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KAT2: C55 Adjustment

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JT Croteau

KAT2: C55 Adjustment

Folks,

I am having a hard time doing the Bridge Null Adjustment (C55) on the
KAT2. The display reads HI pretty much continuously as I adjust C55.
However, every once and awhile it will flicker a number while I am
turning C55 but this is inconsistent. Before installing R6, you are
to also check U4 pin 1 voltage during tune. I am showing 4.61V at
this spot. I have checked, and rechecked T1 multiple times and do not
see any problems. I also don't see any cold solder joints.

However, is it possible to installed C55 the wrong way? The manual
wasn't specific on this like it was when installing variable caps
during K2 assembly.

Thanks

--
JT Croteau, N1ESE - Manchester, NH (FN42gx)
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Don Wilhelm-4

Re: KAT2: C55 Adjustment

JT,

Use a fully insulated tuning tool - both ends of the trimmer cap are
floating above ground, so you will see a lot of 'hand capacitance'
effects with a metallic screwdriver. If you can achieve a null below 9
or 10, that is a normal low value.

Tune slowly and you can obtain the null. If you have a high impedance
input analog reading meter, you can do it easier than the 'no tools'
manual method. Connect your meter from the hot side of R2 to ground and
tune the cap for a minimum reading - the null is easier to see on an
analog meter - just tune for the smallest voltage, then check the value
displayed on the K2 - if it is less than 10, all is good.

If you cannot achieve a decent null, then you may have a significant
amount of residual inductance or capacitance in the KAT2 - usually due
to a non-operating relay. If that is the case, first inspect the
soldering on each of the relays (yes, you will have to lift some
inductors to do that) to be certain all pins are soldered.

73,
Don W3FPR

JT Croteau wrote:

> Folks,
>
> I am having a hard time doing the Bridge Null Adjustment (C55) on the
> KAT2. The display reads HI pretty much continuously as I adjust C55.
> However, every once and awhile it will flicker a number while I am
> turning C55 but this is inconsistent. Before installing R6, you are
> to also check U4 pin 1 voltage during tune. I am showing 4.61V at
> this spot. I have checked, and rechecked T1 multiple times and do not
> see any problems. I also don't see any cold solder joints.
>
> However, is it possible to installed C55 the wrong way? The manual
> wasn't specific on this like it was when installing variable caps
> during K2 assembly.
>
> Thanks
>
>

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Don Brown-4

Re: KAT2: C55 Adjustment

Hi

I have a method that works well for me to adjust the power/SWR bridge.

You will need to make up two additional dummy loads besides your 50 ohm.
These need to be 25 ohms and 100 ohms. I used 2 50 ohm 2 watt MOS resistors
mounted inside a PL239 connector wired in series for the 100 ohm and in
parallel for the 25 ohm. For the kAT2 you will also need UHF to BNC
adapters.

After you do the procedure below from the manual set the forward power pot
per the manual. Set the reverse power pot to about the same position as the
forward pot. Now using about 2-3 watts with the 25 ohm load note the SWR
reading. Now install the 100 ohm load and adjust the null in very small
amounts until you get the same reading with both the 25 and 100 load. It
does not matter what the reading is yet, just so it is the same. Once that
is done adjust the reverse pot so the K2 displays 2.0:1 with either load.
Now go back and check the power and SWR with the 50 ohm load. The SWR should
be 1.0:1. If the power is off adjust the forward power pot to correct it and
recheck the reverse again with the 25 and 100 ohm loads to make sure you
still get 2.0:1. All of this will interact a little so you may need to go
through it several times.

