[Elecraft] K3 won't power up

[Don Wilhelm]

The base K3 is protected from reverse polarity by a series diode as well 
as a diode to ground and a resettable fuse.
So it will not power up if the power leads are reversed - and normally 
will recover with no damage when that condition is corrected.
The KPA3 is not similarly protected - the diode drop would be too large 
and a 20 amp diode would be required.  The PA depends on the circuit 
breaker.

The one element that is not protected is the RFC in series with the DC 
input - RFC6.
If the power were reversed, it is likely that RFC6 could have opened 
before the KPA3 circuit breaker tripped - the reaction of the users in 
the field would be to reset the breaker, so when it returned, the 
breaker would look OK.

A bit of investigation with an ohmmeter following the path from 12V In 
on the K3 schematic (RF Board sheet 3) will verify whether power can get 
to the cathode of D6.

Speculation on my part, but I would investigate based on that scenario.

I doubt the loose nut has anything to do with the problem, but does need 
to be corrected.

73,
Don W3FPR

On 8/4/2014 9:49 AM, Mike K2MK wrote:
> Hi George,
>
> A working K3 didn't power up at field day. That probably means that the
> power leads were reversed and embarrassment causes that information to be
> withheld. People here on the reflector probably know what gets damaged when
> power is reversed. Chances are that is very easy to check if you know what
> you're looking for.
>
> 73,
> Mike K2MK
>
>
> George Thornton wrote
>> I have a k3 that was working fine but now will  not power up.
>>
>> I had lent the unit to someone to use at Field Day.  They reported it
>> would not power up.
>>
>> Going through the unit it appears  that a nut and washer has come off of
>> one of the PL259 antenna plates.  A thorough internal inspection found a
>> loose washer but no nut.
>>
>> There is nothing evidently wrong inside, no flashing, no sparking, no
>> burning odor.  It just won't come on.
>>
>> Fuse did not activate.
>>
>>