[Elecraft] K2: High SWR On Dummy Load

Wed Oct 31 23:35:33 EDT 2012

If you can clip the bad diodes off at the glass enclosures, you may be able
to simply lay the new ones in place and solder their leads to the stubs
sticking up out of the pcb, then trim the excess off. I can tell you  from
years of experience that a lot of field repairs are made that way on
commercial equipment and they are as solid as sucking out the leads and
installing parts in the holes in the pcb. 

If you do remove the heat sink, be aware that the thermal pads between the
transistors and the heat sink may tear with some of the pad sticking to the
transistor and some stuck to the heat sink. That's not a show stopper as
long as you don't lose sections of pads. Just leave the pads alone. When you
put it back together when you're done the pads will fit and work just fine. 

And do NOT forget to retighten the screws in the transistors after about 5
to 10 hours of operation. We are the same age so you'll probably remember
having to re-tighten head gaskets on cars a short while after replacing them
in the "old days". This is much the same, Hi! 

73 Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ariel Jacala
Sent: Wednesday, October 31, 2012 7:35 PM
To: don at w3fpr.com
Cc: elecraft reflector
Subject: Re: [Elecraft] K2: High SWR On Dummy Load

Don,

I took off the shield on the KPA100 and those two diodes are tight against
the back panel and the toroid.  What would you recommend is the best way to
desolder and replace those?  It is tight against the back panel and a
toroid.  I do not want to risk damaging the nearby toroid - so I think I
will remove the the standoffs and remove the PCB from the heat sink and
desolder it from the back side (non component) side.   Is this what you
would do?

Thanks

Ariel NY4G

Sent from my iPad

On Oct 26, 2012, at 8:10 AM, "Don Wilhelm" <w3fpr at embarqmail.com> wrote:

> Ariel,
> 
> You may indeed have a problem with D9 and the 3 associated resistors in
the base K2, BUT when the KAT2 or the KPA100 is installed, that circuit is
ignored.
> Both the KAT2 and the KPA100 contain a true wattmeter that is positioned
right at the output and used for power control.
> 
> The K2 microprocessor reads the actual output and compares that to the
power knob setting.  That result tells the microprocessor how to adjust the
drive.  Unlike other ham transceivers, the K2 uses a closed loop to control
the power instead of simply adjusting the drive.
> 
> What that also means is that the wattmeter in the KPA100 is not working
(you have shown that the one in the KAT2 is working OK).  As I indicated
before (and also indicated by your high SWR indication) there is a high
probability that KPA100 diodes D16 and D17 have been damaged.
> 
> When the diodes are damaged, the output back to the MCU is most likely
zero, and the MCU *thinks* there is no power output - it then increases the
drive to the maximum in an attempt to develop power. The result is a HiCur
warning and stress on the base K2 PA transistors.
> 
> Replace KPA100 D16 and D17 as the first order of business and after that
you can investigate any remaining problems.  Note that those diodes can be
"half bad" and may show as good with a diode test - replacement is the first
order of business.
> 
> 73,
> Don W3FPR
> 
> On 10/25/2012 11:31 PM, Ariel Jacala wrote:
>> Don
>> 
>> The K2 with the ATU behaves as expected - internal meter agrees well with
the external meter, current draw is as expected, and so forth for various
power levels up to 10watts checked out.  It was connected to a dummy load.
>> 
>> Without the ATU and the dummy load connected to the base K2 antenna,
things are rather strange.... At 2 watts set on the power knob, output on
the external meter is 2.7 watts and current draw is 0.96 amps; at a knob
setting of 2.4 watts, output is up to 7 watts on the external meter and
current draw is 1.4 amps; at a knob setting of 2.5 watts, power output is 10
watts and current draw is 1.96 amps; at 3 watts knob setting, I get the high
current warning, and the external meter is pegged.
>> 
>> This leads me to believe that the base K2 is overdriving the KPA100.  At
low power settings - 5 watts, I was pegging the LEDs indicating power on the
LDG100 autotuner, at over 125 watts.
>> 
>> Ariel NY4G
>> 
>> Sent from my iPad
>> 
>> On Oct 25, 2012, at 11:51 AM, "Don Wilhelm" <w3fpr at embarqmail.com> wrote:
>> 
>>> Ariel,
>>> 
>>> Do not be too quick to conclude that there is something wrong with the
base K2.  The basic K2 uses only an RF voltage diode to display the power
output, and it may not be exact - it depends on the load and may vary from
band to band - if it only off by a watt or two, then all is OK   That is
bypassed when the KAT2 or KPA100 is installed.
>>> 
>>> Yes, do check with the external wattmeter - both with and without the
KPA100.  I still think KPA100 diodes D16 and D17 are highest on the suspect
list with other KPA100 wattmeter components being second on that list.
>>> 
>>> 73,
>>> Don W3FPR
>>> 
>>> On 10/25/2012 11:18 AM, Ariel Jacala wrote:
>>>> Thanks to Jim, Vic and Don for the return comments
>>>> 
>>>> I disconnected the KPA100 and put on the K2 top with the KAT2 ATU.
With the K2 ATU connected and the dummy load connected - the SWR correctly
reads 1:1 with menu set for CALS.  With the menu set to CALP, the power
levels appeared to modulate correctly with the PWR knob although this is an
item i have to double check.  With the ATU disconnected, the displayed power
on tune differs from the power setting.  This suggests to me that there is
something wrong with the base K2.  I still have to do what Don suggested
about connection to an external watt meter.
>>>> 
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