[Elecraft] KPA100 power on 20m/30m, update

[Ted Turk]

Vic,

I agree with you, Vic.  As perhaps the third to suffer this in recent 
kits, I have spent much time, with help from experienced guys at my 
club, investigating this.  Increasing the caps (C80,81) cured most but 
not all kits at the recommended level (+900pF).  My caps measured at 
4600 & 4700pF on a Peak meter.  Replacing them both with caps measured 
as closer to the design value did not 'cure' things.  As a last resort 
I changed R19,20 with some British obtained items which happened to be 
from Vishay BC; 1.5 ohms was the only nearest value available.  Imagine 
my surprise when all was cured.

Today, in RADCOM there is an article on Low-inductance resistors as 
part of the 'In practice" series.  This gave me some more insight into 
how metal film resistors are made.  Apparently they can have from 1 to 
10 helical turns cut into the metal film and can vary from manufacturer 
to manufacturer.

You've guessed it - I sanded the coating off one of my original 1.6 ohm 
Elecraft resistors and a spare Vishay 1.5 ohm from the same strip of 10 
that fixed my KPA100.
The score - Elecraft approximately 6 turns and Vishay 1.25 turns.  I 
just got lucky!!

Moral - remember to take some sand paper with you when buying these 
type of resistors to be used for RF purposes in critical areas.  Why 
not???  When we ran a small shop on the Grand Union Canal here in 
England, some folks used to poke or squeeze the bread to see how fresh 
it was!!!

73,
Ted G7BQM K2#4732
On 23 Jun 2005, at 17:16, Vic Rosenthal wrote:

> Bill NY9H wrote:
>> i just received 2 ka100 mod kits...... for both radios...
>> My question, should I be changing out the C80 & C81 while i messing 
>> in there...
>> or do the changes in the "ka100 MOD kit"  eliminate the need for a 
>> change.
>> Looks like the R19 & R20 are changed in the 'mod'  and are sitting 
>> right there.
>
> I wish we could get an official statement from Elecraft on this issue.
>
> <begin MY OPINION>
>
> Apparently the metal-film resistors used in this application vary 
> widely in inductance.  Some resistors have an  inductance that seem to 
> resonate with the capacitors near the 30/20 meter bands, basically 
> putting a parallel trap in series with the drive, and causing the 
> driver stage to have to work harder.  This shows up as high current 
> and/or reduced output on 30/20 meters.  Hence whether or not you need 
> to change the capacitors and if so to what value depends on the 
> resistors that you you have.  Note that this depends on the physical 
> construction of the resistor and it's (unwanted) inductance, not its 
> resistance.
>
> <end MY OPINION>
>
> I guess the question is 'what capacitor value is appropriate for the 
> resistors shipped in the mod kit'?
>
> -- 
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco
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