Posted by
Stewart Baker on
Nov 30, 2004; 6:24am
URL: http://elecraft.85.s1.nabble.com/K2-Strange-Problem-tp372185.html
I think that I have found the problem, but have yet to work out a solution.
To combat the lack of TX audio system gain I have a 1 transistor pre-amp mounted
on the microphone header plug on the front panel board. This gives about 9dB of
gain which provides sufficient audio for the compressor IC and the VOX.
The amplifier is supplied from the 5V pin on the header. This rail is the 5A
supply which is used by the MCU logic, plus other functions.
The amplifier only draws about 1mA, however because of noise on the 5A line,
mainly from the LED bar graph I have had to heavily decouple the pre-amp supply
to stop the noise from being superimposed on my speech.
What I think is happening is that this decoupling is affecting the rise time of
the 5A line which is used for the MCU reset amongst other things.
I have tried reducing the value of decoupling. This seems to stop the "strange"
problem, but I am back with the noise. It is a shame that there is no way to get
at the +8V rail on the front panel board as that would be an ideal mic supply.
The way things are looking I will have to put the pre-amp outside my K2 and
power it separately. The only alternative seems to go back to shouting !
73
Stewart G3RXQ
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