Posted by
Don Wilhelm-4 on
Nov 10, 2009; 11:47pm
URL: http://elecraft.85.s1.nabble.com/K2-signal-tracing-progress-and-questions-arising-tp3982841p3983391.html
Paul,
My only thought on the bandpass filter "weirdness" is that either the
bandpass filter is not properly terminated at the D6/D7 junction, or
that your 'scope probe is picking up something extraneous - change the
routing of the ground on the 'scope probe and see if there is any change.
Your signal generator level may be too low to exclude extraneous noise
pickup from consideration.
Do you have a QRP transmitter that you can feed into a dummy load and
use a pickup wire or coil positioned to provide input to the K2? You
might try using that as your signal generator (use your 'scope to check
the level into a 50 ohm resistor before connecting to the K2) - the
level of the test generator shown in the K2 manual is to be adjusted to
0.14 volts RMS (about 400 mV peak to peak). The K2 will not be damaged
with that level.
If the signal drops drastically at the mixer, it could be related to the
above, or you may not have adequate oscillator injection from the VFO.
The mixer needs a +7 dBm level at the LO port (500 mV RMS or 1.4 volts
peak to peak) - all the mixer ports are 50 ohms.
73,
Don W3FPR
Paul Fletcher wrote:
> Hi all,
>
> Progressing with fault finding the K2 and there are some immediate apparent
> issues. Firstly there is a lot of loss through the low pass filters on
> receive (on 30m at least) - 3dB when the actual loss should be negligible.
> That's obviously one place I need to look. However (weird one this) the
> output measured from the bandpass filters is higher than the input. On my
> scope I'm measuring 78mV pk to pk on the input and 135mV pk to pk on the
> output of the filters at the junction of D6 and D7 which is clearly not what
> is expected (unless this particular K2 has somehow broken the laws of
> physics). I'm wondering if it's possible that something in the transmit
> chain is active? I have checked the 8T and 8R voltages and they are OK.
> Downstream of D6 losses seem comparable with the Signal tracing checklist
> until I get to the composite mixer where everything takes a dive down
> (another area to check).
>
> Any thoughts on the bandpass filter issue?
>
> Cheers,
> Paul.
>
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