A friend of mine was cleaning his mobile radio. When he went to plug
the power back in, the middle pin (offset) was loose and pushed back
into the radio, allowing him to put the connector in backwards. (hot
to ground) This killed the radio. (he had no inline fuse at the time)
I told him I would look inside of it for him. Once inside I found
nothing burnt. After fixing the dummy pin, and connecting power with
the correct polarity to a D.C. power supply on my bench, the radio
still wont come on. When I started checking for voltage, I found that
I had 12 volts on the chassis. It is on both sides of the on/off
switch with the switch off. The 12 volts is everywhere. Does anyone
have any ideas about what to check next? It is hard to use the meter
when everywhere shows 12 volts.... Thanks, Bill

tomagain wrote:
A friend of mine was cleaning his mobile radio. When he went to plug
the power back in, the middle pin (offset) was loose and pushed back
into the radio, allowing him to put the connector in backwards. (hot
to ground) This killed the radio. (he had no inline fuse at the time)
I told him I would look inside of it for him. Once inside I found
nothing burnt. After fixing the dummy pin, and connecting power with
the correct polarity to a D.C. power supply on my bench, the radio
still wont come on. When I started checking for voltage, I found that
I had 12 volts on the chassis. It is on both sides of the on/off
switch with the switch off. The 12 volts is everywhere. Does anyone
have any ideas about what to check next? It is hard to use the meter
when everywhere shows 12 volts.... Thanks, Bill

It's the reverse polarity diode. (hopefully)

Frank might know the repair.

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He is correct about the reverse polarity protection diode.
Follow the Positive (+) circuit board line
in from the power connector on the back of the radio. You should come to a
small diode. Make a note of which side of the diode has the silver band.
Unsolder the diode and head to the nearest electronics store or heaven forbid,
Radio Shack.
It is a standard 1n4000 (low power units) or a 1n4001...very inexpensive.
Put it back in with the silver band on the same side as the one you pulled out.
Should be ready to go.
Try this site.
http://www.njqrp.org/quickies/quickie2.html
Good luck 333

"Got RF?"

In , ojunk
(OZARK333) wrote:

He is correct about the reverse polarity protection diode.

No, he's not. If the diode was shorted, it would have blown the fuse/breaker on
the power supply (or melted something else if the supply wasn't protected). If
the diode was open, the radio would have worked as normal when the power was
connected properly. Neither were the case.

He was measuring from the negative terminal of the power supply to points inside
the radio, where he was getting 12V everywhere. The only way that can happen is
if the negative lead is disconnected. Try it yourself (no, it won't hurt the
radio). What happened is that the reverse-polarity protection diode did it's
job, and did it exceptionally well. When the power is connected backwards, the
diode is supposed to show a short and blow the fuse, but since there wasn't any
fuse it melted a power lead. That's one stout diode!

Anyway, the damage should be minimal, and certainly much less than if the diode
had failed.





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Frank Gilliland wrote in message ...
In , ojunk
(OZARK333) wrote:

He is correct about the reverse polarity protection diode.

No, he's not. If the diode was shorted, it would have blown the
fuse/breaker on
the power supply (or melted something else if the supply wasn't protected).
If
the diode was open, the radio would have worked as normal when the power
was
connected properly. Neither were the case.


True, the diode is not causing the problem he is seeing... but it may well
have been damaged at the same time.
After finding the O/C problem, he should replace the diode as they are cheap
enough - and put a fuse in.



Brainbuster.

To all that replied......Thanks for your help. Here is where I am
now..........
The 1 inch wire from the negative power connector to the pcb board was
not passing the negative connection. I wired in a new jumper. Now when
I put the bench power supply on it, I only get less than 1 volt on the
meter. Unplug it from the radio, and it jumps back up to normal. So I
suspect that diode got fried as well as the wire.I will try and get
the diode replaced as soon as I can get one, and will let you all know
how it came out. So all of you have been a great help. My one question
for now is... If I remove that " suspect " diode, without replacing
it, should the radio power up? or do I need to wait until I replace it
before I put the power back on it?
Thanks again to all!!!


