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K2 Finals - Chronic Blow-Outs

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w7asa

K2 Finals - Chronic Blow-Outs

This post was updated on .

This K2 was a bought from someone on the internet as a working unit & etc. Once received, it blew the finals in about 2 - 3 hours of QSO time. I really like the K2 but it looks it's mine to solve. I've done a lot of radoi building and RF troubleshooting down through the years, and have looked for all the usual suspects of shorts, debris shaken loose during shipment & etc.

BTW: The condition of the board and mounting hardware for the finals and PA bias transistors STRONGLY indicates that they've been changed more than a few times before the rig reached me, but it's also a radio built for fun at home and it might just not have been a good solder day...

Here's what I am seeing:

It has fried three sets of finals in two weeks - I changed them per the manual twice ( am a glutton for punishment ) and VERY carefully re-checked my work and metered for shorts each time. After =< 2 to 3 hours of 'normal' operation each time, the finals blew. This happened on three separate bands with an SWR of 1.2:1 (on average) at 10 Watts indicated both on my SWR/power meter and the K2 digital display. I did not notice a current warning during the last two failures and now am not certain whether I did during the first failure.

Most recently: - I had finished the most recent ragchew then switched to calling CQ using a Vibroplex (as always) and heard a small 'crack' of a transistor case rupturing when immediately the 3 Amp automotive fuse in my power cord blew. The smell told me where to look.

This is a relatively simple circuit and it seems to me that there is not a lot to go wrong: T4 with bad insulation ? Something in the common parts of the LGP/Band switching but not band specific? I am mystified why it would run flawlessly for a few hours of CW then let the smoke out.

Anybody have ideas? I have a ton of Elecraft accessory boards on the way and that's a lot of assessories for a radio that drops dead continuously.

>Ray
W7ASA ..._ ._

Don Wilhelm-4

Re: K2 Finals - Chronic Blow-Outs

Ray,

You apparently have some problem that is destroying your K2 PA
transistors. What I am saying is that the dead PA transistors is the
symptom but not the cause of failure.

When the PA transistors are removed, measure the voltage at the solder
pad for the PA transistor base during a TUNE. If it is greater than
0.63 volts, replace Q11 and Q13. You might just replace Q11 and Q13
"just because".

When you replace the PA transistors, do you use the Elecraft K2PAKIT or
do you purchase the transistors from another source. The Elecraft
K2PAKIT has transistors that are matched, and that is an important
parameter.

Do you have the KAT2, KPA100, or KAT100 installed? If so, check the
wattmeter diodes. Those diodes often become damaged by nearby lightning
(do not leave the antenna connected when not operating), If those diodes
are damaged, the K2 will go to uncontrolled maximum power output, and
the usual result is that the base K2 PA transistors are overloaded and
will sacrifice themselves.

73,
Don W3FPR

On 10/13/2012 7:29 AM, w7asa wrote:

> This K2 was a bought from someone on the internet as a working unit & etc.
> Once received, it blew the finals in about 2 - 3 hours of QSO time. I
> really like the K2 but it looks it's mine to solve. I've done a lot of
> radoi building and RF troubleshooting down through the years, and have
> looked for all the usual suspects of shorts, debris shaken loose during
> shipment & etc.
>
> BTW: The condition of the board and mounting hardware for the finals and PA
> bias transistors STRONGLY indicates that they've been changed more than a
> few times *before* the rig reached me . . . so , in all probability, I was
> probably the only one surprised by it's sudden demise.
>
> Here's what I am seeing:
>
> It has fried three sets of finals in two weeks - I changed them per the
> manual twice ( am a glutton for punishment ) and VERY carefully re-checked
> my work and metered for shorts each time. After =< 2 to 3 hours of 'normal'
> operation each time, the finals blew. This happened on three separate bands
> with an SWR of 1.2:1 (on average) at 10 Watts indicated both on my SWR/power
> meter and the K2 digital display. I did not notice a current warning during
> the last two failures and now am not certain whether I did during the first
> failure.
>
> Most recently: - I had finished the most recent ragchew then switched to
> calling CQ using a Vibroplex (as always) and heard a small 'crack' of a
> transistor case rupturing when immediately the 3 Amp automotive fuse in my
> power cord blew. The smell told me where to look.
>
> This is a relatively simple circuit and it seems to me that there is not a
> lot to go wrong: T4 with bad insulation ? Something in the common parts of
> the LGP/Band switching but not band specific? I am mystified why it would
> run flawlessly for a few hours of CW then let the smoke out.
>
> Anybody have ideas? I have a ton of Elecraft accessory boards on the way and
> that's a lot of assessories for a radio that drops dead continuously.
>

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Jim Wiley

Re: K2 Finals - Chronic Blow-Outs

In reply to this post by w7asa

What is the high current alarm set at? Are there the correct number of
turns on T4? (check against the illustration in the assembly manual)
Perhaps there is a problem with the bias system. Transistors Q11 and Q13
should probably be changed as well as the finals if there is a problem
with the PA stage. You did not say what the set serial number is. I am
not sure if that might make a difference with troubleshooting help, but
it could.

