VHF Radio shows the wrong received channel in Scan Mode

[SVToybox]

Hi All,

I replaced my VHF with a new Icom M504-31, In scan mode when I receive a call I hear the call loud and clear but sometimes (more than half the time) the scan stops on a random channel, not the channel that is transmitting, so I have no idea what channel is being used. A repeat call may show a different wrong channel. It shows the wrong channel most of the time, but not always. The lower channels seem to be more likely to report the wrong channel, then higher channels. In normal mode (not scanning) the radio transmits well on all channels and receives well on all channels when the channel is manually selected. It does this even when all other equipment, including the engine, NMEA networks (0183 & 2000) are turned off or unplugged. Power to the radio is at 12.6 or above, VSWR of the antenna is 1:1, no corrosion or water in the connections or antenna. The radio performs as designed when not in scan mode. It appears that this primarialy occurs when the transmitting radio is near my vessel (less than 1/4th mile away), but I am not certain of this. It acts like the receiver is being overloaded by a strong signal.

Icom had never heard of the problem and was no help and I could not find any similar complains on the net.

In an attempt to isolate the problem to the Icom M504, I replaced the radio with a 10 year old Uniden 525 radio that was in its place before I installed the Icom and which I believed to scan properly when it was the installed for several years. Unfortunately, it does the same thing although it reports the correct channel more often, but still shows the wrong channel in scan mode about half the time.

Any ideas on the problem or where to go from here to isolate the problem?

William L. Bowen
513-425-9388
SVToybox@aol.com

[rwidman]

If it was only the new radio doing this, I would say it's defective and the store should exchange it for you.


With the new radio and an old radio both apparently having this problem, my first reaction is to say that you are nuts. This can't happen.


You probably aren't nuts and I have seen strange things happen in my day (I was an electronics technician).


I think I would take the radio and antenna home and hook it up to a car battery. You could just do it in the marina parking lot. See if you still have the problem. Use a handheld radio if you need a test signal.


I might first take the new radio back and exchange it. It's simpler.

[Lake-Effect]

Are you sure it's not some priority override - eg hearing a call on 16 while in normal scan mode?

Otherwise... given how the radio's "brains" are usually all in one chip, such a fault would be in the firmware and it's very very unlikely that such a gross system fault would have gone unfixed.

And in two very different radios?

If you and a friend have an afternoon to kill, you could call each other on every (permitted) channel while scanning.

[edit - given your information that it seems to be primarily when the transmitting boat is very close... it could be some sort of wierd intermodulation/distortion that causes spurs in different channels, but I think this is also unlikely. In this case the problem is not in the radios. Are you splitting the VHF antenna for use with AIS or the boat stereo? You could try removing any splits, and checking and tightening every connection in the antenna feed. Or you could get or make an emergency VHF antenna and try that]

[SVToybox]

Thanks for the replies, I removed the radio Icom M506 from the boat and confirmed that the problem still exists with new antenna and independant power supply, although primarily on channel 16. Will sent the Icom back to factory for repair, I can't return it to the seller because I had it to long before I install it. It appears the radio tends to show the wrong channel in scan mode the longer it is powered on. It may be over heating. Installed the old radio after re-routing the antenna wire and it now appears to be operating properly with the new and the old antenna.

Thanks for the help.

[Lake-Effect]

Originally Posted by SVToybox

I removed the radio Icom M506 from the boat and confirmed that the problem still exists with new antenna and independant power supply, although primarily on channel 16.

CH 16...Hmmm. Are you absitively, posolutely certain that you're not being fooled by the radio's dual-watch/tri-watch capability?

From the online manual, page 18:

Dualwatch monitors Channel 16 while you are receiving
on another channel; Tri-watch monitors Channel 16 and the
Call channel while receiving another channel. Dualwatch and
Tri-watch are convenient for monitoring Channel 16 when you
are operating on another channel.

Cool radio btw. Did you get the AIS version?

[SVToybox]

Yes, I am certain that it was not a dual or tri watch situation, neither are enabled. I did not get the AIS version because I have an AIS transponder tied to my Garmin MFD and my laptop. Interesting thro, the radio will shown the AIS targets on the AIS screen, but it will not scan in that mode.

Thanks for your help.

William L. Bowen
Nordic Tug NT 32-287
513-425-9388

[wad'nwind]

What scan mode is programmed or used?
Dual, triwatch or regular?

Verstuurd vanaf mijn SM-G903F met Tapatalk

[transmitterdan]

I think it would help to give us a more clear description of the problem. It is not displaying the "wrong" channel based on your description. Watch the radio and try to write down what it does when it stops scanning. Does it stop and display a channel # when a caller is being heard? Is it displaying the "right" channel while the call is being heard? What happens right after the caller stops transmitting? Does the scan remain on that channel for a bit or immediately jump to the next channel in the scan list?

What happens if you increase the squelch setting a couple steps more than what you normally use? Does it help or make the problem worse?

[SVToybox]

In regular scan mode, not dual or tri.

I think it would help to give us a more clear description of the problem.

It is not displaying the "wrong" channel based on your description. Watch the radio and try to write down what it does when it stops scanning. Does it stop and display a channel # when a caller is being heard?

Scan stops, on a random channel, most of the time, the caller is heard loud and clear. so if the call is on channel 16, the radio may stop on one of the other tagged channels, such as 68. The wrong channel is displayed more often when the transmission received is on channel 16 then any other channel.

Is it displaying the "right" channel while the call is being heard?

Most of the time no.

What happens right after the caller stops transmitting? Does the scan remain on that channel for a bit or immediately jump to the next channel in the scan list?

It stops during the transmission, then pauses for the normal time after the transmission is completed.

What happens if you increase the squelch setting a couple steps more than what you normally use? Does it help or make the problem worse?

The squelch setting does not seem to make any difference, hi or low.

In all other respects the radio receives and transmits well.

[transmitterdan]

Originally Posted by SVToybox

Scan stops, on a random channel, most of the time, the caller is heard loud and clear. so if the call is on channel 16, the radio may stop on one of the other tagged channels, such as 68. The wrong channel is displayed more often when the transmission received is on channel 16 then any other channel.

How do you know it is stopping on the wrong channel? Are you testing it with a friend?

[44'cruisingcat]

Do you have it set for the right set of channels? ie. International or US channels.


http://navcen.uscg.gov/?pageName=mtVhf


http://navcen.uscg.gov/?pageName=apps18

[SVToybox]

I verified that the radio was set for US channels and I tested with a friend, walking through various channels, one by one to confirm the transmitted channel with what was received on the Icom, what showed the wrong channel.

[transmitterdan]

William,

Since you have two radios with different software that do the same thing I would look for some commonality between them. I cannot think of anything antenna or coax related that would explain what you observed. The only other common element is the power feed to the radio. I guess I would check the wires carrying power to the radio. Maybe it sags down in voltage when the scan stops and somehow resets something. It is truly odd that two different brand and vintage radios do the same very odd thing.

[SVToybox]

I checked the voltage on the power supply at the radio, it was a constant 12.9 or higher, and after relocating the antenna coax the old radio seems to work fine in all modes. I install the Icom outside the boat with a separate battery and a new antenna, it worked better but still reported the wrong channel when receiving a transmission on channel 16, other channels seemed to work fine, so I mailed the radio back to Icom. I will see if they can identify the problem. Their turn around is 14 business days, so it will be awhile before I heard from them.

Thanks for the help.