Kevin,
May I suggest that you build the test oscillator shown in the
Troubleshooting appendix of the K2 manual, and use that oscillator to
perform the Receiver Signal Tracing steps. That should reveal the
failing stage for you.
Once you know which stage is failing, the final solution is usually
quite easy. Right now, the problem can be anywhere between the antenna
and the speaker.
I can't tell much about your AGC threshold, but certainly 4.1 volts is
too high - set it no higher than 3.8 volts.
73,
Don W3FPR
Kevin Cozens wrote:
Greetings.
A couple of years ago, while working a contest, I realized my K2 seemed a
little deaf. I run the radio using its internal batteries. I was
calling CQ
and got no response. To save batteries on the K2, I turned it off and
turned
on my Grundig Satellite 800 shortwave located near by. Using the
Grundig, I
was surprised to hear a station calling me. Switch back to the K2 I
found I
couldn't hear the station I heard on the Grundig. The Grundig was
using only a
built-in whip antenna and the K2 was connected to a Superantennas MP-1
antenna located just a few feet away.
I acquired this radio from the original builder 6 months after it was
assembled. I may not have built this unit but I have spent a lot of
time going through its guts and applying mods. I have applied all but
the last two of the la3za listed mods to bring my K2 #2172 in line
with the 3000+ K2's. The mods include the A/B kit update, and the PLL
stability mod.
I have downloaded updated versions of some of the pages I had printed
out previously. Namely, the LA3ZA page with mod info for the K2, the
Spectogram alignment procedure. I also downloaded the latest copy of
the Rev. G manual (to compare against my Rev. C), and the one for the
KSB2. I have temporarily removed the KSB2 from my unit until I feel
the base unit is working as expected. I also have a printout of Gary's
message fom 2000 titled "K2: 25 reasons for low RX sensitivity or low
audio".
My sudden push to try and identify, and hopefully fix, problems with
my K2 is due to my having temporary use of a 20MHz handheld
oscilloscope. The scope is on loan from a client while I'm working on
a project. I won't be able to hang on to the scope much longer. I've
been going back over messages I kept when I was originally on this
list back in 2002, plus some more recent messages.
I find I am usually running the K2 with the pre-amp on. When listening
to a station on the K2 I have the volume control set to around the 9
or 10 o'clock position so I don't feel there is a problem with the
audio amp.
My 8V regulator measures 8.45V on its output. I replaced R1 on the
control board with a 5k8 resistor in series with a 20k multi-turn
trim-pot. I have tried AGC voltages from about 3.6 up to 4.1 and still
have a noticeable difference in noise level when turning the AGC on
and off. I curently have the AGC voltage set to 4.1V.
A message I had kept from 2002 said to check for between 2.2 and 2.4V
on TP1 on 160. I went to 80m (my lowest band) and measured about 2.2V.
I removed T5 from the board as I didn't like the spacing of the turns
around it. I discovered two turns of the secondary were side by side.
I removed the secondary, adjusted the spacing of the primary to be
more even, and rewound the secondary (positioning it so its first turn
was between turns 6 and 7 of the primary instead of between 7 and 8).
After re-installing T5, I had 2.5V at TP1.
After having altered T5 I went back and check the VCO control voltage
at the left-side of R30 and read only 5.26V (at 4MHz). I adjusted L30
until I read 6V and verified all the other VCO control voltages are
within range. The lowest reading is 1.99V and highest is 6V.
The last time I adjusted the bandpass filters I used the updated
information that says to use a max. transmit power of 1W. I haven't
re-checked these filters but they should be ok. The simple N0SS signal
tracing method on 40m didn't show much of a problem when I used myself
as an antenna while holding on to one lead of a 22 nF cap.
My main issues at the moment are:
1. What voltage should I be getting at TP1 when tuned to 80m?
2. What other things should I be checking in the AGC circuit that
could account for hearing a change in signal between AGC on/off
regardless of how I set the trim-pot adjustment?
3. Are there some key RF voltage readings I should take to verify all
is well while I still have access to a scope?
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