[K2] ALC and power control on 40m SSB in K2/100

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[K2] ALC and power control on 40m SSB in K2/100

VK7JB
Hello again folks,

Sorry about the barrage of posts from me. Just playing with my K2/100 #6998.

I've noticed that when operating in SSB  on 40m, requesting any power 15-20 W and up will always give me 100W+ output, measured into a dummy load.  Voice peaks are higher and result in Hi Curr warnings occasionally.  I can reduce the power above this threshold only by changing SSBC to 1:1 (normally run at 2:1 with my MH-2 mic) and winding the power out back to just on 11 W.

CW and TUNE modes - all OK:  power requested and power output track very well on all bands, including 40m.

Bands above 40m on SSB are not affected -  I mostly work 20m SSB and haven't noticed the power tracking issue there.  ALC seems to control the power out well there.

I suspect the Tx chain gain on 40m is too high for the ALC to manage when in SSB.  

Is there any way of making the ALC more aggressive on 40m SSB, without affecting the other bands too much?  I remember seeing a previous post about this somewhere but haven't been able to find the details again.

Any suggestions?

73,

John
VK7JB
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Re: [K2] ALC and power control on 40m SSB in K2/100

Steve Kavanagh
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Re: [K2] ALC and power control on 40m SSB in K2/100

VK7JB
Steve, thanks.

I'll investigate the suggested mod to the KSB2.  Have you had to do this mod
yourself?  Did those resistor values work out about right for you (1k0 and
1.5k)?   Did the mod significantly affect the ALC action on higher bands?
I don't want to scarifice too much power out on 20m.

Thanks for the information.

73,

John
VK7JB

> John:
>
> You probably have the diagnosis exactly right....this is a fairly common
> problem with K2/100s. Try these links for more info:
>
> http://elecraft.365791.n2.nabble.com/Re-SSB-Power-control-and-KPA100-
td444463.html#a444463
>
> http://elecraft.365791.n2.nabble.com/K2-100-SSB-Power-Control-Problem-
td384633.html#a384633
>
> 73,
> Steve VE3SMA
>
>
>
>





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Re: [K2] ALC and power control on 40m SSB in K2/100

Don Wilhelm-4
In reply to this post by VK7JB
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Re: [K2] ALC and power control on 40m SSB in K2/100

VK7JB
Don,

The KPA100 is new -from Elecraft in September 2010 - and it therefore is the new
design with the blue toroids @L16/RFC1.  The shield is installed as instructed,
using the 2 solder lugs to the SO-239 socket.

I took the KPA100 off the top of the rig, as you suggested, but there is no
difference in SSB mode running into the dummy load with the KPA100 attached or
flipped off beside the K2.

Looking more systematically, I see the overdrive is most noticable on 40m, but
is also  present on 80 and 30m.  It's not a problem on 20m and up.  Reducing
requested power to 50W (or less) and the power out remains close to 100W on
peaks and the ALC indicator shows 6-7+ bars in ALC mode. Seems the ALC is
working hard but not reducing the drive sufficiently to maintain the requested
output??

Mic is the MH-2 which I run SSBA = 1,  SSBC = 2. The audio sounds very good.
The problem reduces on the low bands with the compression setting @1:1, but then
there is insufficient drive  for 20m and up.  

Does this mean my TX chain has a lot of gain on 80/40/30 - too much for the ALC
to manage?

Thanks for your help,

John
VK7JB





Quoting Don Wilhelm <[hidden email]>:

>   John,
>
> Is this a relatively new KPA100, or did you purchase an older kit from
> someone who never built it?
>
> The reason I ask is because the older T/R switch design had elements
> that could cause parasitic oscillations near 40 meters, and often was
> most noticeable on SSB.
>
> If your KPA100 has blue toroid cores for L16 and RFC1, then it is the
> new design.
>
> If you have the new design, then did you fasten the shield directly to
> the SO-239 jack using the two solder lugs as instructed?  These are
> important.
>
> Make a check - remove the KPA100 from the base K2, but keep it connected
> - in other words turn the KPA100 upside-down on a book or box to the
> right of the K2.  Then try your SSB operation again (into a dummy load
> please, antennas are not consistent).  If it works OK with the KPA100
> upside-down, then look for causes of coupling from the KPA100 into the
> base K2, but if it still mis-behaves, we can look at other things.
>
> 73,
> Don W3FPR
>






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Re: [K2] ALC and power control on 40m SSB in K2/100

Don Wilhelm-4
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Re: [K2] ALC and power control on 40m SSB in K2/100

VK7JB
In reply to this post by VK7JB
Hello Group,

Just to complete the loop and to say thanks to Don, Steve and others who contributed to this discussion.

I've applied the mod to the KSB2 board - 1k0 resistor in series with the base of Q1 and changing the value of R9.  The ALC's can now control the output power of my K2 on 80/40/30m on SSB.

I'm very happy.  Thanks to everyone for your advice.

73,

John
VK7JB
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Re: [K2] ALC and power control on 40m SSB in K2/100

g0kla
In reply to this post by VK7JB
This is an old thread, but is something that I have struggled with on my own
K2.  I also had some RF feedback that was mixed with this, but I have cured
that.

So now I am left with the very similar symptoms where there are too many ALC
lights lit for 40m and it overdrives my KPA500.  I had to play with SSBA,
which I would set to 1 on 40m and 2 on 20m.  But even with SSBA set to 1 I
get much to much power on 40m for the amplifier.  I am using the MH2 mic.

I went through the checks.  I have the new PA.  There are only 1-2 ALC bars
lit on 20m and 15m.  There are typically 7 lit on 40m.

With SSBA set to 1 the voltage at KSB2 U1 pin 10 was not near zero.  It was
2.5V.  I checked the values and soldering of R14 and R15 and all looked ok.
As were the other components listed.

Thinking that the incorrect voltage means that I should change the component
values I updated R9 to 15k and added a 1k between the base of Q1 and P1 pin
6.  The voltage is now near zero, but it has not improved things.

I still need to use SSBA 2 for 20m otherwise the drive is low.  But SSBA 1
on 40m has too much drive.  As soon as the KPA100 clicks in (as I turn from
10-11Watts) the amp is over driven.

Any suggestions?

73
Chris
ac2cz / g0kla





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