AGC Alignment

[Evert Bakker (PA2KW)]

Hi all,

I'm assembling the K2 here (SN 4836) and came to the first
check/alignment.  No smoke here so far, but when adjusting the setting
for the AGC as described on page 47 I became suspicious about the
alignment procedure. Adjust R1 for 3.80 V on the DMM. I turned R1 and
reached the end and got stuck on 3.78V. The absolute value of 3.78
doesn't worry me but that I ended up at the limit of the R1. I thought I
should check all variables like DC voltage which should be 8V (was 7.98V
so OK). Checked the resistors (mainly RP6 / 5.1K) and, RF gain
potentiometer 5K and the R1 (50K). All looked fine. So what's wrong?

I started making calculation on the resistor dividing network and came
to the conclusion that the maximum voltage (theoretically) could be
close to 3.82V when the RF-gain is fully CW  (0 Ohm) and  the R1 is set
fully CCW (50K).

I wonder if something is wrong here and if not, why was the alignement
procerdure setup like this (it could heve been something like: Set
RF-Gain full CW, R1 full CCW and check the voltage at pin 5 of U2 to be
close to3.80V)? Or did I miss some errata info?

Please help.

Thanks in advance

Evert, PA2KW
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[Mark J. Schreiner]

When recently building mine I had a similar problem.  I moved my ground
connection to one of the small loops on the board and that was enough to
give me a higher reading and tweak R1 into range.  Seems that the
"Ground" that I had picked wasn't as good of a ground as I thought!

There certainly may be another reason for your problem as well, but just
thought I would offer my experience.  Hope it helps!

Mark, NK8Q
K2 #4786

Evert Bakker wrote:

> Hi all,
>
> I'm assembling the K2 here (SN 4836) and came to the first
> check/alignment.  No smoke here so far, but when adjusting the setting
> for the AGC as described on page 47 I became suspicious about the
> alignment procedure. Adjust R1 for 3.80 V on the DMM. I turned R1 and
> reached the end and got stuck on 3.78V. The absolute value of 3.78
> doesn't worry me but that I ended up at the limit of the R1. I thought
> I should check all variables like DC voltage which should be 8V (was
> 7.98V so OK). Checked the resistors (mainly RP6 / 5.1K) and, RF gain
> potentiometer 5K and the R1 (50K). All looked fine. So what's wrong?
>

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[Don Wilhelm-3]

Evert,

If RP6 is on the high side of its tolerance and/or R1 is on the low side
combined with a low 8R voltage, you may not be able to achive the 3.80 volt
level.  3.75 to 3.78 volts is probably OK for your initial adjustment.  I
fine-tune my setting later so there is no AGC action with internal receiver
noise and usually end up with a setting somewhere between 3.7 and 3.75
volts, although some particularly hot receivers must be set as low as 3.6
volts.

To assure no AGC action on internal noise, remove the antenna, seto the RF
gain fully CW and AF Gain up until noise can be heard - then toggle the AGC
off/on by pressing AGC and PRE/ATT buttons together.  Adjust the AGC
Threshold just below the point where no change in noise is observed.  Be
certain the set CAL S Lo and CAL S Hi after changing the AGC Threshold
setting.

73,
Don W3FPR

> -----Original Message-----
>
> I'm assembling the K2 here (SN 4836) and came to the first
> check/alignment.  No smoke here so far, but when adjusting the setting
> for the AGC as described on page 47 I became suspicious about the
> alignment procedure. Adjust R1 for 3.80 V on the DMM. I turned R1 and
> reached the end and got stuck on 3.78V. The absolute value of 3.78
> doesn't worry me but that I ended up at the limit of the R1. I thought I
> should check all variables like DC voltage which should be 8V (was 7.98V
> so OK). Checked the resistors (mainly RP6 / 5.1K) and, RF gain
> potentiometer 5K and the R1 (50K). All looked fine. So what's wrong?
>
>

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[Paul Bruneau]

At least a few of us saw exactly the same thing in the last few weeks.
We never heard anyone giving grave warnings that we were in
trouble...so I don't know what to tell you, except that I'm in exactly
the same boat.

I planned on measuring it again when I get to the next alignment/test
where that board is powered up. For now I'm still winding toroids.

-Paul KB8NMZ
K2 #4818

On Apr 9, 2005, at 7:38 AM, Evert Bakker wrote:

> Hi all,
>
> I'm assembling the K2 here (SN 4836) and came to the first
> check/alignment.  No smoke here so far, but when adjusting the setting
> for the AGC as described on page 47 I became suspicious about the
> alignment procedure. Adjust R1 for 3.80 V on the DMM. I turned R1 and
> reached the end and got stuck on 3.78V. The absolute value of 3.78
> doesn't worry me but that I ended up at the limit of the R1. I thought
> I should check all variables like DC voltage which should be 8V (was
> 7.98V so OK). Checked the resistors (mainly RP6 / 5.1K) and, RF gain
> potentiometer 5K and the R1 (50K). All looked fine. So what's wrong?
>
> I started making calculation on the resistor dividing network and came
> to the conclusion that the maximum voltage (theoretically) could be
> close to 3.82V when the RF-gain is fully CW  (0 Ohm) and  the R1 is
> set fully CCW (50K).
>
> I wonder if something is wrong here and if not, why was the alignement
> procerdure setup like this (it could heve been something like: Set
> RF-Gain full CW, R1 full CCW and check the voltage at pin 5 of U2 to
> be close to3.80V)? Or did I miss some errata info?

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[Larry Makoski W2LJ]

Evert,

Same thing happened here last summer as I was building K2 #4090.  I just
let it go; I figured 3.78V was close enough.  My K2's been on the air
for 9 months now with no problems.  I would't worry about it.

73 de Larry W2LJ
K2 #4090  K1 #1647

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