Re: K2 EXCESSIVE PREAMP NOISE?

Posted by Don Wilhelm-4 on Aug 28, 2013; 10:53pm
URL: http://elecraft.85.s1.nabble.com/K2-EXCESSIVE-PREAMP-NOISE-tp7578009p7578359.html

Paul,

That does not indicate excessive preamp noise, but a bit of overall
receiver noise.

Your MDS with the preamp off is a bit low - by 4 to 7 dB, and I would
not expect that much of a difference between preamp on and preamp off.  
That means the preamp is not noisy, because the MDS increases
significantly with the preamp on (the noise level apparently did not
increase significantly).

You might want to check your setup - cables and other connections. Also
remember that the XG2 is not shielded, so there may be leakage.  If you
attempted to use an attenuator between the XG2 and the K2, leakage would
explain your measurements - use the method and formulas in the XG2
manual to approximate the MDS level.

If you are using an attenuator, put the XG2 and its battery inside a
well shielded enclosure - Hammond die cast box is recommended, and that
will allow you to make MDS measurements in the conventional way.

I usually do not do direct MDS measurements on every K2 I repair, but
use my well shielded HP8640B signal generator set for -130 dBm output
and look at the noise floor and the detected signal levels on
Spectrogram.  I expect to see the signal at least 6 dB above the
receiver noise floor.  That gives me a reasonable approximation of a
-136 or better MDS and I will look for the problem if it is anything
less - that is either with the preamp on or off.

Do you have an early K2? - if so, check the winding of T7 - if it is not
the 20 turns red and 5 turns green (with the green winding closest to
the IF crystals), changing it will pick up 3 to 6 dB in your measurements.

If you have the KNB2 installed, check to be certain the 5 dB pad has
been removed (see the KNB2 installation instructions).  I have recently
encountered a few K2 that have not had that pad removed.

73,
Don W3FPR

On 8/28/2013 5:41 PM, N2PD wrote:

> Ok, Don and the gang. I'm back from vacation and made some measurements. I
> measured the S/N using the XG2 sig gen on 14.060, agc off, mode CW, BW 700
> hz.  All measurements at 1 uv.  Here are the results:
>
> Preamp off:  S+N/N = 23db (mds -129)
>
> Preamp on:  S+N/N = 29db (mds -136)
>
> So, what do you think?  Are those numbers in line?
>
> (Still sounds noisy to me, hi)
>
>

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