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K2 AGC problem

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Sjoerd-8

K2 AGC problem

Hello all,

While operating my K2 and switching between bands a lot i noticed my AGC is
a bit strange.

When trying to set the AGC treshold following Don FPR's instructions i find
that the noise level is not changing with the AGC treshold pot with both
preamp on and off. There is a difference in noise level between preamp on or
off but there not enough noise to trigger the AGC, even with preamp on.
Could this be a lack of gain in the receiver or is the ALC circuit wrong?
S-meter seems OK to me and the S-meter readings do change with the ALC
treshold.

Hope to get this nice toy even better..

73,

Sjoerd,

PE2SVN

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Sjoerd-8

RE: K2 AGC problem

Hello all,

Just some added info abt my K2's problem.

Measured Vagc at pin 5 of the IF amp:

-No antenna connected AGC on: 3.83V
-no antenna connected AGC off: 3.82V
-40m antenna connected: 3.90V (my FT-1000D shows abt S5
noise)
-XG2 connected : 3.99V (50uV level, should be
S9)

Found a post from N6IZ in 2004 that stated that Vagc varies about 100mV per
10dB. (
http://ac6rm.net/mailarchive/html/elecraft-list/2004-10/msg00085.html ) In
my case that says my S9 level is only about 16 dB above noise level...

Is there anyone who measured Valc at some point and can tell me if these
values are way off?

Thanks again,

Sjoerd
PE2SVN

-----Oorspronkelijk bericht-----
Van: [hidden email]
[mailto:[hidden email]] Namens Sjoerd
Verzonden: dinsdag 29 augustus 2006 22:56
Aan: [hidden email]
Onderwerp: [Elecraft] K2 AGC problem

Hello all,

While operating my K2 and switching between bands a lot i noticed my AGC is
a bit strange.

When trying to set the AGC treshold following Don FPR's instructions i find
that the noise level is not changing with the AGC treshold pot with both
preamp on and off. There is a difference in noise level between preamp on or
off but there not enough noise to trigger the AGC, even with preamp on.
Could this be a lack of gain in the receiver or is the ALC circuit wrong?
S-meter seems OK to me and the S-meter readings do change with the ALC
treshold.

Hope to get this nice toy even better..

73,

Sjoerd,

PE2SVN

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Don Wilhelm-3

RE: K2 AGC problem

Sjoerd,

If you look at the MC1350 data sheet, you will find the AGC characteristics
for operation wiht a 12 volt supply to the chip and that graph indicates
that the gain changes almost linearly from 0 dB to 20dB with an AGC voltage
change from 5 to 6 volts, or 10 dB for a 500 mV change, and the rate of
change increases abruptly above the 6 volt level.

However, the K2 uses that IF amplifier chip at an 8 volt supply, so as a
rough guess, the voltages should be multiplied by a factor of 2/3. I have
not attempted to characterize this amplifier at an 8 volt supply.

Fact 2 - the gain of the IF Amp is governed by the current rather than the
voltage into the AGC pin, so you should conclude that there are several
dependencies that will modify the actual voltage required for a given gain.

I would suggest that you set the AGC Threshold somewhere between 3.7 and 3.8
volts as measured at Control Board U2 pin 5. You may end up with some
slight reduction of gain between AGC on and AGC off, but lower voltage here
will tend to produce problems with CAL S HI and CAL S LO settings. If you
are willing to live with the consequences of using a voltage lower than 3.7
volts to produce less change between AGC ON/OFF, then you must accept the
fact that you will have a weaker AGC gain reduction overall, and your
S-meter may produce readings that are different from the norm.

You may not have any problem at all, only a consequence of the AGC action
produced by your high noise level.

