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K2 KPA100 SWR Bridge Readings Way Too High

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radiotelegrapher

K2 KPA100 SWR Bridge Readings Way Too High

The SWR bridge on my K2/KPA100 had been working perfectly for years, but I
apparently blew something while making comparison measurements with a
friend's K2 I was trying to fix. Now the K2 panel LCD SWR indicator reads
around 5:1 for a near-perfect dummy load. Adjusting C1 makes almost no
difference. I can force the SWR to read 1:1 by setting R26 toward one end
of its travel and R27 toward the opposite end of its travel (instead of
both at midpoint), but then every load reads as 1:1 SWR. Replacing D16 and
D17 didn't help (I tried it twice). Except for the SWR reading, everything
seems OK. Perhaps I should try replacing U5 and U6? Other ideas? Dick,
K0KK
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Don Wilhelm

Re: K2 KPA100 SWR Bridge Readings Way Too High

Dick,

Make certain you have the diodes D16 and D17 mounted the right way around.

Replace the coax jumper to the dummy load - measure your dummy load with
your ohmmeter to be certain it is a good 50 ohm dummy load (yes, dummy
loads can fail).

Then balance the wattmeter. Easiest way is to power the K2 from only
the coaxial 2.1mm power jack (no power to the KPA100) with power set to
10 watts and adjust C1 for a voltage as close to zero as possible read
at TP4 (should be in the low mV range). If you cannot see a very low
voltage at TP4. then measure U5 pin 3 and adjust C1 for the lowest voltage.
Next with power off preset R26 to 43k ohms read at U5 pin 5 and R27 to
43k read at U5 pin 3.

You can do the initial tests of the wattmeter still in low power mode
(no power to the KPA100). Set for 10 watts and do a TUNE - do not stay
in TUNE for more than 30 seconds at a time, allow a 1 minute cooling off
period between each TUNE.
Check the DC voltage at U5 pin 3. It should be close to zero. The
voltage at U5 pin 6 should also be close to zero. Also check the
voltage at U1 pin 3 which should also be near zero, and the K2 display
should show an SWR of 1.

Connect an external wattmeter between the dummy load and the KPA100 -
make certain you use known good coax jumpers.

Now do a TUNE and set R26 so the display power is the same as your
external wattmeter.

Next, with the same TUNE conditions, measure the DC voltage at U5 pin 5.
You should see the same DC voltage at U5 pin 7, and at U1 pin 2.

Those two voltages at U1 will determine the SWR calculated by the KPA100
firmware and passed along to the base K2 MCU.

If all is well on the DC voltages to this point and the K2 still shows
an SWR greater than 1.0 or 1.1, you have a KPA100 firmware problem.

If the firmware is showing the correct SWR, then set R26 so the K2
display shows the same power as on your external wattmeter. You will
want to refine that power (R26) adjustment at 100 watts later but the 10
watt level will be sufficient for the initial setting.

If you are not able to set the power display without deviating
substantially from the 43k initial setting, then check the DC voltages
during a TUNE at U6. The DC voltage at U6 pin 3 should be the same as
that measured at U5 pin 7, and the DC voltage at U6 pin 6 should be
about twice that at pin 3.

Take it one step at a time and in the order that I indicated. If you
skip around, you will become confused because one thing depends on the
step before it.

If all those conditions are correct, the KPA100 wattmeter is working
correctly. Power the KPA100 and try it at 100 watts (hold TUNE/DISPLAY
together). Quickly adjust R26 to display the actual power. If you have
to move R26 very far, then you have a problem with the scaling.
Transistors Q10 and Q11 may not be working properly.

73,
Don W3FPR

On 10/2/2017 10:58 PM, Dick Lamb wrote:
> The SWR bridge on my K2/KPA100 had been working perfectly for years, but I
> apparently blew something while making comparison measurements with a
> friend's K2 I was trying to fix. Now the K2 panel LCD SWR indicator reads
> around 5:1 for a near-perfect dummy load. Adjusting C1 makes almost no
> difference. I can force the SWR to read 1:1 by setting R26 toward one end
> of its travel and R27 toward the opposite end of its travel (instead of
> both at midpoint), but then every load reads as 1:1 SWR. Replacing D16 and
> D17 didn't help (I tried it twice). Except for the SWR reading, everything
> seems OK. Perhaps I should try replacing U5 and U6? Other ideas? Dick,
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radiotelegrapher

Re: K2 KPA100 SWR Bridge Readings Way Too High

Don,

I set both R26 and R27 to 43K Ohms, applied power directly to the base unit
(no power to KPA100), and set power know to call for 10 W. In TUNE mode
the SWR reads about 9.9:1 regardless of C1 setting. I think the C1 setting
has no effect. I measure the power out at about 15 W, and the K2 says it's
putting out about 5 W. Voltage at U5 Pin 3 is about -1.040 V.

