kpa100 not tracking power with K2

[Pat n8vw]

Having problems with my kpa100 not tracking power with respect to the power
knob on the K2.  For example on 40m if the K2 says 40w the wattmeter shows
100 watts and the current meter on the power supply confirms the latter with
~18 amps.  

The leads me to suspect the power control circuit, but I'm unfamiliar with
its operation.

Any help would be appreciated.

Pat N8vw


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[Don Wilhelm-3]

Pat,

If this condition is confined to 40 meters, check out the KPA100 specific
information regarding the position of RFC10 (the one soldered to the antenna
jack) on Tom Hammond's website www.n0ss.net .

If it happens on all bands, then you have another problem, so ask again and
provide more information and we can lead you on a path of 'problem chasing'.
First you will have to tell us whether the 'wattmeter' you refer to is an
external wattmeter or the internal K2 indication.

73,
Don W3FPR

> -----Original Message-----
>
> Having problems with my kpa100 not tracking power with respect to
> the power
> knob on the K2.  For example on 40m if the K2 says 40w the wattmeter shows
> 100 watts and the current meter on the power supply confirms the
> latter with
> ~18 amps.
>
> The leads me to suspect the power control circuit, but I'm unfamiliar with
> its operation.
>
> Any help would be appreciated.
>
> Pat N8vw
>

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[Pat n8vw]

On Mon, 14 Aug 2006 22:04:16 -0400, "Don Wilhelm" <[hidden email]> wrote :

> Pat,
>
> If this condition is confined to 40 meters, check out the KPA100 specific
> information regarding the position of RFC10 (the one soldered to the antenna
> jack) on Tom Hammond's website www.n0ss.net .
>

Already moved.

> If it happens on all bands, then you have another problem, so ask again and
> provide more information and we can lead you on a path of 'problem chasing'.
> First you will have to tell us whether the 'wattmeter' you refer to is an
> external wattmeter or the internal K2 indication.
>

Both.  Happens on all bands.  Relative to an external wattmeter the power
the K2 shows is wrong and the power the KPA100 outputs in some cases is not
ok.  Max out on the lowbands is almost 150 watts and pulled current is above
20 amps.  On 20m and above output is more normal, output is never above
100w. For example the K2 reads 80, but the external wattmeter reads 100w.
On 15m output is below normal.  The K2 will read 110 and the max out is 90w.
Approachs normal at around 70w.  On 10m ouptput tracks like 20m except max
out is about 110w.


Pat N8vw

> 73,
> Don W3FPR
>
>
> > -----Original Message-----
> >
> > Having problems with my kpa100 not tracking power with respect to
> > the power
> > knob on the K2.  For example on 40m if the K2 says 40w the wattmeter shows
> > 100 watts and the current meter on the power supply confirms the
> > latter with
> > ~18 amps.
> >
> > The leads me to suspect the power control circuit, but I'm unfamiliar with
> > its operation.
> >
> > Any help would be appreciated.
> >
> > Pat N8vw
> >
>
>
>
>

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[Don Wilhelm-3]

Pat,

Something strange is happening here.  First the facts of 'the case':
1) The KPA100 wattmeter should not be frequency dependent, and neither
should your external wattmeter if it is functioning correctly.
2) Your KPA100 wattmeter apparently needs to be calibrated - use a good 50
ohm resistive dummy load and start over at the point where you adjust the
trimmer cap - caution, a dummy load that is reactive will give you an
incorrect indication of the null point.
3) If you use your external wattmeter to set the KPA100 forward power level,
then the K2/100 indication should track with that external wattmeter
afterwards.
4) Be aware that an external wattmeter may be in error by as much as 10 or
20% OF THE FULL SCALE READING!  This is the typical spec for wattmeters.  Be
careful of the scale, on a 100 watt scale, that means that the error could
be as large as 20 watts (at any point on the scale), so do not use a 100
watt scale to read a 20 watt level, use a 20 watt scale where the maximum
error is only 4 watts.  I cannot stress this subtle point strongly enough.
Even a Bird wattmeter is speced at 5% of full scale, and that is only
guaranteed after calibration!

You may have something else wrong there too, but as first steps, check the
components in the wattmeter section and re-do the wattmeter calibration.
Then see if the power control becomes proper.

73,
Don W3FPR


> -----Original Message-----

> > If it happens on all bands, then you have another problem, so
> ask again and
> > provide more information and we can lead you on a path of
> 'problem chasing'.
> > First you will have to tell us whether the 'wattmeter' you
> refer to is an
> > external wattmeter or the internal K2 indication.
> >
>
> Both.  Happens on all bands.  Relative to an external wattmeter the power
> the K2 shows is wrong and the power the KPA100 outputs in some
> cases is not
> ok.  Max out on the lowbands is almost 150 watts and pulled
> current is above
> 20 amps.  On 20m and above output is more normal, output is never above
> 100w. For example the K2 reads 80, but the external wattmeter reads 100w.
> On 15m output is below normal.  The K2 will read 110 and the max
> out is 90w.
> Approachs normal at around 70w.  On 10m ouptput tracks like 20m except max
> out is about 110w.
>
>
> Pat N8vw
>

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[Pat n8vw]

On Tue, 15 Aug 2006 08:53:22 -0400, "Don Wilhelm" <[hidden email]> wrote :

> Pat,
>
> Something strange is happening here.  First the facts of 'the case':
> 1) The KPA100 wattmeter should not be frequency dependent, and neither
> should your external wattmeter if it is functioning correctly.
> 2) Your KPA100 wattmeter apparently needs to be calibrated - use a good 50
> ohm resistive dummy load and start over at the point where you adjust the
> trimmer cap - caution, a dummy load that is reactive will give you an
> incorrect indication of the null point.
> 3) If you use your external wattmeter to set the KPA100 forward power level,
> then the K2/100 indication should track with that external wattmeter
> afterwards.
> 4) Be aware that an external wattmeter may be in error by as much as 10 or
> 20% OF THE FULL SCALE READING!  This is the typical spec for wattmeters.  Be
> careful of the scale, on a 100 watt scale, that means that the error could
> be as large as 20 watts (at any point on the scale), so do not use a 100
> watt scale to read a 20 watt level, use a 20 watt scale where the maximum
> error is only 4 watts.  I cannot stress this subtle point strongly enough.
> Even a Bird wattmeter is speced at 5% of full scale, and that is only
> guaranteed after calibration!
>

Don, I've done all this and taken it into account.  I don't typically post
unless I've spent several hours puzzling over a problem.

> You may have something else wrong there too, but as first steps, check the
> components in the wattmeter section and re-do the wattmeter calibration.
> Then see if the power control becomes proper.
>
Here is what I think the problem is:

U7 Pin 1 is listed as being between 0 and .8 volts on rx mine is 0 volts on
transmit it is about whatever the supply voltage is, in this case 13.5v.
Looking at the circuit, I see that it supplys voltage to Q5 and is part of
the current sense.  On transmit I measured zero volts on the emitter of Q5,
TP1 and that end of R30.  Seems like Q5 might be dead.  

I replaced Q5 and now U7 Pin 1 is about .5v on tx.  However, U7 pin 7 is not
right so guess Q4 might be suspect.


Pat





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