Question about K2's Q7 and Q8 Final Amp

[Tom Mc]

Hi,

  My K2 has been working fine but I decided to fix it anyway...hi.

  Well actually I was a little short on the power output on 10 meters.  I was getting about 7 watts out on 10 meters and Gary Suggested that I replace T2 and T1.  Instead of winding them myself, I contacted 'The Toroid Guy' and he sent me two beauties (along with a T-4) - the guy is great, BTW.

  So yesterday I installed the new T1 and T2 (I did not yet install T4) and replaced the heat sink.  The K2 manual warns that whenever the heat sink is removed and replaced you should verify that the collectors of Q7 and Q8 are not shorted to ground.

  With the heatsink attached I check and sure enough, there was a short between the collector (middle pin) and ground (the ground wire on the RF board) - I used the continuity buzzer on my DMM for this.

  I removed the heatsink and all of the hardware which connects Q7 and Q8 to the heatsink and then again checked for the sort.  It was still there.

  In looking at the schematic, the section of the final which contains Q7 and Q8 seems pretty isolated from everything else, with just T3 and T4 seemingly getting into the action.  Try as a I might I couldn't locate any shorts.  Using the schematic, I started at the collectors of the transistors and worked to the right of the schematic.

  The only options I have presently connected are the Noise Blanker and the SSB module, both of which don't seem to be involved with the finals.

  Obviously the problem must be in the area that I was working on.  

If anyone has any suggestions in tracking this down, I appreciate your help

Tom, WB2QDG
K2 1103



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[Don Wilhelm-3]

Tom,

Quite rare, but if you have a short with the heatsink off, look for a
soldering problem with T4 or T4 might have an internal short (unlikely, but
possible).  You might try lifting the 1 and 2 terminals of T4 to further
isolate the collectors and confirm that T4 is the culprit.

Ask yourself 'What did I change?' - you said the K2 was working before you
replaced the transformers, so it has to be associated with something that
was touched in the process.

Since you said you are using the 'buzzer' of your DMM, it amy be informative
to see what the actual resistance is - the collectors normally show 500 to
600 ohms to ground, and if your DMM buzzes with that value, that would be
misleading.

It is much more likely to have a short only when the heat sink is mounted.
That is most commonly caused by a small bit of aluminum becoming lodged in
the thermal pads.  The bits of aluminum come about when the toothed
lockwashers are loosened.  SOLUTION - replace the thermal pads and be
certain there are no bits of aluminum still clinging to the transistors or
the underside of the heat sink.  As a further precaution, I usually do not
use the toothed lockwashers for locking the heat sink screws and nuts, I use
the split lockwashers instead because they are not as likely to leave those
bits of aluminum behind when they are removed.

73,
Don W3FPR

----- Original Message -----

>
>  My K2 has been working fine but I decided to fix it anyway...hi.
>
>  Well actually I was a little short on the power output on 10 meters.  I
> was getting about 7 watts out on 10 meters and Gary Suggested that I
> replace T2 and T1.  Instead of winding them myself, I contacted 'The
> Toroid Guy' and he sent me two beauties (along with a T-4) - the guy is
> great, BTW.
>
>  So yesterday I installed the new T1 and T2 (I did not yet install T4) and
> replaced the heat sink.  The K2 manual warns that whenever the heat sink
> is removed and replaced you should verify that the collectors of Q7 and Q8
> are not shorted to ground.
>
>  With the heatsink attached I check and sure enough, there was a short
> between the collector (middle pin) and ground (the ground wire on the RF
> board) - I used the continuity buzzer on my DMM for this.
>
>  I removed the heatsink and all of the hardware which connects Q7 and Q8
> to the heatsink and then again checked for the sort.  It was still there.
>
>  In looking at the schematic, the section of the final which contains Q7
> and Q8 seems pretty isolated from everything else, with just T3 and T4
> seemingly getting into the action.  Try as a I might I couldn't locate any
> shorts.  Using the schematic, I started at the collectors of the
> transistors and worked to the right of the schematic.
>
>  The only options I have presently connected are the Noise Blanker and the
> SSB module, both of which don't seem to be involved with the finals.
>
>  Obviously the problem must be in the area that I was working on.
>
> If anyone has any suggestions in tracking this down, I appreciate your
> help
>
> Tom, WB2QDG
> K2 1103
>

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[Terry W9TRB]

On Monday 04 October 2004 08:16, [hidden email] wrote:
> Hi,
>
>   My K2 has been working fine but I decided to fix it anyway...hi.
>
  The K2 manual warns that whenever the heat sink is
> removed and replaced you should verify that the collectors of Q7 and Q8 are
> not shorted to ground.
>

> Tom, WB2QDG
> K2 1103

Hello Tom,

        The way I blew one of mine was due to a very tiny fragment of metal shaving
getting  installed along with the heat transfer pad of Q7, I took only a very
short time to let the smoke out.  
        This happened quite some time back, so I can't remember what I was doing or
trying to accomplish, but I'd say if you've done anything to Q7/Q8 and can't
find the short to ground.   It "might" be something like what I did.  Believe
me it was a tiny, tiny fragment, but when Q7 was tightened down it made
contact and did its damage.

        Just a bit of my experience that you can check and maybe gain from.

73,

Terry  KA9TXE
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[Brian Wruble]

A question on this subject...  I had the same problem, after I installed the
100 watt amp and put everything back together.  I solved it with new pads.
The only symptom was a failure to develop output power.  It was awhile ago.
It may be that the thing refused to power-up at all, I just don't remember.
There was never any smoke, no loss of the finals.  When I loosened the
screws holding Q7 and Q8, everything worked.  After I replaced the pads, I
was able to properly tighten the screws with no problem.

I have assumed there is protective circuitry so that a short to ground would
not blow anything.  Was I just lucky? I don't have the schematics in front
of me, so I can't look to see for myself.  My K2 lives a few states away.

Thanks for any insights.  73 de Brian W3BW

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Terry Bassett
Sent: Monday, October 04, 2004 10:39
To: [hidden email]
Subject: Re: [Elecraft] Question about K2's Q7 and Q8 Final Amp

On Monday 04 October 2004 08:16, [hidden email] wrote:
> Hi,
>
>   My K2 has been working fine but I decided to fix it anyway...hi.
>
  The K2 manual warns that whenever the heat sink is
> removed and replaced you should verify that the collectors of Q7 and Q8
are
> not shorted to ground.
>

> Tom, WB2QDG
> K2 1103

Hello Tom,

        The way I blew one of mine was due to a very tiny fragment of metal
shaving
getting  installed along with the heat transfer pad of Q7, I took only a
very
short time to let the smoke out.  
        This happened quite some time back, so I can't remember what I was
doing or
trying to accomplish, but I'd say if you've done anything to Q7/Q8 and can't

find the short to ground.   It "might" be something like what I did.
Believe
me it was a tiny, tiny fragment, but when Q7 was tightened down it made
contact and did its damage.

        Just a bit of my experience that you can check and maybe gain from.

73,

Terry  KA9TXE
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