K2: SSB issue, excessive drive

[Rob KK4R]

Before getting into the problem description, I have done some homework and know that others have had similar problems, so I'm hoping to tap into some experience.  The problem is with high gain in the SSB transmit chain on 40 meters versus 10 meters.  Folks often perceive these problems as high ALC indications and distortion.

I haven't used my K2/100 much for SSB in the year that I have had it.  Our club has a 10 m net, so I started checking into that and found my Heil mic produced light but adequate audio with around 1 ALC bar showing.  VOX was effectively useless.  A second mic, a homemade electret, had more audio, moved the ALC about 3 bars and operated the VOX.  Mistakenly thinking this was all about mic output level, I made a 15 dB preamp for the Heil, and it seemed to work great.  All that was on 10 meters.

Last weekend, I decided to give it a workout in the SSB SS contest.  Using the Heil and preamp, things seemed to be going great on 15 m, but as I moved down to 20 m, the autotuner kicked in a few times unexpectedly.  I was watching the ALC and had noticed about 5 or 6 bars, which didn't strike me as a problem.  Nobody complained about my audio, but I think the autotuner action was due to some serious distortion.

When I moved down to 40 m, the K2 was clearly red-lined.  The ALC maxed, and the autotuner tried to retune on every transmission, when the antenna was well matched already.  Finally, somebody let me know that my audio was terrible.  The poor fellow was doing me a favor, but he was apologetic.  I guess some folks are sensitive about their audio and must bite your head off if you comment on it.

I started changing things immediately and asking for audio reports as I went.  Compression and gain settings in the K2 didn't really matter.  The preamp had to come out of line and the boom mic was turned out from my face to get the ALC reading below 5 bars, and finally, somebody said my audio was OK.  I think at this point, the K2 was set for the -10 dB mic gain setting.  So I had dropped about 25 dB of gain in the electronics and moved the mic away from my face, pointing it in the wrong direction, before I could get down to 5 bars of ALC.  The antenna tuner still tried to retune once in a while, if I raised my voice, so that was the end of the test.  I switched to my old TS-940 and let it go.

From reading various posts, it is clear that others have had some problems like this.  Either under-drive on 10 meters or extra drive on 40 meters has been talked about on the reflector.  My case seems extreme.  A variable mic output from +15dB down to -30dB might mitigate it, but this seems a bit ridiculous.  Besides, there might be something wrong in the K2 that could be fixed.  I haven't started troubleshooting and am I'm hoping for some hints to point me in the right direction.

Thanks,
Rob

[Don Wilhelm-4]

Rob,

Yes, there does seem to be something extreme in your problem
description.  The K2 does have the greatest TX gain on 40 or 30 meters,
but it should not drop off so fast on the higher bands.

I would start by removing the KPA100 and testing the base K2 for maximum
power output on 40 meters vs. 10 meters.
If you cannot get more than 12 watts on 10 meters with a power supply
voltage of 13.8 volts, then something is wrong.
Have you changed R98 on the bottom of the RF Board to reduce the
oscillation of output power level with the KPA100 added?  If so, you may
have too large a resistor at R98 (the normal value is 270 ohms).  Use no
larger than the value that does not reduce the 10 meter output level.

You can gain better 10 meter output from the base K2 by mounting T2
above the board (use two layers of foam double sided tape) and wind T2
EXACTLY like the diagram in the manual.

If all else fails, replace the 2sc2166 transistor at Q6 with a 2SC5739
which has greater gain at higher frequencies (new K2s use that part),
but along with that change, increase R46 to 330 ohms.

Your 15 dB mic preamp gain is a little too high - to compensate between
a Heil HC4/5 cartridge and an electret mic, you should only need about
10 dB.  Check the KSB2 board to be certain you have the value of R14 at
its normal 1k value if you are using the Heil mic with a preamp.

73,
Don W3FPR

Rob KK4R wrote:

> Before getting into the problem description, I have done some homework and
> know that others have had similar problems, so I'm hoping to tap into some
> experience.  The problem is with high gain in the SSB transmit chain on 40
> meters versus 10 meters.  Folks often perceive these problems as high ALC
> indications and distortion.
>
> I haven't used my K2/100 much for SSB in the year that I have had it.  Our
> club has a 10 m net, so I started checking into that and found my Heil mic
> produced light but adequate audio with around 1 ALC bar showing.  VOX was
> effectively useless.  A second mic, a homemade electret, had more audio,
> moved the ALC about 3 bars and operated the VOX.  Mistakenly thinking this
> was all about mic output level, I made a 15 dB preamp for the Heil, and it
> seemed to work great.  All that was on 10 meters.
>
> Last weekend, I decided to give it a workout in the SSB SS contest.  Using
> the Heil and preamp, things seemed to be going great on 15 m, but as I moved
> down to 20 m, the autotuner kicked in a few times unexpectedly.  I was
> watching the ALC and had noticed about 5 or 6 bars, which didn't strike me
> as a problem.  Nobody complained about my audio, but I think the autotuner
> action was due to some serious distortion.
>
> When I moved down to 40 m, the K2 was clearly red-lined.  The ALC maxed, and
> the autotuner tried to retune on every transmission, when the antenna was
> well matched already.  Finally, somebody let me know that my audio was
> terrible.  The poor fellow was doing me a favor, but he was apologetic.  I
> guess some folks are sensitive about their audio and must bite your head off
> if you comment on it.
>
> I started changing things immediately and asking for audio reports as I
> went.  Compression and gain settings in the K2 didn't really matter.  The
> preamp had to come out of line and the boom mic was turned out from my face
> to get the ALC reading below 5 bars, and finally, somebody said my audio was
> OK.  I think at this point, the K2 was set for the -10 dB mic gain setting.
> So I had dropped about 25 dB of gain in the electronics and moved the mic
> away from my face, pointing it in the wrong direction, before I could get
> down to 5 bars of ALC.  The antenna tuner still tried to retune once in a
> while, if I raised my voice, so that was the end of the test.  I switched to
> my old TS-940 and let it go.
>
> >From reading various posts, it is clear that others have had some problems
> like this.  Either under-drive on 10 meters or extra drive on 40 meters has
> been talked about on the reflector.  My case seems extreme.  A variable mic
> output from +15dB down to -30dB might mitigate it, but this seems a bit
> ridiculous.  Besides, there might be something wrong in the K2 that could be
> fixed.  I haven't started troubleshooting and am I'm hoping for some hints
> to point me in the right direction.
>
> Thanks,
> Rob
>  
>  

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