[Elecraft] Setting bias current on KPA-100

Sun Aug 18 20:06:00 2002

Hi

If your meter does not have a current range or the current range does not=
 give the resolution you need you can measure the current with a 0.1 ohm =
1% 5 or 10 watt resistor. Just connect the resistor in series with the po=
wer supply and read the voltage drop across the resistor. 0.100 volts wil=
l be equal to 1 amp of current. So when adjusting the bias current the re=
ading is going to be around 0.800 to 0.950 volts for 800 to 950 ma. It is=
 best if you can get a 1% or better resistor but if you have several in y=
our junk box get one a little high in resistance and parallel a higher sm=
all resistor until it is as close to .1 ohms as possible. A lot of meters=
 have a relative function that will allow you to subtract out the lead re=
sistance so you can get a accurate reading on the resistor. Use short hea=
vy leads with alligator clips. Short the clips together on the lowest ran=
ge and press the relative button then put the resistor in the clips to me=
asure. =20

I like using a current shunt like the .1 ohm resistor instead of using my=
 meter current ranges. It is to easy to forget and leave the meter on the=
 current function and then try to measure how much current your power sup=
ply can output. Usually blowing the fuse in the meter or worse damaging t=
he internal current shunt. You can make up several of these external shun=
ts in small boxes with 5 way binding posts in 1 ohm .1 ohm and .01 ohm va=
lues

Don Brown
KD5NDB

----- Original Message -----
From: Douglas Westover
Sent: Sunday, August 18, 2002 6:13 PM
To: Doug Person; [email protected]
Subject: Re: [Elecraft] Setting bias current on KPA-100

Hi Doug,

I found the bias setting to be fairly critical. I had a problem
with the KPA becoming very unstable on SSB and spent
a lot of time bugging Gary. Finally turned out to be the bias
adjustment which I had originally set with an old analog
VOM with very poor current resolution.

73,
Doug
W6JD

----- Original Message -----
From: "Doug Person" <[email protected]>
To: <[email protected]>
Sent: Sunday, August 18, 2002 10:05 AM
Subject: [Elecraft] Setting bias current on KPA-100

> How important is it to set the bias current accurately.  None of my met=
ers
> seem to do very well in the necessary range.  They all tend to go 0 to
200ma
> and then jump to 10 amps.  At the higher setting resolution isn't very
good.
> I think I'm OK and near enough but wonder whether to err over or under.
>
> I also notice that KPA is very sensitive to SWR.  It starts to back dow=
n
> power at 1.5:1 or more.  Are the finals really that sensitive?  I notic=
e
too
> that the fan doesn't seen to run much.  Is it better to set it so it ru=
ns
in
> slow all the time? Or is it fine to just leave it in the normal positio=
n?
>
> Doug -- W4DXV
> K2/100 #1920 K1/4 #1297

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