Posted by
Sarah K. on
Dec 02, 2007; 8:58am
URL: http://elecraft.85.s1.nabble.com/K2-AGC-Threshold-Too-Low-tp367004p367005.html
Hi All,
Good news... I got my K2's replacement voltage regulator (LM2930T-8)
from Elecraft on Friday. Today I took out the old one and put in the
new one... the output of the voltage regulator is now 8.04 volts,
which is a whole lot better than the 7.53 volts I had gotten before!
And after adjusting R1, the AGC threshold is reading 3.80, exactly
where it should be!
Elecraft also sent me a replacement R116 which was missing (I think!)
so I was able to finish building Part II and move on to the alignment
and test for Part II.
Someone on the list (NM5B) had recommend I pick up a few of the
Elecraft mini-modules, so I ordered the noise generator, the 50uV
signal generator, and the dummy load. I put those together today too,
with no issues, and used them to help work through the alignment and
test.
On the whole, I was quite happy with the results of the alignment and
test; all of the instructions were fairly straightforward, and, even
better, my K2 seemed to pass all of them perfectly! Yay!
And the noise generator works; I can hear the noise. The signal
generator works, too; when I tune around 7040 kHz, I can hear the 40m
50uV oscillator.
Now comes my strange and possibly embarrassing question...
Um... should I be able to receive anything yet? Keep in mind, this is
my first radio... well, second, if you count my Yaesu VX-7R... and I
don't entirely know what to expect.
I have what is probably a very bad antenna; it's just two pieces of
random wire I had laying around, soldered onto a BNC connector, and
draped around the room. The total length is probably 20-25 feet,
which I *know* isn't the right length for 40 meters... but it was the
longest wire I had laying around. When I hook it to the K2 via a 3ft
BNC patch cable, I definitely hear an increase in the noise level in
the headphones.
But I wasn't able to tune into anything that sounded like an actual
transmission. No voice, no CW. I tried switching between CW, LSB, and
USB. And I fiddled with the filter settings.
I did hear a couple different kinds of things:
I hear whistling noises that increase or decrease in pitch as I tune
around.
I hear "pops" that happen when I tune across some small range of
frequencies. These pops seem to be mostly repeatable, as if I'm
hitting something in the same spot in the band, but I was never able
to tune in to anything.
I do also hear several places where the noise is louder, and the S-
meter increases by a notch or two... and once or twice I heard
something that could possibly have been very, very garbled speech.
But I wasn't ever able to tune into those, either, and in fact it
could have been my imagination.
And, so far... that's about it.
Some other things that might be worth pointing out: I have a high-
voltage transmission line and a substation about a block away from
me... and my antenna is parallel to it. And I connected the VX-7R to
the same antenna and it gets similar kinds of noise, and no signals
on 40m either. Also, I'm on the 2nd floor of an apartment, and an
outside antenna isn't going to be feasible here. I didn't try
grounding the K2 to the electrical ground yet... I certainly can't
install my own ground rod, either! I was running the K2 from 12 volts
worth of D batteries, to minimize power supply noise. And I turned
off all the fluorescent lights, which dropped the noise considerably,
but didn't reveal anything underneath the noise.
So... does this sound appropriate given the current situation? Should
I be hearing more than this? Am I not understanding how the K2 works?
Am I missing something very basic about ham radio? Am I doing
something silly? :-)
Should I try a better/different antenna? I now have about 150ft of
12ga stranded wire that I can use for a second test antenna; if I
run it perpendicular to the high voltage line I think I can go 40ft
in a mostly straight line, and probably 50ft with one right-angle
bend in it. I can't get to 67ft without some major contortions; my
apartment just isn't that big! I have the internal antenna tuner for
my K2 but of course I haven't put it together yet!
Or could there be something wrong with my partly-assembled K2?
To date, I've heard almost nothing on *any* HF band... once or twice
I've been able to hear faint strains of music on the VX-7R, when
using the pre-programmed "Shortwave Broacast" built-in memory. And I
have another little transistor radio that has shortwave bands on
it... and I've never heard anything on that either. Could I somehow
be in a location where I can't receive much of anything? (I do hear
stuff on VHF bands with the VX-7R.)
Suggestions/recommendations welcome!
Thanks,
Sarah AF6FH
On Nov 23, 2007, at 12:53 PM, Sarah K. wrote: [quoted bit corrected
for typos!]
> Hi,
>
> I'm Sarah, AF6FH! I just got my license a couple weeks ago, and I'm
> now assembling K2# 06322, which will be my first HF radio. I started
> assembly on Wednesday night, and so far, everything's been going just
> fine.
>
> But...
>
> I'm having a little issue with setting the AGC Threshold.
>
> The instructions (pg. 48) say to set the RF GAIN control fully
> clockwise, measure the voltage on pin 5 of U2 on the control board,
> and adjust trimmer R1 for a reading of 3.80 volts.
>
> I did my voltage measurements with my own (cheap) DMM, not the
> internal voltmeter.
>
> My concern is that, by adjusting R1, the highest I could get the
> voltage to go was 3.59v, so for the moment, that's where it's set.
>
> I measured the voltage on the LM2930T-8, and it has 12.23V at the
> input, but only 7.53V at the output. The 78M05 has 4.93V at its
> output.
>
> When I rotate the AF GAIN control, I get a range of 4.68V (fully
> counter-clockwise) to 3.59V (fully clockwise) on pin 5 of U2.
>
> My power supply is a small benchtop power supply; it's putting out
> 12.4V, and reports the K2 is drawing about 100mA. When I measure the
> input voltage across the K2 input jack, I get the same 12.4V.
>
> All of the previous checks were fine, and the following step, S-Meter
> Alignment, seemed to work just fine.
>
> So, my question is, do I have a problem? Should I be concerned?
>
> Thanks,
>
> Sarah AF6FH
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