Question about K2 mod for optimizing it for 5W CW

[Alexey Kats]

I have one more question about K2. I noticed that revision C of K2
manual says that if to increase QRP   T4 transfor

--
Alexey Kats (neko)
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[Alexey Kats]

Sorry, I managed to press the wrong key and sent it before I finished it.

Anyway... I know that older version of K2 manual refers to changing
winding of T4 transformer to 2:2 (instead of original 2:3) in order to
increase efficiency of CW performance on 5W or less. Is it still
applicable to the current version of K2? I am not interested in SSB
mode at all, so limiting max power to 5W is acceptable. I simply want
to know whether the rest of tuning instructions will be the same if I
change T4 winding according to those directions from rev C manual?
(25-30% decrease in current drain is quite important when operating
from batteries.) In any case, I am going to add 6.8k resistor to the
R2 (of course it depends on the total resistance of R2 in the current
kit) on control board according to DK3RED mod (which will limit max
power to about 5W anyway), and I want to limit current consumption as
well if I can.

I went through the schematics in manuals from revision C, D and G, and
as far as I can see nothing indicates that this T4 transformer
operation had changed (since it is operating in exactly same output
stage as the older versions of K2 PA), so I am assuming it is safe to
change its winding. But if I am wrong, please, tell me so.

Thank you.

On Mon, May 31, 2010 at 5:56 AM, Alexey Kats <[hidden email]> wrote:
> I have one more question about K2. I noticed that revision C of K2
> manual says that if to increase QRP   T4 transfor
>
> --
> Alexey Kats (neko)
>



--
Alexey Kats (neko)
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[Don Wilhelm-4]

Alexey,

Make that change IF AND ONLY IF 1) you will never install the KSB2, AND
2) you will not operate above the 5 watt level.

That change makes the K2 PA transistors more efficient at 5 watts and
below, but the operation is non-linear and the efficiency is worse if
the power is increased above the 5 watt level.

So it is not a recommended change except for those special
circumstances.  If the major use of the K2 is CW only operation at QRP
levels on batteries, then it is worthy of consideration.

73,
Don W3FPR

Alexey Kats wrote:
> I have one more question about K2. I noticed that revision C of K2
> manual says that if to increase QRP   T4 transfor
>
>  
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[Alexey Kats]

Thank you, Don, for responding so early (I hope you do not have
insomnia as I do). And yes, that's exactly what I am looking for - K2
restricted to 5W and to CW-only operation, so as long as it is safe
and is not going to introduce spurious emission it'll work for me.

On Mon, May 31, 2010 at 6:31 AM, Don Wilhelm <[hidden email]> wrote:

> Alexey,
>
> Make that change IF AND ONLY IF 1) you will never install the KSB2, AND 2)
> you will not operate above the 5 watt level.
>
> That change makes the K2 PA transistors more efficient at 5 watts and below,
> but the operation is non-linear and the efficiency is worse if the power is
> increased above the 5 watt level.
>
> So it is not a recommended change except for those special circumstances.
>  If the major use of the K2 is CW only operation at QRP levels on batteries,
> then it is worthy of consideration.
>
> 73,
> Don W3FPR
>
> Alexey Kats wrote:
>>
>> I have one more question about K2. I noticed that revision C of K2
>> manual says that if to increase QRP   T4 transfor
>>
>>
>

--
Alexey Kats (neko)
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[Don Wilhelm-4]

Alexey,

The R2 that is referred to by DK3RED is on the Front Panel - it is the
power control pot.

There will be no change to any of the alignment procedures with the
change in the T4 winding ratio.

Please put a sticker inside your K2 stating that the T4 winding ratio is
intentionally changed to optimize efficiency at 5 watts.  I have worked
on several K2s where this change was made, then forgotten about (or the
K2 subsequently sold), and then the KSB2 and/or KPA100 was added, and
the owner wondered why it did not operate correctly.

