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K2 -- loss of output power control

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Brian Lloyd-6

K2 -- loss of output power control

Our K2 was working perfectly until I opened it up, tightened the power
transistor mounting screws, realigned the receiver, and put it back
together. Now power output no longer follows the power setting. When
set to a power output less than 10W, I hear the bypass relay but I get
the High Cur display (momentarily) and the display then shows a power
output of 0.3W with an SWR of 3.0. Output power as measured with a W1
is 20W. If I advance the power control, as soon as I pass the 10W
point the PA comes on and the power jumps to 150W output. Turning the
power level control fully CCW (0.1W setting) produces 6W of output.

Looking at pin 7 of U5 on the KPA100 I can see good analog voltage
that varies with power. I can see the value of VFWD change when the
range changes (SCALE changes) also. I can also see varying VFWD at pin
2 of U1 so this does not appear to be an analog problem.

FWIW, voltages at pin 2 of U1 for various power outputs are:

6W -- 0.15V

20W -- 0.30V

150W -- 0.29V

At pin 7 of U5 (U5b):

6W -- 0.15V

20W -- 0.30V

150W -- 0.77V

On the t-r menu item I have "8r hold" set.

I do have output on VRFDET but the value only varies from about
0.3V-0.7V so that appears to be working also.

I am planning to fly to Dominica on Friday and operate from there but
I can't take the K2 unless I solve this problem.

Suggestions?

Brian Lloyd
Granite Bay Montessori School 9330 Sierra College Bl
brian AT gbmontessori DOT com Roseville, CA 95661
+1.916.367.2131 (voice) +1.791.912.8170 (fax)

PGP key ID: 12095C52A32A1B6C
PGP key fingerprint: 3B1D BA11 4913 3254 B6E0 CC09 1209 5C52 A32A 1B6C

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Don Wilhelm-4

Re: K2 -- loss of output power control

Brian,

You have some problem with the wattmeter in the KPA100 - T4, D16, D17,
R26, R27, U5, U6 or on the VRFDET signal pathway back to the K2 control
board.

You seem to have a somewhat normal voltage variation at U5 pin 7, which
(for the moment) says everything is working through U5, but check the U6
pin 6 output under the same conditions and also check for the same
voltage at K2 control board P4 pin 10.

Since this happened only by removing and re-installing the KPA100, be
suspicious of a solder joint that is not well soldered - those may work
for a while and then when touched can fail.

73,
Don W3FPR

Brian Lloyd wrote:

> Our K2 was working perfectly until I opened it up, tightened the power
> transistor mounting screws, realigned the receiver, and put it back
> together. Now power output no longer follows the power setting. When
> set to a power output less than 10W, I hear the bypass relay but I get
> the High Cur display (momentarily) and the display then shows a power
> output of 0.3W with an SWR of 3.0. Output power as measured with a W1
> is 20W. If I advance the power control, as soon as I pass the 10W
> point the PA comes on and the power jumps to 150W output. Turning the
> power level control fully CCW (0.1W setting) produces 6W of output.
>
> Looking at pin 7 of U5 on the KPA100 I can see good analog voltage
> that varies with power. I can see the value of VFWD change when the
> range changes (SCALE changes) also. I can also see varying VFWD at pin
> 2 of U1 so this does not appear to be an analog problem.
>
> FWIW, voltages at pin 2 of U1 for various power outputs are:
>
> 6W -- 0.15V
>
> 20W -- 0.30V
>
> 150W -- 0.29V
>
> At pin 7 of U5 (U5b):
>
> 6W -- 0.15V
>
> 20W -- 0.30V
>
> 150W -- 0.77V
>
> On the t-r menu item I have "8r hold" set.
>
> I do have output on VRFDET but the value only varies from about
> 0.3V-0.7V so that appears to be working also.
>
> I am planning to fly to Dominica on Friday and operate from there but
> I can't take the K2 unless I solve this problem.
>
> Suggestions?
>

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Brian Lloyd-6

Re: K2 -- loss of output power control

On Thu, Dec 11, 2008 at 6:59 AM, Don Wilhelm <[hidden email]> wrote:
> Brian,
>
> You have some problem with the wattmeter in the KPA100 - T4, D16, D17, R26,
> R27, U5, U6 or on the VRFDET signal pathway back to the K2 control board.
>
> You seem to have a somewhat normal voltage variation at U5 pin 7, which (for
> the moment) says everything is working through U5, but check the U6 pin 6
> output under the same conditions and also check for the same voltage at K2
> control board P4 pin 10.

I did. I think I said that in the original message (looking ...), yes,
I did check and the output of U6 pin 6 is the signal VRFDET and tracks
VFWD (output of U5b). It is properly present on header for the aux-I/O
cable to the control board and I have verified the same voltages on
pin 10 of the Aux-I/O connector on the control board.

I did disconnect the Aux-I/O cable from the control board and I tested
the base K2. Power output control of the K2 without the KPA100
connected is working is working properly.

