K2 #6425 low/no sensitivity - suggestions?

[TF3KX]

Hi - I am in the process of constructing my K2 and have two problems that may or may not be related.

Problem 1:  Low, almost nonexistent RX sensitivity during Alignment II.  I can manage to copy a carrier from my MFJ antenna analyzer and I have used this to verify that IF and input circuits are peaked.  Injecting this signal into the antenna terminal I can also verify that PRE and ATT work as expected, as well as the crystal different filter bandwidths.  Using an IF carrier from the MFJ I have poked a low level signal into various parts of the IF chain.  An interesting thing is that I hear this signal much weaker in the K2 audio when I touch it to the INPUT of IF-amp U12 (MC1350) than to its OUTPUT.  In other words, that IF amp does not amplify, but rather attenuate the injected signal.  I have re-checked all connections around U12, re-flowed the solder, checked T7 and removed the overload diodes D40 and D41 - with no improvement.

Problem 2:  Low 8V regulator output, 7.56V.  As a result I could only adjust the U2 pin 5 voltage to 3.60V using R1, in Alignment I.  Probing the voltages derived from the 8V supply, they are of course 5-10% lower than expected.  All DC voltages on U12 are consequently a little low, but otherwise not abnormal.

I plan to get a local replacement for the 8V regulator, just in case.  But does anyone have comments on the low sensitivity?  Would the low 8V level be a possible cause for this?

73 - Kristinn, TF3KX

[David Woolley (E.L)]

TF3KX wrote:

> the IF chain.  An interesting thing is that I hear this signal much weaker
> in the K2 audio when I touch it to the INPUT of IF-amp U12 (MC1350) than to
> its OUTPUT.  In other words, that IF amp does not amplify, but rather
> attenuate the injected signal.  I have re-checked all connections around

That suggests an AGC problem.

I wonder if the AGC mixer is out of balance and you are getting a lot of
its local oscillator getting into the AGC chain?

I also wonder if this problem relates to two recent reports of people
being unable to find the 7MHz birdie.

> U12, re-flowed the solder, checked T7 and removed the overload diodes D40
> and D41 - with no improvement.
>
> Problem 2:  Low 8V regulator output, 7.56V.  As a result I could only adjust
> the U2 pin 5 voltage to 3.60V using R1, in Alignment I.  Probing the

There have been a lot of reports of out of spec 8V regulators recently;
it looks like they've received a bad batch and need to take some quality
control action.  (Note that the U2 pin5 voltage does not allow for
component tolerances, so a low value is not necessarily a problem.)

A low U2 pin 5 voltage, on its own, would actually increase the gain.
On the other hand, they are not operating the IF amplifier at the supply
voltage given in the data sheet and it may be that reducing supply
voltage has the opposite affect on the AGC.


> voltages derived from the 8V supply, they are of course 5-10% lower than
> expected.  All DC voltages on U12 are consequently a little low, but
> otherwise not abnormal.
>
> I plan to get a local replacement for the 8V regulator, just in case.  But
> does anyone have comments on the low sensitivity?  Would the low 8V level be
> a possible cause for this?

As it is out of specification (<7.6V), I believe Elecraft will replace
it under warranty, and doing so will get it formally recorded as a
quality failure.

--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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[TF3KX]

Well, I took my lack-of-sensitivity analysis to the next stage and performed the Signal Tracing checks, as described in the Elecraft K2 Manual.  Injected the 0,14Vrms into the antenna connector when appropriate, and kept the overload diodes (D40,D41) disconnected.

In short, the DC voltages are a little low but probably close enough to normal, except U12 pin 5 (AGC) which measured 3.5V rather than 5.0V.  All RF voltages appeared reasonably close to the K2 suggestions, up to and including U12 pin 4 (U12 input).  The pin 4 voltage was 2.0V rather than the suggested 0.4-0.8V.  However, the output signals from U12 seem to be nonexistent:  U12p8 is 0V at all times, irrespective of the signal level injected into the antenna terminal.  Subsequent signals are zero, as well (U11 input through pin1, etc.).

Again, any suggestion on a plausible cause - a failed component or bad soldering?  Has anyone run into this before, and have suggestions to try out?

73 - Kristinn, TF3KX

TF3KX wrote

Hi - I am in the process of constructing my K2 and have two problems that may or may not be related.

Problem 1:  Low, almost nonexistent RX sensitivity during Alignment II.  I can manage to copy a carrier from my MFJ antenna analyzer and I have used this to verify that IF and input circuits are peaked.  Injecting this signal into the antenna terminal I can also verify that PRE and ATT work as expected, as well as the crystal different filter bandwidths.  Using an IF carrier from the MFJ I have poked a low level signal into various parts of the IF chain.  An interesting thing is that I hear this signal much weaker in the K2 audio when I touch it to the INPUT of IF-amp U12 (MC1350) than to its OUTPUT.  In other words, that IF amp does not amplify, but rather attenuate the injected signal.  I have re-checked all connections around U12, re-flowed the solder, checked T7 and removed the overload diodes D40 and D41 - with no improvement.

Problem 2:  Low 8V regulator output, 7.56V.  As a result I could only adjust the U2 pin 5 voltage to 3.60V using R1, in Alignment I.  Probing the voltages derived from the 8V supply, they are of course 5-10% lower than expected.  All DC voltages on U12 are consequently a little low, but otherwise not abnormal.

I plan to get a local replacement for the 8V regulator, just in case.  But does anyone have comments on the low sensitivity?  Would the low 8V level be a possible cause for this?

73 - Kristinn, TF3KX

[Roelof Bakker]

Hello Kristinn,

Have you checked the value of C182, 180 pF?
I had a sensitivity problem which was caused by mounting a 1800 pF cap!

73,
Roelof, pa0rdt
K2 # 6177

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[TF3KX]

PROBLEM SOLVED.

I found a "full size" MC1350 at a local electronics store (the last one they had, back in old storage...).  Replaced it for the modified SMD-version MC1350 I had received from Elecraft - and this fixed my sensitivity problem!

I still don't feel comfortable with the low 8V level, but will leave it as is for now and probably get a replacement regulator when I'm done building the K2.

Hope others will learn from this.  73 - Kristinn

TF3KX wrote

Hi - I am in the process of constructing my K2 and have two problems that may or may not be related.

Problem 1:  Low, almost nonexistent RX sensitivity during Alignment II.  I can manage to copy a carrier from my MFJ antenna analyzer and I have used this to verify that IF and input circuits are peaked.  Injecting this signal into the antenna terminal I can also verify that PRE and ATT work as expected, as well as the crystal different filter bandwidths.  Using an IF carrier from the MFJ I have poked a low level signal into various parts of the IF chain.  An interesting thing is that I hear this signal much weaker in the K2 audio when I touch it to the INPUT of IF-amp U12 (MC1350) than to its OUTPUT.  In other words, that IF amp does not amplify, but rather attenuate the injected signal.  I have re-checked all connections around U12, re-flowed the solder, checked T7 and removed the overload diodes D40 and D41 - with no improvement.

Problem 2:  Low 8V regulator output, 7.56V.  As a result I could only adjust the U2 pin 5 voltage to 3.60V using R1, in Alignment I.  Probing the voltages derived from the 8V supply, they are of course 5-10% lower than expected.  All DC voltages on U12 are consequently a little low, but otherwise not abnormal.

I plan to get a local replacement for the 8V regulator, just in case.  But does anyone have comments on the low sensitivity?  Would the low 8V level be a possible cause for this?

73 - Kristinn, TF3KX