Hope this helps

Thanks

Don Brown
KD5NDB

----- Original Message -----
From: "Don Wilhelm" <[hidden email]>
To: "JT Croteau" <[hidden email]>
Cc: <[hidden email]>
Sent: Friday, September 28, 2007 8:22 PM
Subject: Re: [Elecraft] KAT2: C55 Adjustment

> JT,
>
> Use a fully insulated tuning tool - both ends of the trimmer cap are
> floating above ground, so you will see a lot of 'hand capacitance'
> effects with a metallic screwdriver. If you can achieve a null below 9
> or 10, that is a normal low value.
>
> Tune slowly and you can obtain the null. If you have a high impedance
> input analog reading meter, you can do it easier than the 'no tools'
> manual method. Connect your meter from the hot side of R2 to ground and
> tune the cap for a minimum reading - the null is easier to see on an
> analog meter - just tune for the smallest voltage, then check the value
> displayed on the K2 - if it is less than 10, all is good.
>
> If you cannot achieve a decent null, then you may have a significant
> amount of residual inductance or capacitance in the KAT2 - usually due
> to a non-operating relay. If that is the case, first inspect the
> soldering on each of the relays (yes, you will have to lift some
> inductors to do that) to be certain all pins are soldered.
>
> 73,
> Don W3FPR
>

Don Wilhelm-4

Re: KAT2: C55 Adjustment

Don and all,

This is exactly the method I have been using on all the Elecraft
wattmeters that I have worked on for several years now. I believe you
mentioned it first, so I do have to give you all the credit for this
great idea.

It does result in a more accurate setting of the trimmer capacitor than
just observing the null, but you have to just nudge the trimmer setting
a bit at a time. As you stated, the 'manual' adjustment done first
will get things close, and these loads can be used to refine the adjustment.

With the KAT2 (and KAT100), I connect each load to the two antenna jacks
and use the ANT 1/2 button to switch between the loads - with the KPA100
it is harder because I have to connect and disconnect the loads for each
comparison. I could devise a switch, but that would introduce an
unknown SWR element.

I have a set of 25 and 100 ohm loads constructed from 30 watt thick film
power resistors ( 50 ohm 1% resistors), so they will handle up to 60
watts continuous, and I like to do a final check at higher power with
the KPA100 and KAT100. If you choose this route, be certain to keep the
connections to them VERY short - connecting to these (non 50 ohm) loads
with even a short piece of 50 ohm coax will alter the SWR due to
transmission line effects. I use male to male connectors.

73,
Don W3FPR

[hidden email] wrote:

> Hi
>
> I have a method that works well for me to adjust the power/SWR bridge.
>
> You will need to make up two additional dummy loads besides your 50 ohm.
> These need to be 25 ohms and 100 ohms. I used 2 50 ohm 2 watt MOS resistors
> mounted inside a PL239 connector wired in series for the 100 ohm and in
> parallel for the 25 ohm. For the kAT2 you will also need UHF to BNC
> adapters.
>
> After you do the procedure below from the manual set the forward power pot
> per the manual. Set the reverse power pot to about the same position as the
> forward pot. Now using about 2-3 watts with the 25 ohm load note the SWR
> reading. Now install the 100 ohm load and adjust the null in very small
> amounts until you get the same reading with both the 25 and 100 load. It
> does not matter what the reading is yet, just so it is the same. Once that
> is done adjust the reverse pot so the K2 displays 2.0:1 with either load.
> Now go back and check the power and SWR with the 50 ohm load. The SWR should
> be 1.0:1. If the power is off adjust the forward power pot to correct it and
> recheck the reverse again with the 25 and 100 ohm loads to make sure you
> still get 2.0:1. All of this will interact a little so you may need to go
> through it several times.
>
> Hope this helps
>
> Thanks
>
> Don Brown
> KD5NDB
>
>

JT Croteau

Re: KAT2: C55 Adjustment

Thanks to all for the replies.

As of right now, I'm going to blame it on a bad relay. I had damaged
one, K8, slightly during install but thought it was only a flesh wound
and that it wouldn't effect operation until I could get it replaced.
Offender removed and I will order a replacement next week.

Meanwhile, FedEx just delivered, as I was typing this, my XV50 and
K60XV so now I have something new to break, er build, in the meantime.

--
JT Croteau, N1ESE - Manchester, NH (FN42gx)
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