73,
Don W3FPR

Alexey Kats wrote:

> Sorry, I managed to press the wrong key and sent it before I finished it.
>
> Anyway... I know that older version of K2 manual refers to changing
> winding of T4 transformer to 2:2 (instead of original 2:3) in order to
> increase efficiency of CW performance on 5W or less. Is it still
> applicable to the current version of K2? I am not interested in SSB
> mode at all, so limiting max power to 5W is acceptable. I simply want
> to know whether the rest of tuning instructions will be the same if I
> change T4 winding according to those directions from rev C manual?
> (25-30% decrease in current drain is quite important when operating
> from batteries.) In any case, I am going to add 6.8k resistor to the
> R2 (of course it depends on the total resistance of R2 in the current
> kit) on control board according to DK3RED mod (which will limit max
> power to about 5W anyway), and I want to limit current consumption as
> well if I can.
>  
>

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[Alexey Kats]

No worries, I'll do that (I hate it myself when I have to work on
customized equipment when the owner either forgot about it, or didn't
even know about it since it's not the original owner). Fortunately,
the last time it happened with me was many years ago. And the most
embarrassing part was that when I figured out what was customized it
was ME who did it. (Even though it was many years ago and even in a
different country I still feel ashamed by that experience.)

On Mon, May 31, 2010 at 6:50 AM, Don Wilhelm <[hidden email]> wrote:

> Alexey,
>
> The R2 that is referred to by DK3RED is on the Front Panel - it is the power
> control pot.
>
> There will be no change to any of the alignment procedures with the change
> in the T4 winding ratio.
>
> Please put a sticker inside your K2 stating that the T4 winding ratio is
> intentionally changed to optimize efficiency at 5 watts.  I have worked on
> several K2s where this change was made, then forgotten about (or the K2
> subsequently sold), and then the KSB2 and/or KPA100 was added, and the owner
> wondered why it did not operate correctly.
>
> 73,
> Don W3FPR
>
> Alexey Kats wrote:
>>
>> Sorry, I managed to press the wrong key and sent it before I finished it.
>>
>> Anyway... I know that older version of K2 manual refers to changing
>> winding of T4 transformer to 2:2 (instead of original 2:3) in order to
>> increase efficiency of CW performance on 5W or less. Is it still
>> applicable to the current version of K2? I am not interested in SSB
>> mode at all, so limiting max power to 5W is acceptable. I simply want
>> to know whether the rest of tuning instructions will be the same if I
>> change T4 winding according to those directions from rev C manual?
>> (25-30% decrease in current drain is quite important when operating
>> from batteries.) In any case, I am going to add 6.8k resistor to the
>> R2 (of course it depends on the total resistance of R2 in the current
>> kit) on control board according to DK3RED mod (which will limit max
>> power to about 5W anyway), and I want to limit current consumption as
>> well if I can.
>>
>

--
Alexey Kats (neko)
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[Vic K2VCO]

On 5/31/2010 6:59 AM, Alexey Kats wrote:

>>> Anyway... I know that older version of K2 manual refers to changing
>>> winding of T4 transformer to 2:2 (instead of original 2:3) in order to
>>> increase efficiency of CW performance on 5W or less.

Keep in mind that the 25% decrease in current draw applies only when the key is down.
Assuming that you are operating in contest mode and running stations you may be
transmitting half of the time. During the time you are transmitting the duty cycle will be
about 50%.

So the key will be down a maximum of 25% of the time (and if you are doing
search-and-pounce, as you must as a QRP contester, it will be significantly less). So you
have a 25% improvement x 50% transmit time x 50% duty cycle, which translates to only a
6.25% reduced current draw overall.

Of course if you are operating casually, listening and ragchewing, the improvement will
even be smaller.

Is it worth permanently crippling your K2 so that it cannot produce the 10-15 watts it is
capable of making, and which in some circumstances you might be happy to have, for this
small improvement?

I've never understood the enthusiasm some have for this modification.
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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