ICOM IC-M402 Radio Rehabilitation

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
Posts: 8610
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

ICOM IC-M402 Radio Rehabilitation

Postby jimh » Tue Dec 11, 2018 11:15 am

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

Author's note: this article is a revision of the original, which appeared in March 2014, with several updates and additions.

For over 17-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 c.2001 the IC-M402 radio was an early digital selective calling (DSC) radio, and was qualified to an early specification called the RTCM SC-101, for Radion Technical Commission on Maritime, Special Committee 101 (for digital selective calling). 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 five significant 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 five problems, returning the radio to a useful condition.

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.

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.

Fig. 1. A sketch of the hand microphone showing the problem with the off-center hole for the microphone sound port.

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.

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.

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—at least mine does. Nothing else works.

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.