- Jim, KL7CC

On 10/13/2012 3:29 AM, w7asa wrote:

w7asa

Re: K2 Finals - Chronic Blow-Outs

This post was updated on .

Man! You guys are really encouraging-and FAST!

The K2 is the ten Watt version, has ONLY the SSB option at this time. I am building the ATU, 160, 60m, and a few more goodies on my bench as we speak.

---> Believe me, I FULLY understand that finals, like fuses are not the problem, they are the result. Good advice though.

>>> I bought this on the internet very recently and have basicaly no information available from the seller.

When I ordered the ATU kit from Elecraft, I also ordered two K2PAKIT packs just in case, figuring that I would never need them

I installed all K2PAKIT components, including Q11 & Q13, and the 1.5 Ohm resistor R50. My bench has a grounded ESD mat and wrist strap and I've made a career working to IPC610/620 and mil electronics standards. So, I am really comfortable inside of this radio.

The general workmanship on the main board was not very good but I was told that it was functioning fine. After I have it running, I intend to do a lot of flux removal, reflow & rework to clean it up, but fo now, keeping it stable until I can trouble shoot it is important.

>>> However the SSB unit is really excellent: shiney joints, no voids, great fillets... so I knew that it was built by someone else as soon as I saw it. I found paperwork in the maual and the EXCELENT SSB board was made by Don Wilhelm! Great Work Don! Rest assured, I used to do a lot of nit-picky QA & rework for DoD and even through my magnifiers, your SSB board is really superb work ! ... small world, eh? (and no, I do not work for Don...)

Thanks for the settings and reading info. I will do the current, bias voltage and settings checks as indicated by you guys and thanks for the direction. I am brand new to the K2. My next two replacement parts kits are ETA next week, so I have plenty of time to snoop, measure, poke & prod until then, but I'll go straight for the settings - being the low hanging fruit.

Thank You & I'll keep you posted -

73 de Ray
W7ASA ..._ ._

Don Wilhelm-4

Re: K2 Finals - Chronic Blow-Outs

Ray,

Even though you have changed Q11 and Q13, do check the voltage at the
base of Q7 or Q8 both during receive and transmit. It should be zero
during receive and close to 0.6 during transmit.
If that voltage is something different, you will need to look for the
reason before installing (and blowing) another set of PA transistors.

Thanks for the good words on the KSB2 option. You can always tell one
of my KSB2 builds at a glance from the top. I ground the filter
crystals near the bottom of the crystal cans instead of on the top.

73,
Don W3FPR

On 10/13/2012 9:08 AM, w7asa wrote:

> Man! You guys are really encouraging-and FAST!
>
> The K2 has ONLY the SSB option at this time. I am building the ATU, 160,
> 60m, and a few more goodies on my bench as we speak.
>
> ---
>>>> I bought this on the internet very recently and have basicaly no
>>>> information available from the seller. When I ordered the ATU kit from
>>>> Elecraft, I also ordered two K2PAKIT packs just in case, *figuring that
>>>> I would never need them*
> I installed all K2PAKIT components, including Q11 & Q13, and the 1.5 Ohm
> resistor R50. My bench has a grounded ESD mat and wrist strap and I've made
> a career working to IPC610/620 and mil electronics standards. So, I am
> really comfortable inside of this radio.
>
> The general workmanship on the main board was not very good but I am told
> that it was functioning for years. After I have it running, I intend to do
> a lot of flux removal, reflow & rework to clean it up. However the SSB unit
> *is really excellent*: shiney joints, no voids, great fillets... so I knew
> that it was built by someone else as soon as I saw it. I found paperwork in
> the maual and the EXCELENT SSB board was made by Don Wilhelm! Great Work
> Don! Rest assured, I used to do a lot of nit-picky QA & rework for DoD and
> even through my magnifiers, your SSB board is really superb work ! ...
> small world, eh? (and no, I do not work for Don...)
>
> Thanks for the settings and reading info. I will do the current, bias
> voltage and settings checks as indicated by you guys and thanks for the
> direction. I am brand new to the K2. My next two replacement parts kits are
> ETA next week, so I have plenty of time to snoop, measure, poke & prod until
> then, but I'll go straight for the settings - being the low hanging fruit.
>
>
> Thank You & I'll keep you posted -
>
>
> 73 de Ray
> W7ASA ..._ ._
>
>
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/K2-Finals-Chronic-Blow-Outs-tp7564052p7564055.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> 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 Finals - Chronic Blow-Outs

In reply to this post by w7asa

Ray,

There is one more thing to check. Use an external wattmeter and a dummy
load - check to be certain the actual output power is somewhere near
what the K2 indicates on the display during a TUNE.