On Wed, 08 Oct 2003 16:06:05 -0700, Frank Gilliland
wrote:

In , ojunk
(OZARK333) wrote:

He is correct about the reverse polarity protection diode.

No, he's not. If the diode was shorted, it would have blown the fuse/breaker on
the power supply (or melted something else if the supply wasn't protected). If
the diode was open, the radio would have worked as normal when the power was
connected properly. Neither were the case.

He was measuring from the negative terminal of the power supply to points inside
the radio, where he was getting 12V everywhere. The only way that can happen is
if the negative lead is disconnected. Try it yourself (no, it won't hurt the
radio). What happened is that the reverse-polarity protection diode did it's
job, and did it exceptionally well. When the power is connected backwards, the
diode is supposed to show a short and blow the fuse, but since there wasn't any
fuse it melted a power lead. That's one stout diode!

Anyway, the damage should be minimal, and certainly much less than if the diode
had failed.





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In , tomagain
wrote:

To all that replied......Thanks for your help. Here is where I am
now..........
The 1 inch wire from the negative power connector to the pcb board was
not passing the negative connection. I wired in a new jumper. Now when
I put the bench power supply on it, I only get less than 1 volt on the
meter. Unplug it from the radio, and it jumps back up to normal. So I
suspect that diode got fried as well as the wire.I will try and get
the diode replaced as soon as I can get one, and will let you all know
how it came out. So all of you have been a great help. My one question
for now is... If I remove that " suspect " diode, without replacing
it, should the radio power up? or do I need to wait until I replace it
before I put the power back on it?
Thanks again to all!!!

The radio will power up, but unless you replace it, and put a fuse connecter on
the power lead, you will have lost your reverse-voltage protection.






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After fixing the negative wire and replacing the diode, the radio
powers up just fine. I'll find out Monday when Ronnie puts it back in
his truck if it works like it used to. Thanks especially to the both
of you, because you both were dead on in what you suggested. If I
missed thanking anyone else, I apoligize. Total cost of repair
excluding my time is 59 cents for the diode and $3 for an inline blade
fuse holder. I think I'll just let Ronnie buy me lunch......
Have a great day............


On Fri, 10 Oct 2003 21:15:02 -0400, tomagain
wrote:

To all that replied......Thanks for your help. Here is where I am
now..........
The 1 inch wire from the negative power connector to the pcb board was
not passing the negative connection. I wired in a new jumper. Now when
I put the bench power supply on it, I only get less than 1 volt on the
meter. Unplug it from the radio, and it jumps back up to normal. So I
suspect that diode got fried as well as the wire.I will try and get
the diode replaced as soon as I can get one, and will let you all know
how it came out. So all of you have been a great help. My one question
for now is... If I remove that " suspect " diode, without replacing
it, should the radio power up? or do I need to wait until I replace it
before I put the power back on it?
Thanks again to all!!!


On Wed, 08 Oct 2003 16:06:05 -0700, Frank Gilliland
wrote:

In , ojunk
(OZARK333) wrote:

He is correct about the reverse polarity protection diode.

No, he's not. If the diode was shorted, it would have blown the fuse/breaker on
the power supply (or melted something else if the supply wasn't protected). If
the diode was open, the radio would have worked as normal when the power was
connected properly. Neither were the case.

He was measuring from the negative terminal of the power supply to points inside
the radio, where he was getting 12V everywhere. The only way that can happen is
if the negative lead is disconnected. Try it yourself (no, it won't hurt the
radio). What happened is that the reverse-polarity protection diode did it's
job, and did it exceptionally well. When the power is connected backwards, the
diode is supposed to show a short and blow the fuse, but since there wasn't any
fuse it melted a power lead. That's one stout diode!

Anyway, the damage should be minimal, and certainly much less than if the diode
had failed.





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