build # 6952 smoke! yikes

[John Fritze]

Before turning on power the resistance checks were all good.
Everything looked good, didn't see any problems.  The first 2 tests
went without a hitch, so perhaps I was a bit cocky.

I turned on the power to do alignment and test part 3, got a loud
snap, bit of phenolic smell....

Not exactly what I would have liked to happen:-(   Q7 was toast.  I
thought that perhaps I didn't get the insulator on the screw properly
as it's a bit tricky. I also found I had inadvertently forgot to
install Z1.  So I put it in, and I replaced Q7 with one from the
second K2 kit, but no joy.

I basically now have a 12V short to ground:

Q7 collector 24.5 Ohms
Q6 base 118 Ohms- right where it was before
Q7 base 2 Ohms
U 10, 11, 12 are all as before.

Anyone have any thoughts before I just start pulling the surrounding parts?

John
K2QY

OH NO!  The electricicals are escaping!
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[Don Wilhelm-4]

John,

You had a collector to base short (or leakage) on one of the PA
transistors, which caused Q7 to draw a LOT of current and even explode.
That does not say that Q7 is the only problem.  Other potential
candidates are Q8 and Q11 and Q13.

Q11 and Q13 set the bias for Q7 and Q8 - so remove both Q7 and Q8 and
during transmit (or TUNE), measure the voltage at the Q7 or Q8 solder
pads - if it is quite different from 0.62 volts, then you must find the
problem (likely Q11 or Q13) before installing Q7 and Q8.

Secondly, Elecraft has recently been matching Q7 and Q8 for all kits and
replacement PA transistor kits, so you should have replaced both Q7 and
Q8 as a pair - you stated that you just replaced Q7, and that may have
caused a secondary failure if the HFE of Q8 was quite different than the
Q7 that you installed.  It looks like you now need 2 K2PAKITs to be
certain of the matched transistors.

BTW - the K2PAKIT does contain replacements for Q7, Q8, Q11 and Q13 as
well as new thermal pads and PA mounting hardware should that be
needed.  There is also a replacement for R50 if that is needed too.

73,
Don W3FPR

John Fritze wrote:

> Before turning on power the resistance checks were all good.
> Everything looked good, didn't see any problems.  The first 2 tests
> went without a hitch, so perhaps I was a bit cocky.
>
> I turned on the power to do alignment and test part 3, got a loud
> snap, bit of phenolic smell....
>
> Not exactly what I would have liked to happen:-(   Q7 was toast.  I
> thought that perhaps I didn't get the insulator on the screw properly
> as it's a bit tricky. I also found I had inadvertently forgot to
> install Z1.  So I put it in, and I replaced Q7 with one from the
> second K2 kit, but no joy.
>
> I basically now have a 12V short to ground:
>
> Q7 collector 24.5 Ohms
> Q6 base 118 Ohms- right where it was before
> Q7 base 2 Ohms
> U 10, 11, 12 are all as before.
>
> Anyone have any thoughts before I just start pulling the surrounding parts?
>
> John
> K2QY
>
> OH NO!  The electricicals are escaping!
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>  

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