Re: K2 - Strange Problem ?
Posted by
Stewart Baker on
Nov 30, 2004; 9:20am
URL: http://elecraft.85.s1.nabble.com/K2-Strange-Problem-tp372185p372194.html
On Tue, 30 Nov 2004 08:47:38 GMT, Stewart Baker wrote:
> On Tue, 30 Nov 2004 07:59:31 +0000, Andy Pevy wrote:
>> Hi Stewart
>> Try Rather than directly connecting the decoupling cap to the 5v supply, try
>> feeding it via a 100 ohm (or so) resistor. at 1ma you will only drop .1 v
>> but you have created a low pass filter for the noise.
>> Rgds
>> Andy Pevy
>> G4XYW
>> A future K2 owner (when the company bonus arrives).
>> On Tuesday 30 November 2004 06:24, ext Stewart Baker wrote:
>>> 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
> Hi Andy,
> Yes, that's what I have been trying. The noise is really a pulse as the LEDs
> in
> the display go on and off. Got up to 1.5k with a 47uF cap, however I now get
> the
> VOX switching on/off by it's self very occasionally, so I think that the 5A
> line
> does not appreciate any time constant being hung off it.
> Latest thoughts are to use an op-amp instead of a transistor as I believe that
> they have a greater immunity to supply noise than a simple common emitter
> transistor amplifier. That should mean even less RC.
> 73
> Stewart G3RXQ
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