K2 EXCESSIVE PREAMP NOISE?

[N2PD]

I've been wondering about this for awhile and would like to get some opinions. When I switch in the K2 preamp, I get a substantial increase in noise....not band noise...I'm talking about no antenna connected. I haven't measured it as yet, but the noise increase is great enough to make me suspect I have a noisy transistor. Think that's possible?  Any comments appreciated.

[Don Wilhelm-4]

Paul,

How much is the noise increase?  If you can measure it that would be
helpful.
The K2 Preamp adds about 12 dB of gain, and that will show up as a 12 dB
increase in receiver noise.
You can use Spectrogram or other audio spectrum analyzers to measure the
increase in noise.

This is exactly why one should turn the preamp off on bands where it is
not needed.  If the noise level increases when the antenna is connected,
you have enough usable gain.  Test it with the ATTenuator on, then with
the Preamp off, and finally with the preamp on.  The results will depend
on the band and your antenna as well as the local noise level.

73,
Don W3FPR

On 8/20/2013 8:56 PM, Pdn0528 wrote:

> I've been wondering about this for awhile and would like to get some
> opinions. When I switch in the K2 preamp, I get a substantial increase in
> noise....not band noise...I'm talking about no antenna connected. I haven't
> measured it as yet, but the noise increase is great enough to make me
> suspect I have a noisy transistor. Think that's possible?  Any comments
> appreciated.
>
>
>
> -----
> N2PD
> --
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[N2PD]

Don, thanks for your response.  I am out of town on vacation at present and the rig is home, so can't measure now.  FYI, I rarely use the preamp but when I do it seems that it is excessively noisy. I've searched the reflector and it seems no one else has made this observation, which makes me think even more that there is something amiss.  I have what I feel is adequate audio so I don't use the preamp just to make things louder. The sensitivity is also adequate but, I would just like to know if I have a prob with that preamp. Will make measurements next week when I get home.  BTW, thanks for all the great info you have provided in the past.
Tnx
Paul

[G3TCT]

Paul
The key question is whether the signal to noise ratio is the same or
better with the preamp on than with it off.  If not, it's faulty.

Graham

On 19:59, N2PD wrote:
> Don, thanks for your response.  I am out of town on vacation at present and
> the rig is home, so can't measure now.  FYI, I rarely use the preamp but
> when I do it seems that it is excessively noisy.
> [snip]
>    
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[N2PD]

Thanks for your comments, Graham.  Yes, I'm going to measure the S/N with preamp in and out as soon as I get home next week
Paul

[N2PD]

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)

[Don Wilhelm-4]

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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