If the displayed power is nowhere close to the actual power, then you
need to fix the power measuring circuit - for the base K2, that is D9,
R66, R67, R68, R69 and C77. If you have the KAT2 or the KPA100 or
KAT100, then the wattmeter in those options may not be working correctly.

One other thing I might mention - are you operating the K2 into a high
SWR load? If so, the voltages in the PA stage may be excessively high
causing failure of the finals.

73,
Don W3FPR

On 10/13/2012 9:08 AM, w7asa wrote:

R Thompson

Re: K2 Finals - Chronic Blow-Outs

In reply to this post by w7asa

When I read this today it reminded me of a problem that I had with my
TenTec 6M transverter. Sometime after TenTec ceased production of that
kit, they did another run of them. I picked up one from the last
production run, and it performed as advertised. Then out of nowhere the
finals failed for no understandable reason. After taking a close look
at what remained of the factory supplied finals, 2SC1969's, and checking
it out on the internet, I now think the kit came with bogus (fake) final
transistors.

I wouldn't begin to suggest this if you hadn't said that it appears
that the finals were replaced before you got the radio.

Here is a link I found on the subject:

http://www.4hamradio.com/counterfeit-mitsubishi-2sc19692sc2312-transistor-test/

Eventually I did find some, well I think they're close enough to the
real thing, they work and they take a lot less drive than the old ones
did to get the desired output. I would strongly suggest, if you're not
already doing this, that you get replacements from Elecraft and not off
of an auction site.

Ron VE8RT

On Sat, 2012-10-13 at 06:08 -0700, w7asa wrote:

> Man! You guys are really encouraging-and FAST!
>
> The K2 has ONLY the SSB option at this time. I am building the ATU, 160,
> 60m, and a few more goodies on my bench as we speak.
>
> ---
> >>> I bought this on the internet very recently and have basicaly no
> >>> information available from the seller. When I ordered the ATU kit from
> >>> Elecraft, I also ordered two K2PAKIT packs just in case, *figuring that
> >>> I would never need them*
>
> I installed all K2PAKIT components, including Q11 & Q13, and the 1.5 Ohm
> resistor R50. My bench has a grounded ESD mat and wrist strap and I've made
> a career working to IPC610/620 and mil electronics standards. So, I am
> really comfortable inside of this radio.
>
> The general workmanship on the main board was not very good but I am told
> that it was functioning for years. After I have it running, I intend to do
> a lot of flux removal, reflow & rework to clean it up. However the SSB unit
> *is really excellent*: shiney joints, no voids, great fillets... so I knew
> that it was built by someone else as soon as I saw it. I found paperwork in
> the maual and the EXCELENT SSB board was made by Don Wilhelm! Great Work
> Don! Rest assured, I used to do a lot of nit-picky QA & rework for DoD and
> even through my magnifiers, your SSB board is really superb work ! ...
> small world, eh? (and no, I do not work for Don...)
>
> Thanks for the settings and reading info. I will do the current, bias
> voltage and settings checks as indicated by you guys and thanks for the
> direction. I am brand new to the K2. My next two replacement parts kits are
> ETA next week, so I have plenty of time to snoop, measure, poke & prod until
> then, but I'll go straight for the settings - being the low hanging fruit.
>
>
> Thank You & I'll keep you posted -
>
>
> 73 de Ray
> W7ASA ..._ ._
>
>
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/K2-Finals-Chronic-Blow-Outs-tp7564052p7564055.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> ______________________________________________________________
> 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

John Magliacane

Re: K2 Finals - Chronic Blow-Outs

In reply to this post by w7asa

Hi Ray.

One possibility might be improper assembly of the transceiver that is causing the transistors to not mate properly with the heatsink on the bottom chassis.

Been there... done that. :-)

Here's the story behind repeated 2SC1969 failures in my K2 back in 2003, and the mechanics behind the cause:

http://mailman.qth.net/pipermail/elecraft/2003-September/023145.html

http://mailman.qth.net/pipermail/elecraft/2003-September/023307.html

There were never any over-current issues, and it took several transistor replacements before I realized the problem was due to the incorrect placement of the lock washers associated with the 2-D fasteners.

Good luck!

73, de John, KD2BD

--
Visit John on the Web at:

http://www.qsl.net/kd2bd/

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Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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Please help support this email list: http://www.qsl.net/donate.html