K2: KPA 100 Capacitor Specification?

[jferg977]

Felow Elecrafters,
I have just replaced the second-to-fail .01uF cap on the KPA/100 low band  
filter relay circuitry of #5087.  In this case, C13.  Failure is a  partial
short which allows that relay pair (k3 and k4) to close even though  U2 hasn't so
ordered.  Symptom that this has happened is inability to  tune down SWR and
power runaway.  This would make sense because  in this condition - particularly
this one - since I don't have a 160 meter setup  - the low band filters are
triggered for 160 plus whatever other band I thought  I was going to work, two
filters for the price of one - not a bargain.
 
This radio has suffered some hard-knocks before I understood how to deal  
with ESD and I feel that I owe
Elecraft far more than they owe me, But.........
 
Should I replace the relay coil caps with ones with better specs? If so,  
what should I use?
 
73   AI4TO    John Ferguson   M/V  Arcadian
 
 
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[Don Wilhelm-4]

John,

I have never found one of those caps shorted, and I believe the DC
rating on them is 50 volts, so they should have an adequate safety
margin since they are used in a 12 volt DC circuit.
I have trouble imagining a condition that would cause them to fail, but
then I recall that your 'shipboard' operation is unusual and may
introduce 'perils' that I cannot imagine.
There would be no harm in replacing those capacitors with 100 volt caps
*if* you can get them to fit physically.  You may have to solder them
flat against the bottom of the board.  The other problem is that 100
volt caps may be difficult to procure with 0.1 inch lead spacing and you
may have to get creative to substitute caps with 0.2 inch lead spacing.

73,
Don W3FPR

[hidden email] wrote:

> Felow Elecrafters,
> I have just replaced the second-to-fail .01uF cap on the KPA/100 low band  
> filter relay circuitry of #5087.  In this case, C13.  Failure is a  partial
> short which allows that relay pair (k3 and k4) to close even though  U2 hasn't so
> ordered.  Symptom that this has happened is inability to  tune down SWR and
> power runaway.  This would make sense because  in this condition - particularly
> this one - since I don't have a 160 meter setup  - the low band filters are
> triggered for 160 plus whatever other band I thought  I was going to work, two
> filters for the price of one - not a bargain.
>  
> This radio has suffered some hard-knocks before I understood how to deal  
> with ESD and I feel that I owe
> Elecraft far more than they owe me, But.........
>  
> Should I replace the relay coil caps with ones with better specs? If so,  
> what should I use?
>  
> 73   AI4TO    John Ferguson   M/V  Arcadian
>  
>

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[Geoffrey Mackenzie-Kennedy]

John,

If the voltage rating of these relay coil bypass caps is somehow being
exceeded in your shipboard installation causing the caps to fail, and if you
cannot find suitable higher DC voltage rated leaded replacements, you might
like to consider using SMD (chip) capacitors instead.

AVX include in their product line of multilayer ceramic SMD capacitors for
low frequency use, as opposed to microwave rated devices, .01uF  100 VDC
caps in both 0805 and 1206 sizes, and 0.015 uF caps rated at 500 and 600 VDC
in 1206 packages.

I have no financial interest in AVX, but do use their microwave rated
capacitors.

73,
Geoff
GM4ESD


Don Wilhelm wrote on Friday, April 03, 2009 at 3:29 PM.

> John,
>
> I have never found one of those caps shorted, and I believe the DC
> rating on them is 50 volts, so they should have an adequate safety
> margin since they are used in a 12 volt DC circuit.
> I have trouble imagining a condition that would cause them to fail, but
> then I recall that your 'shipboard' operation is unusual and may
> introduce 'perils' that I cannot imagine.
> There would be no harm in replacing those capacitors with 100 volt caps
> *if* you can get them to fit physically.  You may have to solder them
> flat against the bottom of the board.  The other problem is that 100
> volt caps may be difficult to procure with 0.1 inch lead spacing and you
> may have to get creative to substitute caps with 0.2 inch lead spacing.
>
> 73,
> Don W3FPR

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