[Elecraft] K-2 5 volts on Mic Jack

[Jack Brindle]

Don is essentially right as to what is happening, although the MCU is  
not being "scrambled." The 5A line is connected directly to the  
18C452's reset line (MCLR). When 5A drops below about 4.5 volts for at  
least 200 microseconds, it triggers a reset for the microcontroller. It  
doesn't take much draw on the 5A line to trigger this condition.

Don and I have extensively discussed the use of the microphone 5 volt  
line. I fully agree with his strong suggestion to not use it. In my  
case I found problems with RF Feedback that I have been able to  
directly associate with my use of the power pin, even though I heavily  
bypassed it in my interface box. The symptoms were bad audio reports on  
my SSB transmissions on both 20 and 40 meters. Replacing the interface  
box with a direct connection (and Mic 5V not connected) solved the  
problem.

Like Don, I have moved to power my audio interface box from the main  
station +12V supply, and removed the jumper from the MIC header to help  
make sure the feedback problems do not return. I agree that it is nice  
to have one feed point for power to the peripheral devices, but that  
one feed point should not be the K2 - it simply was not designed for  
that purpose.

On Mar 18, 2005, at 7:45 AM, W3FPR - Don Wilhelm wrote:

> If your SignalLink interface has large capacitors on its voltage input  
> line,
> the answer is YES - it will draw significantly more than 10 ma when  
> you turn
> the switch on.
>
> YES it is likely that the SignalLink is causing the +5 volt rail on the
> front panel and Control board to drop long enough the cycle the K2  
> through a
> reset condition.  The microprocessor directly uses this same voltage,  
> and if
> it drops even momentarily, there is the possibility that the K2's  
> 'brain'
> can become scrambled.

> I would tend to believe that the K2 mic connector power was considered  
> only
> for powering such things as microphones and mic pre-amps which draw  
> only
> microamps.

- Jack Brindle, W6FB
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