73,
Don W3FPR

> -----Original Message-----
>
> Hello all,
>
> Just some added info abt my K2's problem.
>
> Measured Vagc at pin 5 of the IF amp:
>
> -No antenna connected AGC on: 3.83V
> -no antenna connected AGC off: 3.82V
> -40m antenna connected: 3.90V (my FT-1000D shows abt S5
> noise)
> -XG2 connected : 3.99V (50uV level, should be
> S9)
>
> Found a post from N6IZ in 2004 that stated that Vagc varies about
> 100mV per
> 10dB. (
> http://ac6rm.net/mailarchive/html/elecraft-list/2004-10/msg00085.html ) In
> my case that says my S9 level is only about 16 dB above noise level...
>
> Is there anyone who measured Valc at some point and can tell me if these
> values are way off?
>
> Thanks again,
>
> Sjoerd
> PE2SVN
>

Sjoerd-8

RE: K2 AGC problem

Don,

Ok about the MC1350 specs, and my noise level is very high, but i am afraid
that with less noise there is a problem.
I did some more measurements end followed the signal tracing procedure,
where i find 2 questionable things:

- Crystal filter output seems quite low, it reads 0,17 Vrms, instead of the
given 0,35V .. tried this with different BFO settings, no significant
difference. The input of the filter is about 2,15 Vrms with a jumper over
the NB connector (5 dB more than normal)

- AGC @ saturation seems way too low, it reads about 4,3 volts where it
should be 6,9V.

My opinion is that there is some sort of fault in the AGC circuit, that
causes no difference in noise level with AGC on or off, and also a big
difference in audio level between a S3 and a S9+ station.

I have changed C8 a while ago to stop the receive from muting with AGC off,
Is it possible that the ACG oscillator level is too low because of this?

The receive seems to hear well, compared to my FT-1000. Just the big
difference in audio levels worries me..

Thanks again,

Sjoerd
PE2SVN

-----Oorspronkelijk bericht-----
Van: Don Wilhelm [mailto:[hidden email]]
Verzonden: woensdag 30 augustus 2006 14:02
Aan: Sjoerd; [hidden email]
Onderwerp: RE: [Elecraft] K2 AGC problem

Sjoerd,

If you look at the MC1350 data sheet, you will find the AGC characteristics
for operation wiht a 12 volt supply to the chip and that graph indicates
that the gain changes almost linearly from 0 dB to 20dB with an AGC voltage
change from 5 to 6 volts, or 10 dB for a 500 mV change, and the rate of
change increases abruptly above the 6 volt level.

However, the K2 uses that IF amplifier chip at an 8 volt supply, so as a
rough guess, the voltages should be multiplied by a factor of 2/3. I have
not attempted to characterize this amplifier at an 8 volt supply.

Fact 2 - the gain of the IF Amp is governed by the current rather than the
voltage into the AGC pin, so you should conclude that there are several
dependencies that will modify the actual voltage required for a given gain.

I would suggest that you set the AGC Threshold somewhere between 3.7 and 3.8
volts as measured at Control Board U2 pin 5. You may end up with some
slight reduction of gain between AGC on and AGC off, but lower voltage here
will tend to produce problems with CAL S HI and CAL S LO settings. If you
are willing to live with the consequences of using a voltage lower than 3.7
volts to produce less change between AGC ON/OFF, then you must accept the
fact that you will have a weaker AGC gain reduction overall, and your
S-meter may produce readings that are different from the norm.

You may not have any problem at all, only a consequence of the AGC action
produced by your high noise level.

73,
Don W3FPR

Don Wilhelm-3

RE: K2 AGC problem

Sjoerd,

Your first point about the low output from the crystal filter with an input
that is normal would indicate that either you have a crystal problem or the
filter is not properly matched.
Check T7 to see if it has the proper turns ratio (20:5), and the 5 turn
winding goes toward the crystal filter.

The 5 dB higher than normal at the filter input with the KNB2 removed and
jumpered is about right, the KNB2 contains a 6 dB pad which is not in the
circuit with just a jumper. You may want to measure the filter input and
output again with the KNB2 in place to provide a better match for the filter
input.

Solve the filter loss problem first and then look at the AGC. You may be
getting an incorrect reading for the AGC Saturation level unless you ramped
up the signal level into the IF amp to compensate for the filter loss and as
a result are not able to saturate the AGC.

The S-meter readings will follow the AGC action directly (output of Control
Board U2A) except for differences in the resistances following the op amp
output, so if your S-meter is responding in a somewhat normal fashion, the
AGC voltage applied should also follow - all the rest of the circuitry is
insode the IF amp chip.

