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KPA100 Hi Cur

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Jonathan Taylor, K1RFD

KPA100 Hi Cur

K2 #2462

I just upgraded the KPA100 from Rev A to Rev D, and installed the
upgraded shield, under the philosophy of "better late than never" :)

I am noticing two problems, when connected to a 50-ohm dummy load:

1) In TUNE, the display briefly shows "Hi Cur", followed by a strange
power/SWR indication such as "7 5.0-1"
2) At any setting above 10W, the Power knob seems to have no effect --
the wattmeter always reads approximately 100W output. This is true
both in TUNE and in CW.

The actual power output appears to be about 100W, when measured with
an external wattmeter (although a bit low on 10M).

Do you think re-alignment is required?

Thanks
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Don Wilhelm

Re: KPA100 Hi Cur

Jonathan,

The normal cause of that HiCur message is a failure in the wattmeter of
the KPA100. Re-alignment will not help.

To test for that possibility set the power knob for about 50 watts,
connect an external wattmeter and a dummy load. Then go Keydown and see
what the actual power output is. If it is in excess of 100 watts, then
that is proof that the KPA100 wattmeter has a problem.

The most common cause is that the diodes D16 and D17 have been damaged
by static from the feedline, so replacing D16 and D17 is the first step
in attempting to correct the problem. Check again for actual power
output with the power set for 50 watts after replacing those diodes.

If you still have a problem after replacing those diodes, I can help
with further troubleshooting.

73,
Don W3FPR

On 1/3/2020 9:15 AM, Jonathan Taylor, K1RFD wrote:

> K2 #2462
>
> I just upgraded the KPA100 from Rev A to Rev D, and installed the
> upgraded shield, under the philosophy of "better late than never" :)
>
> I am noticing two problems, when connected to a 50-ohm dummy load:
>
> 1) In TUNE, the display briefly shows "Hi Cur", followed by a strange
> power/SWR indication such as "7 5.0-1"
> 2) At any setting above 10W, the Power knob seems to have no effect --
> the wattmeter always reads approximately 100W output. This is true
> both in TUNE and in CW.
>
> The actual power output appears to be about 100W, when measured with
> an external wattmeter (although a bit low on 10M).
>

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Jonathan Taylor, K1RFD

Re: KPA100 Hi Cur

Don,

Thanks for the tip -- that was exactly it. I checked and replaced D16
and D17 and all is well now. (Both were showing the same voltage drop
in both directions.) I gather from your description that the built-in
wattmeter forms a negative feedback loop with the power adjustment.

My diodes and I are definitely static-challenged -- I have had a
similar problem at least twice in the KAT100 despite my best efforts
to remember to disconnect the antenna after operating.

73
Jonathan K1RFD

On Fri, Jan 3, 2020 at 10:44 AM Don Wilhelm <[hidden email]> wrote:

>
> Jonathan,
>
> The normal cause of that HiCur message is a failure in the wattmeter of
> the KPA100. Re-alignment will not help.
>
> To test for that possibility set the power knob for about 50 watts,
> connect an external wattmeter and a dummy load. Then go Keydown and see
> what the actual power output is. If it is in excess of 100 watts, then
> that is proof that the KPA100 wattmeter has a problem.
>
> The most common cause is that the diodes D16 and D17 have been damaged
> by static from the feedline, so replacing D16 and D17 is the first step
> in attempting to correct the problem. Check again for actual power
> output with the power set for 50 watts after replacing those diodes.
>
> If you still have a problem after replacing those diodes, I can help
> with further troubleshooting.
>
> 73,
> Don W3FPR
>
> On 1/3/2020 9:15 AM, Jonathan Taylor, K1RFD wrote:
> > K2 #2462
> >
> > I just upgraded the KPA100 from Rev A to Rev D, and installed the
> > upgraded shield, under the philosophy of "better late than never" :)
> >
> > I am noticing two problems, when connected to a 50-ohm dummy load:
> >
> > 1) In TUNE, the display briefly shows "Hi Cur", followed by a strange
> > power/SWR indication such as "7 5.0-1"
> > 2) At any setting above 10W, the Power knob seems to have no effect --
> > the wattmeter always reads approximately 100W output. This is true
> > both in TUNE and in CW.
> >
> > The actual power output appears to be about 100W, when measured with
> > an external wattmeter (although a bit low on 10M).
> >

Don Wilhelm

Re: KPA100 Hi Cur

Johnathan and all,

Actually it is not negative feedback for power control, but it is a
closed circuit control loop providing positive control of the power level.
The actual power is measured at the KPA100 output and the result is sent
to the MCU (via the VRFDET signal line) where it is compared with the
requested power setting. The result of that comparison is used to
control the level of BFO injection in order to regulate the power output.

When the KPA100 wattmeter diodes are faulty, the MCU thinks there is low
or no power output, and ramps up the drive to maximum. The result is
uncontrolled maximum power output. If operated in that condition for
some time, the base K2 PA transistors will be stressed and will
eventually fail. That condition also causes distortion, especially
noted in SSB transmissions, but also increased IMD on the transmitted
signal because the base K2 PA transistors are no longer being operated
in a linear fashion.

With the KPA100 in the circuit, the KAT100 power sensing is not used,
and the KAT100 wattmeter is used only to light the SWR indicators.
The KAT100 wattmeter is used if the KPA100 is not present. The KAT2
wattmeter is used if neither the KPA100 or KAT100 wattmeters are not
present. In the basic K2 (without the KAT2 or KPA100 or KAT100), there
is an RF Detector to measure the power output, but that detector is
dependent on the load - a 50 ohm resistivity load is assumed by the MCU
calculation of power so it will not be correct if the load is other than
50 ohms non-reactive.

So that is the story of how the K2 controls the power output. The K1
and K3/K3S use a similar system. As far as I know, this system of power
control is only used by some commercial transceivers and most other
amateur transceivers simply let the operator control the drive level or
provide only a fixed drive level.

73,
Don W3FPR

On 1/3/2020 10:15 PM, Jonathan Taylor, K1RFD wrote:

> Don,
>
> Thanks for the tip -- that was exactly it. I checked and replaced D16
> and D17 and all is well now. (Both were showing the same voltage drop
> in both directions.) I gather from your description that the built-in
> wattmeter forms a negative feedback loop with the power adjustment.
>
> My diodes and I are definitely static-challenged -- I have had a
> similar problem at least twice in the KAT100 despite my best efforts
> to remember to disconnect the antenna after operating.
>
>