K2 signal tracing...

[G4JQT]

Hi all,

In a quest to get to the bottom of my K2 not having enough volume (after ascertaining the audio section is ok) I started upon the Signal Tracing section of the manual, made up the Elecraft RF probe and even made the 10.000 MHz xtal oscillator - appendix E page 9. (Note: My K2 is not insensitive, just barely adequate in volume. An extra audio amp would probably sort it out, but at about $700 for the basic kit I shouldn't need to; in fact most users DON'T need to so I guess there's something not quite right with my kit.)

First 'issue': The RF probe doesn't come close to providing RMS readings on my DMM. Three volts p-p of the 10MHz oscillator measured with 100MHz scope (measured when connected to the K2 antenna input) input should give an RMS of about 1.06 volts by my reckoning. DDM 'RMS' using probe reads 200 mV, confirmed by connecting probe to DC-coupled scope. No good for recording the 'actual' against the 'expected' measurements in the manual. Am I missing something here?

I decided to ditch the probe and use the scope and derive the RMS from the peak-to-peak measurements.

Mixer, IF Amplifiers and crystal Filter section Appendix E page 11

Most readings were near enough to those in the manual until I got to "8: IF saturated output" measurements. I could only get close to the figures with RF max and AGC off otherwise no output at U11. Is AGC off for this test is correct or do have I a fault? (Such a fault would surely make the K2 worse than mine is...?) 2nd crystal filter output seemed ok, but product detector saturated output (with oscillator output at max or set to give 0.15 Vrms as in para 8?) had no significant gain.

AGC tests also App E, page 11

This was ok but I couldn't get the U2 pin 1 volts above 4.8V even after peaking the tuning. It should be 6.9V but at what oscillator input level?

Either I'm not interpreting the instructions here correctly (they seem a bit vague regarding AGC on or off settings), my K2 is within the spread of voltage tolerances or I have a fault, although seemingly not a serious one - I don't know.

I'm not going to do any more checks is case I am already 'barking up he wrong tree'! So perhaps someone could clarify some of the tests or suggest other checks.

Thanks and regards,

Ian, G4JQT


     
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[Don Wilhelm-4]

Ian,

If you have the "Extremely Strong Signal Handling" mod installed on your
K2, the diodes will limit the signal at the input to the IF amplifier
and you will not be able to do the 'saturated' tests.

It sounds like your K2 is working OK.  The AGC will make no difference
in your readings until you progress to the output of the IF amplifier.  
So there are two things limiting its output - the limiting diodes and
the AGC.

The signal tracing procedures have not been updated to consider the
addition of diodes D40 and D41.  Lift one end of them if you wish to
continue with the tests.

You may find that AGC action is keeping your K2 at lower than normal
audio levels.  I would suggest lowering the DC voltage at CB U2 pin 5 to
about 3.7 volts and re-doing the CAL S HI and CAL S LO parameters to see
if that helps any.  You could lower it even more, but the S-meter
alignment gets a bit strange if you go too far.  If your K2 is below SN
3000, you do not have the AGC Threshold pot to adjust, but you can
decrease the value of R1 to reduce the AGC Threshold voltage.

If you have an older K2, check the turns on T7 - the newer windings have
20 turns of red wire and 5 turns of green.  Also check to be certain T7
is not mounted backwards - the red winding connects to U12.

I would check the diode in your RF Probe, and also check the value of
the series resistor.  Then check the specs for your DMM - the probe
expects a DMM input resistance of 11 Megohms (note that most 'scopes
have a 1 megohm input resistance, so the 'scope will not read the RF
Probe DC output correctly).
Hint - the series resistor in the probe and the input resistance of the
DMM form a voltage divider such that the conversion to RMS is done with
the resistive values.

73,
Don W3FPR

On 9/19/2011 9:25 AM, Ian Liston-Smith wrote:

> In a quest to get to the bottom of my K2 not having enough volume (after ascertaining the audio section is ok) I started upon the Signal Tracing section of the manual, made up the Elecraft RF probe and even made the 10.000 MHz xtal oscillator - appendix E page 9. (Note: My K2 is not insensitive, just barely adequate in volume. An extra audio amp would probably sort it out, but at about $700 for the basic kit I shouldn't need to; in fact most users DON'T need to so I guess there's something not quite right with my kit.)
>
> First 'issue': The RF probe doesn't come close to providing RMS readings on my DMM. Three volts p-p of the 10MHz oscillator measured with 100MHz scope (measured when connected to the K2 antenna input) input should give an RMS of about 1.06 volts by my reckoning. DDM 'RMS' using probe reads 200 mV, confirmed by connecting probe to DC-coupled scope. No good for recording the 'actual' against the 'expected' measurements in the manual. Am I missing something here?
>
> I decided to ditch the probe and use the scope and derive the RMS from the peak-to-peak measurements.
>
> Mixer, IF Amplifiers and crystal Filter section Appendix E page 11
>
> Most readings were near enough to those in the manual until I got to "8: IF saturated output" measurements. I could only get close to the figures with RF max and AGC off otherwise no output at U11. Is AGC off for this test is correct or do have I a fault? (Such a fault would surely make the K2 worse than mine is...?) 2nd crystal filter output seemed ok, but product detector saturated output (with oscillator output at max or set to give 0.15 Vrms as in para 8?) had no significant gain.
>
> AGC tests also App E, page 11
>
> This was ok but I couldn't get the U2 pin 1 volts above 4.8V even after peaking the tuning. It should be 6.9V but at what oscillator input level?
>
> Either I'm not interpreting the instructions here correctly (they seem a bit vague regarding AGC on or off settings), my K2 is within the spread of voltage tolerances or I have a fault, although seemingly not a serious one - I don't know.
>
> I'm not going to do any more checks is case I am already 'barking up he wrong tree'! So perhaps someone could clarify some of the tests or suggest other checks.
>
> Thanks and regards,
>
> Ian, G4JQT
>
>
>    
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html