K2 - Do Builder Alert 22a Upgrade ?

[Steve Kavanagh]

I purchased a recently built K2/100 this summer and am
gradually working my way through careful evaluations.
Today I checked the key-down current and found some
variations that look rather like those mentioned in
Builder Alert #22a.  Here are the current readings in
amps, at 75 and 100 watts requested output, on CW,
after allowing the power control loop to settle:

MHz  75 W 100 W

3.5  15   17.5
7    13   15
10   14   16.5
14   13   15.5
18   17   (not tested to avoid blowing 20A fuse)
21   16   19
24   15   17
28   15   18

I observed noticeably high current on 18 and 21 MHz,
as opposed to 14 and 18 MHz as indicated in the
Builder Alert.  On the other hand, when I looked at
the current indicated on the K2 display (which should
give an indication of drive power to the PA) I got (in
amps, for 75 watts output requested):

MHz  K2 Display Current

3.5   1.38
7     1.44
10    1.44
14    1.98
18    2.00
21    1.58
24    1.66
28    1.62

This looks exactly like the low gain situation
indicated in the Builder Alert.

The supply voltage for these tests, indicated on the
K2 display (key down at 17.5 A), was 13.0 V.

It looks to me like I have a borderline case of the
resistor problem described in the Builder Alert.  I
assume the original builder did not know about this
issue as I bought the rig before the Alert came out.  

Do you think it would be useful to make the upgrade to
get it working as well as it can ?  Has anyone else
observed if the current on 15m is also affected by the
resistor problem or is the higher current I observed
there likely due to something else ?  Has anyone got
similar current data before and after making the
Builder Alert 22a upgrade ?

73,
Steve VE3SMA


       

       
               
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[Don Wilhelm-3]

Steve,

I would be tempted to change the resistors to see if it makes any
difference - it certainly will do no harm and has promise of correcting your
problem.  While you are inside the KPA100 shield, I would recommend that you
also check the low pass filter for 15/12/10 meters - look for the proper
number of turns on the toroids (count the turns that pass through the center
of the core) and verify that the LPF capacitor values are correct.

I have not seen this low gain behavior on 15 meters and above, but the exact
frequency depends on just how much inductance the resistors actually
possess.

73,
Don W3FPR

> -----Original Message-----
>
> I purchased a recently built K2/100 this summer and am
> gradually working my way through careful evaluations.
> Today I checked the key-down current and found some
> variations that look rather like those mentioned in
> Builder Alert #22a.
> ...
> Do you think it would be useful to make the upgrade to
> get it working as well as it can ?  Has anyone else
> observed if the current on 15m is also affected by the
> resistor problem or is the higher current I observed
> there likely due to something else ?  Has anyone got
> similar current data before and after making the
> Builder Alert 22a upgrade ?
>
> 73,
> Steve VE3SMA
>

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