Fixing partially deaf K2.

[Kevin Cozens-2]

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?

--
Cheers!

Kevin.

http://www.ve3syb.ca/           |"What are we going to do today, Borg?"
Owner of Elecraft K2 #2172      |"Same thing we always do, Pinkutus:
                                 |  Try to assimilate the world!"
#include <disclaimer/favourite> |              -Pinkutus & the Borg


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[Don Wilhelm-4]

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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[Kevin Cozens-2]

Don Wilhelm wrote:
> 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.

I would except I probably don't have the right resistor values. The Cheap 'N
Dirty signal tracing from N0SS hasn't turned up any noticeable change in
signal at any of the text points. I do have some crystal oscillator packages
which I could use with a pot on the output to do create a signal source.

The noise generator and 3-band signal generator Elecraft mini-modules are on
my Christmas wish list. I'm hoping to find one or both under the tree this
year. If not, I'll have to order them myself.

> 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.

The Rev. G manual says to set it to 3.8 during the construction phase. The
Rev. C and G manuals both say to set it to 3.6V in the signal tracing section.
Messages on the mailing list suggest setting the AGC voltage as high as
possible as long as you can still get no noticeable change in audio between
AGC on and off. Since my 8V regulator is putting out 8.45V, I have decided to
use the experimental formula for setting the AGC voltage so I have set it to
3.98V for now.

--
Cheers!

Kevin.

http://www.ve3syb.ca/           |"What are we going to do today, Borg?"
Owner of Elecraft K2 #2172      |"Same thing we always do, Pinkutus:
                                 |  Try to assimilate the world!"
#include <disclaimer/favourite> |              -Pinkutus & the Borg

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Post to: [hidden email]
You must be a subscriber to post to the list.
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[Kevin Cozens-2]

Bruce Bowman wrote:
> Just curious... did you ever get this resolved?

I'm not certain yet. Since my original message, I have dropped my AGC voltage
to 3.98V, re-adjusted L34, and recalibrated the filters using the most recent
notes for using Spectograph, and recalibrated the S-meter.

When I connected the K2 to an external antenna on Saturday night, the K2
seemed to be hearing some stations in the DX window of 80m about as well my
Grundig Satellite 800 shortwave radio (which was just using its built-in
whip). The results are promising but I need more time to listen to stations
and compare to my shortwave receivers.

While turning the slug of L34 clockwise, the internal signal peaks. Turning it
a bit more and the noise to the right of the internal signal begins to drop.
About the time it drops the slug appears to bottom out. I am wonders if I may
need to change a cap around L34 to bring the point of best S/N ratio to
somewhere more in the middle of the slugs travel range.

I didn't get any response to whether 2.5V rms on TP1 when the rig is tuned to
80m is withing the expected range or not. For now, I will just have to assume
it is.

I still think I am getting more of a change in audio level between AGC on and
off than I should. To see if the 8V regulator output is causing the problem
may require replacing the regulator. I'm not yet sure what voltages/parts I
should check in the IF amp or AGC circuit to determine if there is a problem
with one of these circuits.

Now that the receiver seems to be performing a little better, I will move on
to the transmitter. I want to retest the amount of maximum RF I can get from
the radio on the various bands and see if the change in VCO output has made
any noticeable change. I will also be rewiring T1 and T3 as I don't think they
are wound as well as they could be. They could be the cause for the slightly
lower RF output (barely 10W) on 10m.

--
Cheers!

Kevin.

http://www.ve3syb.ca/           |"What are we going to do today, Borg?"
Owner of Elecraft K2 #2172      |"Same thing we always do, Pinkutus:
                                 |  Try to assimilate the world!"
#include <disclaimer/favourite> |              -Pinkutus & the Borg
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