> Since this happened only by removing and re-installing the KPA100, be
> suspicious of a solder joint that is not well soldered - those may work for
> a while and then when touched can fail.

I did carefully remove the PA board from the heatsink and check all
connections visually and specifically all of those between U5, U6, and
the Aux-I/O connector. I even rang out the connections. That is all
good.

I have also verified the entire path from U6 pin 6 (KPA100) all the
way to pin 2 on the microcontroller, U6, on the control board. Voltage
is present there and it does follow power output. So VRFDET is
reaching the control board.

Just for grins, I looked at the values for VRFDET coming form the K2
RF board RF output detector. They are *much* higher than what I am
getting from VRFDET coming from the KPA100. At full output I have
something like 3.5V on VRFDET from the K2 RF board output but only
about 0.3V on VRFDET coming from the KPA100. I am thinking maybe D16?
What kind of voltage should I see at the cathode of D16? I see about
1.7V at full power output from the PA.

73 de Brian, WB6RQN

>
> 73,
> Don W3FPR
>
>
> Brian Lloyd wrote:
>>
>> Our K2 was working perfectly until I opened it up, tightened the power
>> transistor mounting screws, realigned the receiver, and put it back
>> together. Now power output no longer follows the power setting. When set to
>> a power output less than 10W, I hear the bypass relay but I get the High Cur
>> display (momentarily) and the display then shows a power output of 0.3W with
>> an SWR of 3.0. Output power as measured with a W1 is 20W. If I advance the
>> power control, as soon as I pass the 10W point the PA comes on and the power
>> jumps to 150W output. Turning the power level control fully CCW (0.1W
>> setting) produces 6W of output.
>>
>> Looking at pin 7 of U5 on the KPA100 I can see good analog voltage that
>> varies with power. I can see the value of VFWD change when the range changes
>> (SCALE changes) also. I can also see varying VFWD at pin 2 of U1 so this
>> does not appear to be an analog problem.
>>
>> FWIW, voltages at pin 2 of U1 for various power outputs are:
>>
>> 6W -- 0.15V
>>
>> 20W -- 0.30V
>>
>> 150W -- 0.29V
>>
>> At pin 7 of U5 (U5b):
>>
>> 6W -- 0.15V
>>
>> 20W -- 0.30V
>>
>> 150W -- 0.77V
>>
>> On the t-r menu item I have "8r hold" set.
>>
>> I do have output on VRFDET but the value only varies from about 0.3V-0.7V
>> so that appears to be working also.
>>
>> I am planning to fly to Dominica on Friday and operate from there but I
>> can't take the K2 unless I solve this problem.
>>
>> Suggestions?
>>
>

Don Wilhelm-4

Re: K2 -- loss of output power control

Brian,

Yes, that is not enough voltage from D16. The K2 power control circuits
think that there is not enough power output and ramps up the power in an
effort to see more voltage on the VRFDET line.

I would recommend that you 'bite the bullet' and remove the KPA100 board
from the heat sink and check T4 carefully. Replace D16 and D17 both.
They may have been damaged by a lightning surge, but they usually just
short and fail to work at all - your failure is a bit different in that
you do get some voltage (and that is why you should check T4 sand the
other components in the wattmeter sensing area.

If you do not care a lot about power accuracy at low power levels, you
can use 1N4148 diodes in place of the 1N5711 type. The 1N4148s are a
little more 'hardy'.

73,
Don W3FPR

Brian Lloyd wrote:

