K2/100 - Hi Curr and loss of power control - help! de VK7JB

[VK7JB]

Hi Group,

Using my K2/100 #6998 on 20m SSB phone today, running it into an experimental small loop antenna.  SWR was 1.5:1.   Then:  

Hi Curr warning on LCD - shows continuously in TX.
Maximum power out (>100W) independent of power control setting
LED Bargraph stopped displaying RF output (ie no bars lit) despite > 100W output measured on external power meter.

Disconnected antenna and attached dummy load - same symptoms.

Could I have somehow fried D16 & D17 in the KPA100?  I checked these diodes in-circuit with my multimeter "diode test" function and there is no voltage drop across either.  Does this mean they've both failed open?

Is there anything else I should check/look for/measure before I dismantle the KPA100 and replace D16 and 17?

Any advice appreciated as always,

73,
John
VK7JB

[Gary Gregory]

John,

Do a full reset of the K3....in the manual, follow the steps and let me know
how it goes.

Gary

On 17 June 2011 15:59, VK7JB <[hidden email]> wrote:

> Hi Group,
>
> Using my K2/100 #6998 on 20m SSB phone today, running it into an
> experimental small loop antenna.  SWR was 1.5:1.   Then:
>
> Hi Curr warning on LCD - shows continuously in TX.
> Maximum power out (>100W) independent of power control setting
> LED Bargraph stopped displaying RF output (ie no bars lit) despite > 100W
> output measured on external power meter.
>
> Disconnected antenna and attached dummy load - same symptoms.
>
> Could I have somehow fried D16 & D17 in the KPA100?  I checked these diodes
> in-circuit with my multimeter "diode test" function and there is no voltage
> drop across either.  Does this mean they've both failed open?
>
> Is there anything else I should check/look for/measure before I dismantle
> the KPA100 and replace D16 and 17?
>
> Any advice appreciated as always,
>
> 73,
> John
> VK7JB
>
>
>
>
> --
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> Sent from the Elecraft mailing list archive at Nabble.com.
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--

VK4FD - Motorhome Mobile
Elecraft Equipment
K3 #679, KPA-500 #018
Living the dream!!!
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[VK7JB]

Hi Gary,

It's my K2, not my K3, that's in trouble.  Mercifully, my K3 is playing very happily!

73,

John

[Mark Bayern]

> Could I have somehow fried D16 & D17 in the KPA100?  I checked these diodes
> in-circuit with my multimeter "diode test" function and there is no voltage
> drop across either.  Does this mean they've both failed open?

No voltage drop across a diode should mean it failed shorted. However,
I would be leery of the in-circuit measurement. Why not lift one end
of each diode so you can be positive that there is nothing besides the
diode in the measurement?

Mark  AD5SS



On Fri, Jun 17, 2011 at 12:59 AM, VK7JB <[hidden email]> wrote:

> Hi Group,
>
> Using my K2/100 #6998 on 20m SSB phone today, running it into an
> experimental small loop antenna.  SWR was 1.5:1.   Then:
>
> Hi Curr warning on LCD - shows continuously in TX.
> Maximum power out (>100W) independent of power control setting
> LED Bargraph stopped displaying RF output (ie no bars lit) despite > 100W
> output measured on external power meter.
>
> Disconnected antenna and attached dummy load - same symptoms.
>
> Could I have somehow fried D16 & D17 in the KPA100?  I checked these diodes
> in-circuit with my multimeter "diode test" function and there is no voltage
> drop across either.  Does this mean they've both failed open?
>
> Is there anything else I should check/look for/measure before I dismantle
> the KPA100 and replace D16 and 17?
>
> Any advice appreciated as always,
>
> 73,
> John
> VK7JB
>

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[Don Wilhelm-4]

  John,

Yes, that is the symptom of damaged diodes D16 and D17.  There is also
the possibility that U5 and or U6 is not responding properly, but if you
measure no voltage drop across those diodes, they are for certain
toasted.  Start with the diodes, and after that we can move on to the
other components if you still have trouble.

73,
Don W3FPR

On 6/17/2011 1:59 AM, VK7JB wrote:

> Hi Group,
>
> Using my K2/100 #6998 on 20m SSB phone today, running it into an
> experimental small loop antenna.  SWR was 1.5:1.   Then:
>
> Hi Curr warning on LCD - shows continuously in TX.
> Maximum power out (>100W) independent of power control setting
> LED Bargraph stopped displaying RF output (ie no bars lit) despite>  100W
> output measured on external power meter.
>
> Disconnected antenna and attached dummy load - same symptoms.
>
> Could I have somehow fried D16&  D17 in the KPA100?  I checked these diodes
> in-circuit with my multimeter "diode test" function and there is no voltage
> drop across either.  Does this mean they've both failed open?
>
>

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[VK7JB]

Thanks Don & Mark,

That's what I suspected, but I wondered about U5 & 6 too, whether they may may have been stung  further down the schematic.  I'll replace the diodes tomorrow when it's light.  I have a small supply of 1N5711's here in my junk box.  Then it'll be clear if there's more than just the diodes that've turned to toast.

Mark, looking at the dismantling involved to get to the fiddly location of D16 and 17, I think it'll be just as easy to replace the diodes and be done with it.  Lifting one end to check would be a good idea though, if they were in a less awkward location.  

This is an interesting failure.  I wonder if the high voltage you'd expect on the capacitor of this small loop antenna might have caused this diode-frying??  I did disconnect the coax from the back of the loop tuner (de-energised of course) at one stage to connect my antenna analyser to retune it to another band.  The problem manifest after reconnecting the coax to the K2.  The SWR was low throughout, but I guess that's not related to high charge on the variable cap, which would be several kV.    I won't be using the K2 on this loop antenna again, just in case.  Pity, because it's great to get a 57 reports working into Austria and Russia and 59 into Florida, running 50W into a diminutive 1m diameter loop sitting on my picnic table :)

I'll report on the patient after the surgery...




73,
John
VK7JB

[VK7JB]

Hi All,

I replaced D16 and D17 on the KPA100  board and all is back to normal.  

I'm still stumped how I managed to cook them.

Just in case I ever have to do this repair again, I mounted the diodes on the underside of the board.  There's plenty of clearance.  When mounted on top of the board, they're tucked away in a tricky to access spot and I found removing the old diodes the most difficult part of the repair.  Next time it'll be easier to get at them and to pull them out from underneath.

73,

John
VK7JB

[aj4tf]

Hi John,

I just made the eaxct same repair last week on K2/100 # 7006.  I
installed some milled IC socket pins in the diode mounting holes, and
pressed the diode leads into the sockets.  Not the best solution for a
radio that will be subject to vibration (mine is not), but it will make
it very easy to do the repair again if necessary without de-soldering
and disassembly.

73,

David AJ4TF



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[VK7JB]

Hi David,

What a great idea.  I wish I'd thought of that before I did my repair.  I've put your idea in my notebook and if/when this happens again, I'll install your solution.  I do worry about the longevity of the PCB holes and traces should this repair become an ongoing maintenance issue (hopefully not!).

Were you able to work out what zapped your diodes?

Can you buy single milled IC sockets, or did you cut up a strip to get them?

Thanks again for a great idea.

73,
John
VK7JB