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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