and probably many other M402 microphones) the little hole is far off center. This is significant.

To provide some protection against ingress of water, the inner face of the housing had a gauze material held in place by a ring of double-sided plastic adhesive. The plastic adhesive area is in a ring around the center of the gauze, leaving only a thin film of gauze in the middle, through which air and sound waves can easily pass. The problem occurs because the hole in the housing is far off center. When the gauze was inserted into the circular cup, the plastic adhesive material completely covered the off-center small hole, effectively sealing off any passage of sound waves. This drastically reduced the sensitivity and frequency response of the microphone, causing the transmitter to sound terrible.

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I removed the original gauze material and its adhesive ring. In its place I inserted a very similar small piece of gauze that I harvested from a tiny plastic circular Band-Aid I found in my medicine closet. I did not use any adhesive on the gauze, because the rubber gasket was sufficient to hold it in place.

I reassembled the microphone. The difference in transmitter audio was astonishing. The improvement was remarkable. The problem of bad transmitter audio was remedied in five minutes at the cost of one small Band-Aid.

DSC FEATURES UNUSABLE. The DSC features of the radio were essentially unusable when the display contrast was too low. However, even with the display contrast restored, the DSC features are barely usable. The only remedy for this is to always have the instruction manual about six inches away from the radio. I found it necessary to (literally) read instructions step by step in order to cause the radio to perform some DSC function. The radio surprised me with the set of DSC features it contained, but accessing them was extremely difficult. Other than memorization of an astonishing number of button press sequences involving buttons whose legends are marked for other, non-DSC functions, I don't think there is a remedy. But, with the instruction manual as an immediately available reference, I could coax the radio to perform many DSC functions that I did not realize it had.

NMEA INPUT IGNORED. The ICOM M402 seemed to be ignoring all NMEA input from a GPS receiver. This problem was resolved by persistence and experimentation. The radio will only respond to $GPGGA sentences. This problem was also discussed with ICOM technical support. They did not acknowledge this behavior in the M402, and wanted to insist the radio could recognize other NMEA sentences besides $GPGGA. Extensive experimentation showed that was not true. The M402 wants only to hear $GPGGA on its NMEA input--well, at least mine does. Nothing else works.

LACK OF CHANNEL 70 DSC MONITORING. The radio appeared to be completely deaf to any DSC transmission on the DSC channel, Channel 70, unless the radio happened to be manually tuned to Channel 70. This turned out to be another problem in the SET MODE configuration. Like the DISPLAY CONTRAST, the SET MODE special configuration menu can set DSC WATCH to on or off. The manual says that the default mode is for DSC WATCH to be set to ON. I have no recollection of ever changing this parameter, and I found it set to OFF. Once I found SET MODE and changed DSC WATCH to ON, the radio began to monitor the DSC channel and respond to DSC transmissions it heard.

Over about two or three days of working with the old ICOM IC-M402 radio, I was able to find remedies for all of these problems. The radio was on its way to the trash bin a few nights ago after it seemed frustratingly unusable, but it has now been rehabilitated. I hope some of the problems and their remedies that I describe may be useful to other owners of an ICOM IC-M402 radio.

[Note: This article was originally published several years ago in the old forum. I have reposted it here in the new forum to make it easier to find.]

jimh
Posts: 6251
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: ICOM IC-M402: Rehabilitating an older VHF Marine Band Radio

Postby jimh » Wed Jan 17, 2018 5:12 am

After I modified the microphone of the ICOM IC-M402 radio, the quality of the audio modulation improved dramatically, as I noted above. I then made some comparison tests of the transmitter modulation of the M402 compared to my Standard-Horizon GX1500S. To my surprise, I heard the ICOM radio sounding clearer and better than my Standard-Horizon.

I used a third radio as a neutral test receiver, and I recorded its output into a digital audio recorder. This allowed me to replay the test transmissions many times to compare them. I even applied some amplitude leveling transforms to make the two signals more equal in loudness. Loudness always plays a part in deciding what sounds better. The ICOM still sounded better. This was quite an eye opener--or maybe an ear opener.

I decided to investigate the microphone of the Standard-Horizon radio. Upon disassembly, I found the microphone employed a similar water-resistant membrane in the sound path to the actual microphone element inside the case of the microphone, with a similar rubber cylinder gasket. The membrane was more porous than the one found originally in the ICOM. I removed this membrane, and, as I did with the ICOM, I replaced it with some water-resistance gauze material pulled from a plastic Band-Aid strip's pad. Because of the arrangement of the rubber gasket inside the Standard-Horizon microphone, I could not put the gauze in the cup-recess. Instead, I just pushed some gauze into the narrow center passage of the rubber gasket. I reassembled the microphone and retested.

The result of removing the membrane and substituting gauze was an improvement in the transmitter modulation of the Standard-Horizon radio. It was now the equal to or perhaps even better than the modified ICOM radio.

I am sure that removing the membrane in the sound path has compromised the resistance of these microphones to immersion in water. I will have to take more care with these microphones in any sort of wet environment. The improvement in the quality of the transmitter modulation is worth the risk of decreased water resistance, in my opinion and for my boat's operating environment, which is typically not completely open to rain or spray.