K2 and KPA 100- How Is Hi Current Generated

[Mike Walkington-3]

Hi All,

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?

Mike
VK1KCK
K2 #2599




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[Don Wilhelm-3]

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.

The Hi-Cur indication is coming from the base K2 and not the KPA100 - it
indicates that the base K2 has to drive the KPA100 too hard (either the
KPA100 has inadequate gain or it is really putting out more than 100
watts) - that is why we need to know about your power measurement (R26/R27
calibration) techniques and equipment.

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?

To properly sweep the low pass filters in the KPA100, you can connect the
ribbon cable to the K2 and put a 51 ohm resistor across the pins of J1 (the
K2 stays in receive condition).  Switch to the proper band and do the sweep
from the KPA100 antenna jack - the base K2 must be powered on, no need to
power the KPA100 except through the 12VCTRL signal in the ribbon cable.

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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