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Re: K2 PA Failure

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john petters

Re: K2 PA Failure

-------

Hi Folks,
I have had another PA transistor failure in my 2003 vintage K2. The
transistors have been replaced several times over the years and the
board is now a mess.
I'm planning to rebuild the PA on an external board with a proper heat
sink mounted on the back panel of the K2. This will involve taking the
secondary from the driver transformer on a piece of coax about 6 inches
long.

My query is what impedance is the secondary of T2 designed to work into
and will I have a problem taking this to the external board?

I've never liked the design of the mechanical fitting of the K2 PA and
always regarded this as the Achilles Heel of the rig.
Suggestions appreciated.
73

--
John Petters
www.traditional-jazz.com
Amateur Radio Station G3YPZ
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wayne burdick

Re: K2 PA Failure

Administrator

John,

K2 PAs are generally extremely reliable, and the heat sinking is
entirely adequate as determined by extensive temperature tests of the
PA device itself and the heat sink. There are some 7000 K2s in the
field and few failures.

This suggests that you may have a defective part elsewhere in the
circuit (bias, etc.). If you have any measurements, please send them
to us. We could also take a look at it if you wanted to send it in.

I would discourage remote mounting of the PA. This might result in HF/
VHF/UHF parasitics that take quite a bit of effort to eliminate.

73,
Wayne
N6KR

On Feb 16, 2011, at 12:15 PM, john petters wrote:

>
>
> -------
>
>
> Hi Folks,
> I have had another PA transistor failure in my 2003 vintage K2. The
> transistors have been replaced several times over the years and the
> board is now a mess.
> I'm planning to rebuild the PA on an external board with a proper heat
> sink mounted on the back panel of the K2. This will involve taking the
> secondary from the driver transformer on a piece of coax about 6
> inches
> long.
>
> My query is what impedance is the secondary of T2 designed to work
> into
> and will I have a problem taking this to the external board?
>
> I've never liked the design of the mechanical fitting of the K2 PA and
> always regarded this as the Achilles Heel of the rig.
> Suggestions appreciated.
> 73
>
>
> --
> John Petters
> www.traditional-jazz.com
> Amateur Radio Station G3YPZ
> ______________________________________________________________
> 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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Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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Johnny Siu

K2 PA Failure

In reply to this post by john petters

Hello John,

I built quite a number of K2 in the past. The PA of K2 is fairly tough. It is
nothing demanding for that pair of PA transistors for an output of merely
10-15W.

I don't think it is a good idea to build external PA board. I do suggest you
take a digital photo of the PCB section related to the PA. Then send your photo
to [hidden email] and seek their advice.

K2 is a very mature product and all the production bugs have been
overcome. Elecraft knows it inside out and can give you all the required
support. The beauty of Elecraft product is that most of the repair can be done
on the field and without the trouble of sending back for repair.

You may wish to take this chance to investigate why the PA of your K2 keeps on
failing. It is a very unusual circumstances.

cheers,

Johnny VR2XMC

________________________________
寄件人﹕ john petters <[hidden email]>
收件人﹕ Elecraft <[hidden email]>
傳送日期﹕ 2011/2/17 (四) 4:15:25 AM
主題： Re: [Elecraft] K2 PA Failure

-------

Hi Folks,
I have had another PA transistor failure in my 2003 vintage K2. The
transistors have been replaced several times over the years and the
board is now a mess.
I'm planning to rebuild the PA on an external board with a proper heat
sink mounted on the back panel of the K2. This will involve taking the
secondary from the driver transformer on a piece of coax about 6 inches
long.

My query is what impedance is the secondary of T2 designed to work into
and will I have a problem taking this to the external board?

I've never liked the design of the mechanical fitting of the K2 PA and
always regarded this as the Achilles Heel of the rig.
Suggestions appreciated.
73

--
John Petters
www.traditional-jazz.com
Amateur Radio Station G3YPZ

______________________________________________________________
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

Don Wilhelm-4

Re: K2 PA Failure

John,

I agree with Johnny and Wayne. I have built and repaired a LOT of K2s,
and in general the PA transistors are quite robust. Due to an
unfortunate set of circumstances about 2 years ago, the PA transistors
were found to be quite variable in gain which placed stress on one of
the PA transistor pairs. Replacements (and new kits) are now being
provided as pairs matched for hfe.

I would suggest that you obtain a new K2PAKIT from Elecraft and use
those transistors in their original configuration. If the solder pads
to your PA transistors are damaged, repair them with wire laid in the
same track as the original PC Board traces. The traces are on the
bottom side of the RF Board.

While you have the PA transistors removed from the board, do a TUNE and
measure the DC voltage at the base solder pads. If it is above 0.63
volts (or lower than 0.60 volts) also replace Q11 and Q13 and
remeasure. If it is still out of bounds, check the resistor values
associated with Q11 and Q13 (see the schematic).

73,
Don W3FPR

On 2/16/2011 7:48 PM, Johnny Siu wrote:

> Hello John,
>
> I built quite a number of K2 in the past. The PA of K2 is fairly tough. It is
> nothing demanding for that pair of PA transistors for an output of merely
> 10-15W.
>
> I don't think it is a good idea to build external PA board. I do suggest you
> take a digital photo of the PCB section related to the PA. Then send your photo
> to [hidden email] and seek their advice.
>
> K2 is a very mature product and all the production bugs have been
> overcome. Elecraft knows it inside out and can give you all the required
> support. The beauty of Elecraft product is that most of the repair can be done
> on the field and without the trouble of sending back for repair.
>
> You may wish to take this chance to investigate why the PA of your K2 keeps on
> failing. It is a very unusual circumstances.
>
> cheers,
>
>
> Johnny VR2XMC
>
>
>
>
> ________________________________
> 寄件人﹕ john petters<[hidden email]>
> 收件人﹕ Elecraft<[hidden email]>
> 傳送日期﹕ 2011/2/17 (四) 4:15:25 AM
> 主題： Re: [Elecraft] K2 PA Failure
>
>
>
> -------
>
>
> Hi Folks,
> I have had another PA transistor failure in my 2003 vintage K2. The
> transistors have been replaced several times over the years and the
> board is now a mess.
> I'm planning to rebuild the PA on an external board with a proper heat
> sink mounted on the back panel of the K2. This will involve taking the
> secondary from the driver transformer on a piece of coax about 6 inches
> long.
>
> My query is what impedance is the secondary of T2 designed to work into
> and will I have a problem taking this to the external board?
>
> I've never liked the design of the mechanical fitting of the K2 PA and
> always regarded this as the Achilles Heel of the rig.
> Suggestions appreciated.
> 73
>
>