73,
Don W3FPR

> -----Original Message-----
>
> Ok about the MC1350 specs, and my noise level is very high, but i
> am afraid
> that with less noise there is a problem.
> I did some more measurements end followed the signal tracing procedure,
> where i find 2 questionable things:
>
> - Crystal filter output seems quite low, it reads 0,17 Vrms,
> instead of the
> given 0,35V .. tried this with different BFO settings, no significant
> difference. The input of the filter is about 2,15 Vrms with a jumper over
> the NB connector (5 dB more than normal)
>
> - AGC @ saturation seems way too low, it reads about 4,3 volts where it
> should be 6,9V.
>
> My opinion is that there is some sort of fault in the AGC circuit, that
> causes no difference in noise level with AGC on or off, and also a big
> difference in audio level between a S3 and a S9+ station.
>
> I have changed C8 a while ago to stop the receive from muting
> with AGC off,
> Is it possible that the ACG oscillator level is too low because of this?
>
> The receive seems to hear well, compared to my FT-1000. Just the big
> difference in audio levels worries me..
>
> Thanks again,
>
>
> Sjoerd
> PE2SVN
>
>
> -----Oorspronkelijk bericht-----
> Van: Don Wilhelm [mailto:[hidden email]]
> Verzonden: woensdag 30 augustus 2006 14:02
> Aan: Sjoerd; [hidden email]
> Onderwerp: RE: [Elecraft] K2 AGC problem
>
> Sjoerd,
>
> If you look at the MC1350 data sheet, you will find the AGC
> characteristics
> for operation wiht a 12 volt supply to the chip and that graph indicates
> that the gain changes almost linearly from 0 dB to 20dB with an
> AGC voltage
> change from 5 to 6 volts, or 10 dB for a 500 mV change, and the rate of
> change increases abruptly above the 6 volt level.
>
> However, the K2 uses that IF amplifier chip at an 8 volt supply, so as a
> rough guess, the voltages should be multiplied by a factor of 2/3. I have
> not attempted to characterize this amplifier at an 8 volt supply.
>
> Fact 2 - the gain of the IF Amp is governed by the current rather than the
> voltage into the AGC pin, so you should conclude that there are several
> dependencies that will modify the actual voltage required for a
> given gain.
>
> I would suggest that you set the AGC Threshold somewhere between
> 3.7 and 3.8
> volts as measured at Control Board U2 pin 5. You may end up with some
> slight reduction of gain between AGC on and AGC off, but lower
> voltage here
> will tend to produce problems with CAL S HI and CAL S LO settings. If you
> are willing to live with the consequences of using a voltage
> lower than 3.7
> volts to produce less change between AGC ON/OFF, then you must accept the
> fact that you will have a weaker AGC gain reduction overall, and your
> S-meter may produce readings that are different from the norm.
>
> You may not have any problem at all, only a consequence of the AGC action
> produced by your high noise level.
>
> 73,
> Don W3FPR
>
>
> > -----Original Message-----
> >
> > Hello all,
> >
> > Just some added info abt my K2's problem.
> >
> > Measured Vagc at pin 5 of the IF amp:
> >
> > -No antenna connected AGC on: 3.83V
> > -no antenna connected AGC off: 3.82V
> > -40m antenna connected: 3.90V (my FT-1000D shows abt S5
> > noise)
> > -XG2 connected : 3.99V (50uV level, should be
> > S9)
> >
> > Found a post from N6IZ in 2004 that stated that Vagc varies about
> > 100mV per
> > 10dB. (
> >

http://ac6rm.net/mailarchive/html/elecraft-list/2004-10/msg00085.html ) In

> my case that says my S9 level is only about 16 dB above noise level...
>
> Is there anyone who measured Valc at some point and can tell me if these
> values are way off?
>
> Thanks again,
>
> Sjoerd
> PE2SVN
>

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Chris Kantarjiev K6DBG

RE: K2 AGC problem

In reply to this post by Sjoerd-8

Sjoerd,

Your AGC symptoms sound similar to some of what I found wrong with s/n 3641
when I bought it used. You can find a lot of details in the elecraft
archive - (search for "low audio on K2 s/n 3641").

Among other problems, I had a leaky C10 cap in the AGC circuit, upsetting
the AGC control circuit /AGCOFF. You might want to check there, to make
sure there is no signal on pin U1-2.

73 de chris K6DBG
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