Question about cheap and dirty signal tracing

[Kevin Cozens-2]

Greetings.

I have barely used my K2 going on about two years now. It seemed to work
well when I first got it off the person who had assembled it. I got
occasional comments about good audio when operating on SSB. I then
started applying the mods to bring it in line with the post 3000 serial
number units, applied the PLL stability mode, and replaced the crystals
with a more closely matching set, and completely re-aligned the radio.

Since then, the radio seems to have become slightly deaf, and I also
have some issues with transmit audio. I've considered selling it to
someone who could fix it and starting over. Having acquired a good
digital storage oscilloscope some months back, I'm going to have another
shot at trying to fix the problems. I'll start by working on the receive
issues.

I'm planning to get one of the Elecraft signal generator kits to help
with tracking down the receiver issues. Since I don't currently have
one, I'm starting with the cheap and dirty signal tracing notes from
Tom, N0SS. I pulled out the KSB2 and disconnected the KAT2 to eliminate
them as a source of problem. I have noticed one odd thing when using a
random length of wire and touching one end of it to the indicated test
points.

Touching the wire to either side of C6 (RF Board page 3), the S meter
reading on the front panel goes up by about 2 LED's. What is odd is that
if I touch W6, or either end of D6 (the previous and next points in the
signal chain) I don't see this level of increase in signal level.

This isn't making sense to me right now. I would think I would get the
same signal level increase at either W6, or D6, but not at neither point
compared to both sides of C6. Could this indicate bad, or wrong value
caps, for C5 and/or C7? Any other thoughts about this rather odd
behaviour? Seems to be pointing towards something to investigate further
but I hope I don't have to removed that bottom rear panel as its more of
a pain to remove.

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[Don Wilhelm-4]

Kevin,

It is normal to see a higher response touching a random, wire to C6
compared to touching the same wire to W6.  The C6 location is a high
impedance point while the W6 and D6 locations are low impedance - the
capacitive dividers (C7,8 and C4,5) reduce the impedance.
The end of a random wire is a high impedance and matches better at C6.  
If you were connecting the center conductor and ground the shield of a
properly matched feedline close to the test point, you would likely find
a better response at W6 than at C6.
That is why that testing technique is called "Cheap and Dirty" - it does
have some eccentricities that the user needs to be aware of (like this
one), but overall is a quick troubleshooting aid.

If you are getting about the same response at D6 and W6, then the
bandpass filter is probably OK.

73,
Don W3FPR

Kevin Cozens wrote:

> Greetings.
>
> I have barely used my K2 going on about two years now. It seemed to work
> well when I first got it off the person who had assembled it. I got
> occasional comments about good audio when operating on SSB. I then
> started applying the mods to bring it in line with the post 3000 serial
> number units, applied the PLL stability mode, and replaced the crystals
> with a more closely matching set, and completely re-aligned the radio.
>
> Since then, the radio seems to have become slightly deaf, and I also
> have some issues with transmit audio. I've considered selling it to
> someone who could fix it and starting over. Having acquired a good
> digital storage oscilloscope some months back, I'm going to have another
> shot at trying to fix the problems. I'll start by working on the receive
> issues.
>
> I'm planning to get one of the Elecraft signal generator kits to help
> with tracking down the receiver issues. Since I don't currently have
> one, I'm starting with the cheap and dirty signal tracing notes from
> Tom, N0SS. I pulled out the KSB2 and disconnected the KAT2 to eliminate
> them as a source of problem. I have noticed one odd thing when using a
> random length of wire and touching one end of it to the indicated test
> points.
>
> Touching the wire to either side of C6 (RF Board page 3), the S meter
> reading on the front panel goes up by about 2 LED's. What is odd is that
> if I touch W6, or either end of D6 (the previous and next points in the
> signal chain) I don't see this level of increase in signal level.
>
> This isn't making sense to me right now. I would think I would get the
> same signal level increase at either W6, or D6, but not at neither point
> compared to both sides of C6. Could this indicate bad, or wrong value
> caps, for C5 and/or C7? Any other thoughts about this rather odd
> behaviour? Seems to be pointing towards something to investigate further
> but I hope I don't have to removed that bottom rear panel as its more of
> a pain to remove.
>  
>

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[Kevin Cozens-2]

Thank you Don, and Tom, for your replies.

Don Wilhelm wrote:
> It is normal to see a higher response touching a random, wire to C6
> compared to touching the same wire to W6.  The C6 location is a high
> impedance point while the W6 and D6 locations are low impedance - the
> capacitive dividers (C7,8 and C4,5) reduce the impedance.

That makes sense. I knew something was odd when the signal levels at the
test points before and after C6 gave the same but lower audible signal.

> If you are getting about the same response at D6 and W6, then the
> bandpass filter is probably OK.

It would be nice to get my problems resolved in time for the local
provincial QSO party which takes place around the third weekend in
April. I think the only way to be sure whether the receiver of my K2 is
up to snuff or not is to order an XG2 signal generator kit. That will
help me to get a more accurate idea on the receivers sensitivity when
running with and without the KAT2 (in case the KAT2 has been
contributing in any way to apparent sensitivity problems).


If the cheap and dirty tracing doesn't seem to turn up any issues I'll
have to start getting in there with the scope and check the voltage
measurements at various points noted in the manual.
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