> On Thu, Dec 11, 2008 at 6:59 AM, Don Wilhelm <[hidden email]> wrote:
>
>> Brian,
>>
>> You have some problem with the wattmeter in the KPA100 - T4, D16, D17, R26,
>> R27, U5, U6 or on the VRFDET signal pathway back to the K2 control board.
>>
>> You seem to have a somewhat normal voltage variation at U5 pin 7, which (for
>> the moment) says everything is working through U5, but check the U6 pin 6
>> output under the same conditions and also check for the same voltage at K2
>> control board P4 pin 10.
>>
>
> I did. I think I said that in the original message (looking ...), yes,
> I did check and the output of U6 pin 6 is the signal VRFDET and tracks
> VFWD (output of U5b). It is properly present on header for the aux-I/O
> cable to the control board and I have verified the same voltages on
> pin 10 of the Aux-I/O connector on the control board.
>
> I did disconnect the Aux-I/O cable from the control board and I tested
> the base K2. Power output control of the K2 without the KPA100
> connected is working is working properly.
>
>
>> Since this happened only by removing and re-installing the KPA100, be
>> suspicious of a solder joint that is not well soldered - those may work for
>> a while and then when touched can fail.
>>
>
> I did carefully remove the PA board from the heatsink and check all
> connections visually and specifically all of those between U5, U6, and
> the Aux-I/O connector. I even rang out the connections. That is all
> good.
>
> I have also verified the entire path from U6 pin 6 (KPA100) all the
> way to pin 2 on the microcontroller, U6, on the control board. Voltage
> is present there and it does follow power output. So VRFDET is
> reaching the control board.
>
> Just for grins, I looked at the values for VRFDET coming form the K2
> RF board RF output detector. They are *much* higher than what I am
> getting from VRFDET coming from the KPA100. At full output I have
> something like 3.5V on VRFDET from the K2 RF board output but only
> about 0.3V on VRFDET coming from the KPA100. I am thinking maybe D16?
> What kind of voltage should I see at the cathode of D16? I see about
> 1.7V at full power output from the PA.
>
> 73 de Brian, WB6RQN
>
>
>
>> 73,
>> Don W3FPR
>>
>>
>> Brian Lloyd wrote:
>>
>>> Our K2 was working perfectly until I opened it up, tightened the power
>>> transistor mounting screws, realigned the receiver, and put it back
>>> together. Now power output no longer follows the power setting. When set to
>>> a power output less than 10W, I hear the bypass relay but I get the High Cur
>>> display (momentarily) and the display then shows a power output of 0.3W with
>>> an SWR of 3.0. Output power as measured with a W1 is 20W. If I advance the
>>> power control, as soon as I pass the 10W point the PA comes on and the power
>>> jumps to 150W output. Turning the power level control fully CCW (0.1W
>>> setting) produces 6W of output.
>>>
>>> Looking at pin 7 of U5 on the KPA100 I can see good analog voltage that
>>> varies with power. I can see the value of VFWD change when the range changes
>>> (SCALE changes) also. I can also see varying VFWD at pin 2 of U1 so this
>>> does not appear to be an analog problem.
>>>
>>> FWIW, voltages at pin 2 of U1 for various power outputs are:
>>>
>>> 6W -- 0.15V
>>>
>>> 20W -- 0.30V
>>>
>>> 150W -- 0.29V
>>>
>>> At pin 7 of U5 (U5b):
>>>
>>> 6W -- 0.15V
>>>
>>> 20W -- 0.30V
>>>
>>> 150W -- 0.77V
>>>
>>> On the t-r menu item I have "8r hold" set.
>>>
>>> I do have output on VRFDET but the value only varies from about 0.3V-0.7V
>>> so that appears to be working also.
>>>
>>> I am planning to fly to Dominica on Friday and operate from there but I
>>> can't take the K2 unless I solve this problem.
>>>
>>> Suggestions?
>>>
>>>
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Brian Lloyd-6

Re: K2 -- loss of output power control

On Thu, Dec 11, 2008 at 10:47 AM, Don Wilhelm <[hidden email]> wrote:
> Brian,
>
> Yes, that is not enough voltage from D16. The K2 power control circuits
> think that there is not enough power output and ramps up the power in an
> effort to see more voltage on the VRFDET line.

That is what I am thinking also.

> I would recommend that you 'bite the bullet' and remove the KPA100 board
> from the heat sink and check T4 carefully.

I did that but now I am going to do it again. Grr.

> Replace D16 and D17 both. They
> may have been damaged by a lightning surge, but they usually just short and
> fail to work at all - your failure is a bit different in that you do get
> some voltage (and that is why you should check T4 sand the other components
> in the wattmeter sensing area.

I have checked T4 and it shows proper continuity to the board so I
believe that the soldering is good. (The solder joints look good under
a magnifier too.) D16 and D17 both show the same forward and reverse
resistance once they conduct which makes them suspect to me. They do
show open at low voltage so they are not shorted. Seems they are the
most likely culprits.

> If you do not care a lot about power accuracy at low power levels, you can
> use 1N4148 diodes in place of the 1N5711 type. The 1N4148s are a little
> more 'hardy'.

But they are standard silicon small signal diodes. They would not work
well at low power levels due to their greater forward drop. Can I sub
in some other small-signal schottky diode? If I can't find a 1N5711?

>
> 73,
> Don W3FPR

73 de Brian, WB6RQN
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Don Wilhelm-4

Re: K2 -- loss of output power control

Brian,

The real mystery is that this initially occurred when you removed the
KPA100 to tighten the PA transistor screws. I really would not think
that would trigger a component failure, so I encourage you strongly to
look for a bad solder connection.
Be certain the wire from the center of the SO239 to hole 6 of the T4
connections is in place and well soldered - that one is easily
overlooked. Do the rest of the T4 connections look 'like mountains, but
not volcanoes'? Re-strip if they have that volcano appearance.
You might also check R28 and R29 too. A bad capacitor is a difficult
thing to find - fortunately they are not encountered often, but check
anyway.

Measure the resistance from the wiper of R26 to ground - it should be
close to 43k.
A possibility is that R26 itself was ready to fail - it perhaps worked
until it was touched.

You could possibly use some other diode at D16 and D17, but I don't know
the response characteristics vs. frequency. The 1N4148s will work fine
down to about the 2 watt level, I don't know how low you wish to have
accurate power control and indication. Elecraft will likely send you
1N5711s if you can't find them locally.

73,
Don W3FPR

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