Incoming DSC emergency call blew my instruments (I think!)

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steve1963

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I was out at the weekend, when I received an incoming DSC VHF emergency call. I have never had one of these, and, as I was single handing, I just popped below to cancel the alarm (by pressing the channel 16 button). I then went back on deck for a minute or two, before going back below to monitor what was happening. At this point, I noticed a slight burning smell, which scared the hell out me. Couldn't find where it was coming from, and it quickly disappeared, so I decided to investigate when I returned to the mooring. It was then that I noticed that the instruments (depth/log multi, and a wind instrument) had gone down (fuse blown). When I investigated further, the plotter/GPS had also blown. This was not switched on at the time. I'm really confused as to what happened. Any ideas? When using DSC connected to a GPS(plotter) is it necessary for the plotter to be switched on?
 
There is no requirement for anything to be switched on - other than the fact that you may not have full functionality with one or more components switched off. The only issue I can think of in circumstances such as this is if the triggering of the alarm resulted in an increased current demand and the fuse fitted was on the borderline for the installation. Our DSC radio will turn the volume up very high if you don't acknowledge the call reasonably quickly. It is not uncommon for either the DSC radio or the plotter to include an output that is intended to drive an external klaxon in the event of an alarm and this may be able to deliver a significant current - if your installation has that output and it is short-circuited, then that could also result in the fuse blowing when an alarm sounds.
 
Hi Maby,

Thanks for the reply. The fuse to the VHF remained intact, but the fuse to the instruments blew. I noticed that the plotter (which blew, even though it was not switched on) also has an additional in-line fuse (2A i think) which did not blow. I was wondering if there could have been a current surge via the NMEA connection?
 
Sounds like a co-incidence, especially as the radio survived. All that would happen with an incoming DSC alert is that the radio would send out a message. Assuming NMEA0183 that would be the DSC & DSE sentences. Should cause no consternation to your chart plotter - it'll either understand them and display the position or it won't and will ignore them.
 
steve1963 said:
Hi Maby,

Thanks for the reply. The fuse to the VHF remained intact, but the fuse to the instruments blew. I noticed that the plotter (which blew, even though it was not switched on) also has an additional in-line fuse (2A i think) which did not blow. I was wondering if there could have been a current surge via the NMEA connection?
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Did it all work after you replaced the fuses? If so, I doubt very much that it was anything to do with the NMEA connection - that would suffer permanent damage if it carried anywhere near enough current to blow fuses. I would suspect that there is an output somewhere on the network that is intended to drive an external klaxon and that it is shorted.
 
Hi Maby,

No, instruments now no longer work. I'm really confused, as the plotter appears to be damaged, even though it was not even switched on. It also has an in-line fuse (2A) which was unaffected. Multi-instrument display and wind-instrument also appear to have been destroyed.
 
Was there an actual dsc message? Maybe the 'alarm' was actually a cry of pain from the vhf, which survived the electrical problem unlike your other instruments.
 
steve1963 said:
Hi Maby,

No, instruments now no longer work. I'm really confused, as the plotter appears to be damaged, even though it was not even switched on. It also has an in-line fuse (2A) which was unaffected. Multi-instrument display and wind-instrument also appear to have been destroyed.
Click to expand...

Hmmm, well, there is no valid operational condition that would produce that result. Something must have pushed a significant current into a circuit that was not designed for it! What brand of equipment is it?
 
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