The actual power out stays high when I adjust the power knob (exiting TUNE
mode between attempts), but it starts falling off rapidly when I call for
less than 1 W. At calling for 0.1 W (full CCW on the power knob), U5 Pin 3
is about 70 mV, SWR reads 4:1, the K2 display says 0.1W output. (I'd guess
real output at that point is around 1.0 W--hard to interpolate that low on
my Heathkit HM-2140 power/SWR meter.

I've checked and tried a couple different dummy loads and jumper sets, and
I confirmed D16 and D17 are mounted in the right direction.

Dick, K0KK
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Don Wilhelm

Re: K2 KPA100 SWR Bridge Readings Way Too High

Dick,

Is that voltage at U5 pin 3 actually negative? If so diode D17 is
backwards - the voltage should be positive in all cases. Yes, C1 will
not adjust if the D17 polatity is wrong.
You can refer to the parts placement diagram at the back of the KPA100
manual if you cannot see the silkscreen clearly.

After putting D17 in correctly, recheck the 43k initial setting of R27
and go from there.

73,
Don W3FPR

On 10/3/2017 2:08 PM, Dick Lamb wrote:

> Don,
>
> I set both R26 and R27 to 43K Ohms, applied power directly to the base
> unit (no power to KPA100), and set power know to call for 10 W. In
> TUNE mode the SWR reads about 9.9:1 regardless of C1 setting. I think
> the C1 setting has no effect. I measure the power out at about 15 W,
> and the K2 says it's putting out about 5 W. Voltage at U5 Pin 3 is
> about -1.040 V.
>
>

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radiotelegrapher

Re: K2 KPA100 SWR Bridge Readings Way Too High

Yes, the voltage is negative on U5 Pin 3, but D17 is not installed
backward--i.e., the cathode band is on the side away from the coax
connector, as shown in the manual. I tried reversing the diodes (cathode
band on the side of the the coax connector), and Pin 3 voltage does drop to
millivolt positive range, and the SWR shows 1:1, but the power output
doesn't change regardless of the "power" knob setting. I verified that the
diodes are banded correctly with a semiconductor tester. Dick, K0KK

On Tue, Oct 3, 2017 at 1:20 PM, Don Wilhelm <[hidden email]> wrote:

> Dick,
>
> Is that voltage at U5 pin 3 actually negative? If so diode D17 is
> backwards - the voltage should be positive in all cases. Yes, C1 will not
> adjust if the D17 polatity is wrong.
> You can refer to the parts placement diagram at the back of the KPA100
> manual if you cannot see the silkscreen clearly.
>
> After putting D17 in correctly, recheck the 43k initial setting of R27 and
> go from there.
>
> 73,
> Don W3FPR
>
>
> On 10/3/2017 2:08 PM, Dick Lamb wrote:
>
>> Don,
>>
>> I set both R26 and R27 to 43K Ohms, applied power directly to the base
>> unit (no power to KPA100), and set power know to call for 10 W. In TUNE
>> mode the SWR reads about 9.9:1 regardless of C1 setting. I think the C1
>> setting has no effect. I measure the power out at about 15 W, and the K2
>> says it's putting out about 5 W. Voltage at U5 Pin 3 is about -1.040 V.
>>
>>
>>
>

Don Wilhelm

Re: K2 KPA100 SWR Bridge Readings Way Too High

Dick,

That is mystifying. With the diode "backwards", you get a positive
voltage at U5 pin 3 - that should be the correct orientation for D17.

For the power indication, check the polarity of the voltage at U5 pin 5
- if that is negative, then D16 is also backwards.

You may have a situation where T4 is not wound with the correct sense,
although the orientation of the diodes should still produce a positive
voltage at U5.
Something is strange here.

73,
Don W3FPR

On 10/3/2017 3:44 PM, Dick Lamb wrote:

> Yes, the voltage is negative on U5 Pin 3, but D17 is not installed
> backward--i.e., the cathode band is on the side away from the coax
> connector, as shown in the manual. I tried reversing the diodes
> (cathode band on the side of the the coax connector), and Pin 3
> voltage does drop to millivolt positive range, and the SWR shows 1:1,
> but the power output doesn't change regardless of the "power" knob
> setting. I verified that the diodes are banded correctly with a
> semiconductor tester. Dick, K0KK
>
> On Tue, Oct 3, 2017 at 1:20 PM, Don Wilhelm <[hidden email]
> <mailto:[hidden email]>> wrote:
>
> Dick,
>
> Is that voltage at U5 pin 3 actually negative? If so diode D17 is
> backwards - the voltage should be positive in all cases. Yes, C1
> will not adjust if the D17 polatity is wrong.
> You can refer to the parts placement diagram at the back of the
> KPA100 manual if you cannot see the silkscreen clearly.
>
> After putting D17 in correctly, recheck the 43k initial setting of
> R27 and go from there.
>

radiotelegrapher

Re: K2 KPA100 SWR Bridge Readings Way Too High

Don,

I found the problem: the short wire that connects the ungrounded side of
C1 to the SO259 jack had a poor solder connection.

