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Old 11 May 2011, 08:34   #1
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VHF reverse polarity diode

VHF keeps blowing fuses every time it's switched on. From research I suspect it may be the reverse polarity diode that has blown (which would mean the polarity was reversed at dome stage????). I've nothing to lose now so going to Open up the radio (cobra 55) and remove the diode.
Anyone ever try this???
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Old 11 May 2011, 08:49   #2
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VHF keeps blowing fuses every time it's switched on. From research I suspect it may be the reverse polarity diode that has blown (which would mean the polarity was reversed at dome stage????). I've nothing to lose now so going to Open up the radio (cobra 55) and remove the diode.
Anyone ever try this???
Doubt it very much, Normal failure mode on a Diode is to go open circuit!!
The system works by the Diode being normally reverse biased, when it sees reverse polarity the diode conducts thus blowing the fuse and not the radio. I assume polarity is correct.
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Old 11 May 2011, 10:51   #3
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Yes and by removing this now damaged diode I am hoping the radio will function again although I will now not have the benefit of this check valve fail safe and if polarity is reversed again the entire circuit will fry.....

I think its worth a shot, out of curiosity if nothing else, as otherwise it's heading for the bin.
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Old 11 May 2011, 10:57   #4
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As I said the normal failure mode for FMECA purposes is that a diode fails OPEN !!! i.e blown !!!
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Old 11 May 2011, 11:25   #5
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If you're going to open it up, do a bit more poking around than simply removing the diode that, as Ashby says, likely isn't the problem.

I'd suspect (though I've never investigated) that the radio has some voltage regulating circuitry just in from the power feed; that's a more likely culprit than the diode, which would simply block any voltage induced reverse current.

You may be able to find schematics on-line, if you're lucky.

jky
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Old 11 May 2011, 12:57   #6
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Found this
VHF blowing fuses... - The Hull Truth - Boating and Fishing Forum
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Old 11 May 2011, 17:40   #7
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Many years ago, I was involved in CB radios (when they were a bit illegal!!!) Reverse polarity was a common problem..
Yes you are right, you should be able to remove the diode - hopefully it did its job and protected the radio - if correctly rated fuse all should be well!

Try and replace diode (easily got from Maplins or RS components) as without it there will be no protection..

Mike
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Old 11 May 2011, 17:58   #8
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I don't understand the hull truth thread... I suspect it's not the whole truth. Nor do I understand the comments in this thread re the diode. Normally a reverse polarity protection diode is simply in series with the power supply. Reversing the supply polarity should do no damage because the diode will not conduct in that direction. Diodes do not fail when reverse biased, that's their job, they behave as one way valves for electrical current.
I guess it's possible your radio is in some way unusual but I suspect a different fault if it keeps blowing fuses.
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Old 11 May 2011, 18:19   #9
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Nor do I understand the comments in this thread re the diode. Normally a reverse polarity protection diode is simply in series with the power supply.
The diode is in parallel with the power supply.

When radio is connected properly current cannot flow through diode as it only allows power to flow one way.

If you connect the supply wrong way round, it allows the current to flow directly across the input causing the fuse and normally the diode to blow - normally quick enough to protect the equipment.

Although you may then connect power supply correctly, the fuse will continue to blow as the diode has "blown" and is allowing current to flow both ways.

Over 30 years or so ago, I used to make a fair bit of pocket money 'Fixing' broken CB radios that had been connected wrongly. (about 1 in 25 were damaged beyond repair, normally because the wrong in-line fuse had been used!)

Hope you can understand this explanation


Mike
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Old 11 May 2011, 18:42   #10
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Yes it took me a while to get my head around it too but bingomike has explained it very well. Cheers Mike.

Took the radio apart this evening and removed the diode this. I will try fitting it tomorrow and see how it goes.

As regards replacing it, I may look into it but reckon you would need to be fairly confident with your soldering to to it, it is in an awkward spot and tiny,
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