RE: K2 and KPA 100- How Is Hi Current Generated

[Mike Walkington-3]

Hi Don,

Thanks for your reply. First the easy answers:
I'm using two dummy loads, the primary one being an old fashioned tubular
carbon resistor - about half an inch across and two inches long. Both show a
flat response across the spectrum when scanned with an MFJ-259 analyzer. I'm
using the power meter built into a Kenwood AT-200 Antenna Tuner to measure
power. I use a KAT100 as a tuner though. The AT-200 handbook claims it has
plus/minus 10% accuracy at full scale (20W and 200W) ranges.

By the way, I rechecked the K2 power output and its fine 15W at 160M falling
to 12 or so at 10m.

KPA100 low pass filters were swept with MFJ259 as described in manual and by
you. I used a 51 ohm resistor. Sweeps on other bands seemed OK 1:1.1 and
1:1.2.

My concern re high SWR on 30m and 10m was that the KPA100 might reduce power
if it detected SWR greater than 1.5, but I wasn't sure where this would be
detected.  I'll need to recheck LPF components values, but believe they were
OK. Still, I wouldn't be the first person on the reflector to have misplaced
confidence ;).  I'll also try spreading the torroid coils.  Do you have any
suggestion on which torroid to start with, ie one closest to antenna, or tx
or the one in the middle?

I had fixed the HI Current warning by resetting R26/27 and adjusting bias,
but still wasn't getting the specified power out. Reading back through the
archives there was a suggestion that when setting R26 you should change the
power output each time (to reset the ALC). I did a bit of this but can't
recollect if I did it every time. Still would failing to do this make much
of a difference?

Mike
VK1KCK
K2 #2599


> -----Original Reply-----

Mike,

How are you measuring power out?  and how good is your dummy load? (check it
with your antenna analyzer) - that will get the first things out of the way
for us.

OK, you said you swept the KPA100 low pass filters for 30 through 10 and
found them OK, (well, you found 1.5 or less and I would expect 1.2 or 1.3,
but you will have to tell me how you set up for the sweep before I draw any
conclusions).  What do the sweeps for 160 through 40 meters show?

If you do the sweep as above, you should find the SWR 1.2 or below in most
cases - if not, look carefully at your toroids (count the turns) and triple
check the capacitor values.  I have found on occasion that the SWR on 10
meters can be reduced a bit by judicious spreading/compressing the turns of
the 10/12/15 meter LPF, but the other bands fall into place without any
'tweaking'.  In any case, such 'tweaking' will not improve the KPA100 power
output, but will allow the base K2 to drive through the high power LPF a bit
easier when at the QRP levels. Bottom line here - such 'tweaking' will not
solve whatever problem you have.

I doubt that the new capacitors will solve this particular problem either -
indeed you may end up installing the new capacitors, but if you have a
problem with 160 through 40 meters, the capacitors are not likely to fix it.

The bias control setting on the KPA100 will not produce the symptoms you
describe either - yes it will affect the current drawn by the KPA100, but
remembere that the HI-CUR indication means that the base K2 is drawing too
much current (it is being asked to produce more power than the current limit
will allow)

73,
Don W3FPR

> -----Original Message-----
>
> I've just spent the day fine tuning my KPA100 - measuring the output power
> on each band and trying to improve it. Initially I was getting a
> HI Current
> warning on each of the bands from 160 through to 40 (all into a
> dummy load),
> but not on the rest. In all cases the maximum power I could get
> out on each
> band was 50-65W. I reset R26/R27 and the bias current and now the
> HI Current
> warning has gone and power output has improved on most bands to
> 70+W, but is
> not yet 100W. Some observations and questions that came up:
>
> Can someone please explain to me how the HI Current warning is generated?
> Is it likely that my bias current is a little low?
> I swept the low pass filters with an antenna analyser and the SWR
> on 30m and
> 10m is about 1.5.  All the components look to be right, and I feel that
> spreading or compressing the turns on the applicable torroids may
> upset the
> associated filters. Is 1.5 too high (I have the previous version of the
> KPA100 firmware)?
> Is the new KPA Capacitor mod likely to help?
>
>

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[Mike Walkington-3]

Hi Jim,

Good question.  While I have been troubleshooting the KPA I have left the
KAT100 out of circuit - I don't need it while transmitting into a dummy
load.  The AT-200 has a straight through mode for power measurement that I
am using. I only borrowed the AT-200 for power meter as the KAT100 is
excellent :).

Mike
VK1KCK
K2 #2599

-----Original Message-----
From: k4zm [mailto:[hidden email]]
Sent: Wednesday, 29 June 2005 9:44 AM
To: Mike Walkington
Subject: Re: [Elecraft] RE: K2 and KPA 100- How Is Hi Current Generated

Don:

Do you have the Kenwood AT-200 installed between the KPA100 and the KAT100
or beween the KAT100 Output and the antenna?  If connected on the output of
the KAT100 you cannot expect to see an accurate SWR/POWER indication on the
AT-200.

73
Jim Younce K4ZM
K2  SN: 18





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