I figured that since C1 wouldn't tune, there had to be a problem in that
bridge circuit prior to U5. I started tracing each component's
connections with my Ohmmeter, and I finally discovered that the ungrounded
solder pad for C1 didn't connect.

Thanks for your help, Don. You're a terrific resource and your
extraordinary efforts and patience in helping bewildered folks like me are
really appreciated.

73
Dick, K0KK

On Tue, Oct 3, 2017 at 3:10 PM, Don Wilhelm <[hidden email]> wrote:

> Dick,
>
> That is mystifying. With the diode "backwards", you get a positive
> voltage at U5 pin 3 - that should be the correct orientation for D17.
>
> For the power indication, check the polarity of the voltage at U5 pin 5 -
> if that is negative, then D16 is also backwards.
>
> You may have a situation where T4 is not wound with the correct sense,
> although the orientation of the diodes should still produce a positive
> voltage at U5.
> Something is strange here.
>
> 73,
> Don W3FPR
>
> On 10/3/2017 3:44 PM, Dick Lamb wrote:
>
>> Yes, the voltage is negative on U5 Pin 3, but D17 is not installed
>> backward--i.e., the cathode band is on the side away from the coax
>> connector, as shown in the manual. I tried reversing the diodes (cathode
>> band on the side of the the coax connector), and Pin 3 voltage does drop to
>> millivolt positive range, and the SWR shows 1:1, but the power output
>> doesn't change regardless of the "power" knob setting. I verified that the
>> diodes are banded correctly with a semiconductor tester. Dick, K0KK
>>
>> On Tue, Oct 3, 2017 at 1:20 PM, Don Wilhelm <[hidden email]
>> <mailto:[hidden email]>> wrote:
>>
>> Dick,
>>
>> Is that voltage at U5 pin 3 actually negative? If so diode D17 is
>> backwards - the voltage should be positive in all cases. Yes, C1
>> will not adjust if the D17 polatity is wrong.
>> You can refer to the parts placement diagram at the back of the
>> KPA100 manual if you cannot see the silkscreen clearly.
>>
>> After putting D17 in correctly, recheck the 43k initial setting of
>> R27 and go from there.
>>
>>
>

Don Wilhelm

Re: K2 KPA100 SWR Bridge Readings Way Too High

Dick,

With that corrected, do you have some semblance of power control?
You should, but there are other pieces of the KPA100 wattmeter sending
the VRFDET signal back to the base K2 MCU that could be producing a
problem with the power output.

73,
Don W3FPR

On 10/3/2017 9:49 PM, Dick Lamb wrote:

> Don,
>
> I found the problem: the short wire that connects the ungrounded side
> of C1 to the SO259 jack had a poor solder connection.
>
> I figured that since C1 wouldn't tune, there had to be a problem in
> that bridge circuit prior to U5. I started tracing each component's
> connections with my Ohmmeter, and I finally discovered that the
> ungrounded solder pad for C1 didn't connect.
>
> Thanks for your help, Don. You're a terrific resource and your
> extraordinary efforts and patience in helping bewildered folks like me
> are really appreciated.
>
>

radiotelegrapher

Re: K2 KPA100 SWR Bridge Readings Way Too High

Yes, smooth, complete control on power level from 1 W to 100 W, and the K2
power display and SWR track closely with my external meter. I think
everything is working perfectly.

Thanks again.

73
Dick, K0KK

On Tue, Oct 3, 2017 at 8:54 PM, Don Wilhelm <[hidden email]> wrote:

> Dick,
>
> With that corrected, do you have some semblance of power control?
> You should, but there are other pieces of the KPA100 wattmeter sending the
> VRFDET signal back to the base K2 MCU that could be producing a problem
> with the power output.
>
> 73,
> Don W3FPR
>
>
> On 10/3/2017 9:49 PM, Dick Lamb wrote:
>
>> Don,
>>
>> I found the problem: the short wire that connects the ungrounded side of
>> C1 to the SO259 jack had a poor solder connection.
>>
>> I figured that since C1 wouldn't tune, there had to be a problem in that
>> bridge circuit prior to U5. I started tracing each component's
>> connections with my Ohmmeter, and I finally discovered that the ungrounded
>> solder pad for C1 didn't connect.
>>
>> Thanks for your help, Don. You're a terrific resource and your
>> extraordinary efforts and patience in helping bewildered folks like me are
>> really appreciated.
>>
>>
>>
>