K2 - Need tips on troubleshooting audio intermittent problem

[dw-4]

Hi Gang,
I got my K2 a couple of years ago.
Once in a great while I would notice the audio volume drop to about 30%.
It would typically happen after I had moved the unit or had the cover
off doing filter alignments.
I found I would simply tap the unit and the audio volume would be
normal.

I figured I would eventually get around to checking the circuit when I
had the unit cover off again.

Well last week I replaced the audio and RF gain pots.
Those are working sweetly.

At the time I had the control PC dis-assembled I did some re-soldering
of all of the joints I could find around the LM380.

I thought I had it fixed by doing that but last night it did it again.

Any hints or tips on best way to troubleshoot this?

Maybe tracing the audio circuit with a signal source at the time it
drops?
Your suggestions welcome!!

Thanks
N1BBR
--
 [hidden email]

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[Don Wilhelm-4]

DW,

You have a bad solder connection somewhere, and it is not necessarily in
the audio section - it can be anywhere in the receive chain, including
the noise blanker and the KSB2 option if those are installed.

Use a constant signal source such as a XG1, XG2, XG3 signal generator
set for a 50 uV output.  Tune in the generator output and make some
observations - does the S-meter drop when the audio drops?  If so, the
problem is prior to the output of the IF amplifier.  OTOH, if the
S-meter stays constant and the audio drops, the problem is after the IF
amplifier output.

I can't help much with specifics beyond that to further locate the
problem.  You might "shotgun" it by re-flowing the soldering with a hot
(750 degF) soldering iron - add a tiny bit of flux if necessary.  
Examine your solder connections carefully.  I know nothing of your
soldering capability, so please no insult if you are an expert.  If your
soldering level is expert, the most likely is that you missed a solder
connection somewhere.

A good solder connection will have the solder flowing out to an almost
invisible edge on both the solder pad and the component lead.  There may
be a small concave fillet at the connection.  If your solder looks more
like "balls" (convex) or there is a ring around a component lead, that
is  a problem.

73,
Don W3FPR

On 9/25/2012 5:20 PM, dw wrote:

> Hi Gang,
> I got my K2 a couple of years ago.
> Once in a great while I would notice the audio volume drop to about 30%.
> It would typically happen after I had moved the unit or had the cover
> off doing filter alignments.
> I found I would simply tap the unit and the audio volume would be
> normal.
>
> I figured I would eventually get around to checking the circuit when I
> had the unit cover off again.
>
> Well last week I replaced the audio and RF gain pots.
> Those are working sweetly.
>
> At the time I had the control PC dis-assembled I did some re-soldering
> of all of the joints I could find around the LM380.
>
> I thought I had it fixed by doing that but last night it did it again.
>
> Any hints or tips on best way to troubleshoot this?
>
> Maybe tracing the audio circuit with a signal source at the time it
> drops?
> Your suggestions welcome!!
>

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[Don Wilhelm-4]

DW,

Additional information about Receive Signal Tracing when your problem
occurs.  Yes, that can be done using the RX signal tracing steps in the
manual Appendix E.  You will need a high level signal generator - the
xtal oscillator shown in the manual will do just fine, or the attenuated
output of an MFJ259 antenna analyzer.  You want 0.14 volts at the
antenna as read on an RF probe (2.8 times that if using the peak to peak
RF voltage on an oscilloscope.

Caution:  The addition of diodes D40 and D41 have made the IF Amp
Saturated Output results to be unobtainable (the IF Amp will not
saturate with the diodes in place).  You can temporarily lift one end of
those diodes and go on with the tracing.

The first stage where you find the RF voltage significantly less than
expected is the troublesome stage - any reading after that point are
meaningless until that problem is resolved.

Good luck on it.  Intermittent problems are difficult - one never know
whether the problem has been solved or it just went away to rear its
ugly head at a later time..

73,
Don W3FPR

On 9/25/2012 5:20 PM, dw wrote:

> Hi Gang,
> I got my K2 a couple of years ago.
> Once in a great while I would notice the audio volume drop to about 30%.
> It would typically happen after I had moved the unit or had the cover
> off doing filter alignments.
> I found I would simply tap the unit and the audio volume would be
> normal.
>
> I figured I would eventually get around to checking the circuit when I
> had the unit cover off again.
>
> Well last week I replaced the audio and RF gain pots.
> Those are working sweetly.
>
> At the time I had the control PC dis-assembled I did some re-soldering
> of all of the joints I could find around the LM380.
>
> I thought I had it fixed by doing that but last night it did it again.
>
> Any hints or tips on best way to troubleshoot this?
>
> Maybe tracing the audio circuit with a signal source at the time it
> drops?
> Your suggestions welcome!!
>
> Thanks
> N1BBR

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[Bill Clarke]

Three very important items in the technicians tool box when searching
for intermittents:

1 - a can of Freeze (a freon product)
2 - a hair dryer
3 - a non-conductive probe

Explanation: The cold causes bad solder joints/PCB traces to open -
causing the circuit to fail. The heat will cause expansion and close the
opens in the circuits. The probe can sometimes assist by placing
physical pressure on a component/joint/PCB/socket/etc. - thus opening or
closing a circuit.

Bill W2BLC

--
IN GOD I TRUST (but, NOT a single politician)
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