K2 / KPA100 - net shorted on KPA100 --> C31 / D13 ... failure modes ?

[OZ2BRN Brian Lodahl]

Hi there,

After 100 % flawless work with the whole K2 journey, the KPA100 and all
the add-on modules with bell's and whistles, it seems my turn has come
now to have a build error on # 6936, or more specificly; the KPA-100:

When doing the 2nd alignment step of KPA100, I measure all the
resistances OK but one; The cathode of D13 seems very low resistance to
ground and not >10 kohms as expected. I measured some 6-7 ohms.

The intial checkout in 1st alignment step round confirmed all diodes
pass and having the expected forward and reverse voltages / resistances
as expected, so I assume for now the Diodes are OK still.

Now I may have myself an idea to the failure mode, but can any of you
think of other (known) failure modes to this ?

My assumption is that I have shorted out the two foil layers of
capacitor C31; effectively putting both sides of C31 to ground - as I
managed to nick the house of C31 with my auxillary 30W solder iron, when
soldering the emitters of Q1 and Q2, and the plastic housing of C31 is
now slightly open and the internal foil visible.

( and yes; I'm givin' myself a hard time and cursing my own lack of
being careful :-) !! )

Any other bright ideas beyond the obvious C31 defect, hence replace ?

73'
Brian OZ2BRN
K2 # 6936



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[Don Wilhelm-4]

  Brian,

Why speculate, just remove C31 (it needs to be replaced anyway) and
remeasure to confirm that C31 is indeed the problem - it quite likely is.

73,
Don W3FPR

On 1/22/2011 3:11 PM, OZ2BRN Brian Lodahl wrote:

> Hi there,
>
> After 100 % flawless work with the whole K2 journey, the KPA100 and all
> the add-on modules with bell's and whistles, it seems my turn has come
> now to have a build error on # 6936, or more specificly; the KPA-100:
>
> When doing the 2nd alignment step of KPA100, I measure all the
> resistances OK but one; The cathode of D13 seems very low resistance to
> ground and not>10 kohms as expected. I measured some 6-7 ohms.
>
> The intial checkout in 1st alignment step round confirmed all diodes
> pass and having the expected forward and reverse voltages / resistances
> as expected, so I assume for now the Diodes are OK still.
>
> Now I may have myself an idea to the failure mode, but can any of you
> think of other (known) failure modes to this ?
>
> My assumption is that I have shorted out the two foil layers of
> capacitor C31; effectively putting both sides of C31 to ground - as I
> managed to nick the house of C31 with my auxillary 30W solder iron, when
> soldering the emitters of Q1 and Q2, and the plastic housing of C31 is
> now slightly open and the internal foil visible.
>
> ( and yes; I'm givin' myself a hard time and cursing my own lack of
> being careful :-) !! )
>
> Any other bright ideas beyond the obvious C31 defect, hence replace ?
>
> 73'
> Brian OZ2BRN
>

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[OZ2BRN Brian Lodahl]

Yep, that was it.

You can add this to the list of do's and particularly the DON'Ts of the
KPA-100 assembly:

* When soldering Q1 and Q2, be careful not to nick the nearby components
with the solder iron(s).
* If the checkout step of resistance measuring D13 cathode fails and you
read something like a short to ground on this net, this could be caused
by C31 shorting out as a consequence of the solder process failure above.

C31 had shorted out  - I guess the two foils were shorted when I hit
them with my auxillary solder iron.

Replacement done with 2x100 nF / 63V polyester caps in parallel, and
we're back again in the alignment buisness.

73'
Brian OZ2BRN
K2 / KPA100 # 6936

PS: and Don; Reason for asking was to check any other issues the group
would have suggested at the same time, now that I had to take off the
whole heat sink anyway, in order not to take it off too many times and
risk damaging the heat transfer pads in the process.



Den 22-01-2011 21:40, Don Wilhelm skrev:

>  Brian,
>
> Why speculate, just remove C31 (it needs to be replaced anyway) and
> remeasure to confirm that C31 is indeed the problem - it quite likely is.
>
> 73,
> Don W3FPR
>
> On 1/22/2011 3:11 PM, OZ2BRN Brian Lodahl wrote:
>> Hi there,
>>
>> After 100 % flawless work with the whole K2 journey, the KPA100 and all
>> the add-on modules with bell's and whistles, it seems my turn has come
>> now to have a build error on # 6936, or more specificly; the KPA-100:
>>
>> When doing the 2nd alignment step of KPA100, I measure all the
>> resistances OK but one; The cathode of D13 seems very low resistance to
>> ground and not>10 kohms as expected. I measured some 6-7 ohms.
>>
>> The intial checkout in 1st alignment step round confirmed all diodes
>> pass and having the expected forward and reverse voltages / resistances
>> as expected, so I assume for now the Diodes are OK still.
>>
>> Now I may have myself an idea to the failure mode, but can any of you
>> think of other (known) failure modes to this ?
>>
>> My assumption is that I have shorted out the two foil layers of
>> capacitor C31; effectively putting both sides of C31 to ground - as I
>> managed to nick the house of C31 with my auxillary 30W solder iron, when
>> soldering the emitters of Q1 and Q2, and the plastic housing of C31 is
>> now slightly open and the internal foil visible.
>>
>> ( and yes; I'm givin' myself a hard time and cursing my own lack of
>> being careful :-) !! )
>>
>> Any other bright ideas beyond the obvious C31 defect, hence replace ?
>>
>> 73'
>> Brian OZ2BRN
>>
>
>

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