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KPA100 100% Duty Cycle - Operating Temperature

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K1FFX

KPA100 100% Duty Cycle - Operating Temperature

I started running some 100% duty cycle operation with my KPA100 (PSK31 and
Olivia). I was running 20 watts ... I'm not sure what power folks normally
run in these modes (I'd be happy to get feedback on that), but, in any case,
I was running well below the KPA100's 100 watt maximum. After around 90
seconds, the little fan on the KPA100 kicked in and really rev'ed up. The
heat sink was, not surprisingly, quite warm to the touch.

So, considering I was running at 20 watts, would that level of heat be
normal, i.e., was it actually no surprise that the fan kicked in? And is
there any danger to the output transistors running key-down at the 20 watt
level?

Thanks for any tips or insight.

Bruce K1FFX

-----
Bruce Rosen
K1FFX
K2/100 6982 KSB2 KAT100-1

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Bruce Rosen
K1FFX
K2/100 6982 KSB2 KAT100-1 KAF2 KIO2

Don Wilhelm

Re: KPA100 100% Duty Cycle - Operating Temperature

Bruce,

If you had the fan turn on at 20 watts, there is likely a problem.
Get an external wattmeter and check the actual power output.
I suspect the actual power is not controlled, and can exceed 150 watts.

The usual cause of that problem is damaged diodes D16 and D17 in the KPA100.

There will usually be no harm to the KPA100 PA transistors, but the PA
transistors in the base K2 can be damaged because they are being
stressed to produce their maximum output.

Also that condition is likely to give you a distorted signal, and with
Data Modes that usually means a high IMD.

73,
Don W3FPR

On 11/4/2017 6:55 PM, K1FFX wrote:

> I started running some 100% duty cycle operation with my KPA100 (PSK31 and
> Olivia). I was running 20 watts ... I'm not sure what power folks normally
> run in these modes (I'd be happy to get feedback on that), but, in any case,
> I was running well below the KPA100's 100 watt maximum. After around 90
> seconds, the little fan on the KPA100 kicked in and really rev'ed up. The
> heat sink was, not surprisingly, quite warm to the touch.
>
> So, considering I was running at 20 watts, would that level of heat be
> normal, i.e., was it actually no surprise that the fan kicked in? And is
> there any danger to the output transistors running key-down at the 20 watt
> level?

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Lynn W. Taylor, WB6UUT-2

Re: KPA100 100% Duty Cycle - Operating Temperature

In reply to this post by K1FFX

20 watts is a lot of power on PSK-31.

10 watts is a lot of power on PSK-31.

73 -- Lynn

On 11/4/2017 3:55 PM, K1FFX wrote:
> I started running some 100% duty cycle operation with my KPA100 (PSK31 and
> Olivia). I was running 20 watts ..
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Rick WA6NHC-2

Re: KPA100 100% Duty Cycle - Operating Temperature

Pffft.

Use only enough power to allow the completion of the contact. Even if it’s legal limit.

It’s a common misconception.

Rick WA6NHC

Smell Czech happens

> On Nov 4, 2017, at 4:59 PM, Lynn W. Taylor, WB6UUT <[hidden email]> wrote:
>
> 20 watts is a lot of power on PSK-31.
>
> 10 watts is a lot of power on PSK-31.
>
> 73 -- Lynn
>
>> On 11/4/2017 3:55 PM, K1FFX wrote:
>> I started running some 100% duty cycle operation with my KPA100 (PSK31 and
>> Olivia). I was running 20 watts ..
> ______________________________________________________________
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Bob McGraw - K4TAX

Re: KPA100 100% Duty Cycle - Operating Temperature

Digital modes lend themselves to lower power contacts successfully. It gets messy for all when a station jumps in with excessive power. I'm seeing more and more stations with poor to very poor quality signals. It takes very little effort to be loud, but requires more effort and skill to be good.

Bob, K4TAX

Sent from my iPhone

> On Nov 4, 2017, at 7:04 PM, Rick Bates (WA6NHC) <[hidden email]> wrote:
>
> Pffft.
>
> Use only enough power to allow the completion of the contact. Even if it’s legal limit.
>
> It’s a common misconception.
>
> Rick WA6NHC
>
> Smell Czech happens
>
>> On Nov 4, 2017, at 4:59 PM, Lynn W. Taylor, WB6UUT <[hidden email]> wrote:
>>
>> 20 watts is a lot of power on PSK-31.
>>
>> 10 watts is a lot of power on PSK-31.
>>
>> 73 -- Lynn
>>
>>> On 11/4/2017 3:55 PM, K1FFX wrote:
>>> I started running some 100% duty cycle operation with my KPA100 (PSK31 and
>>> Olivia). I was running 20 watts ..
>> ______________________________________________________________
>> 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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> ______________________________________________________________
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K1FFX-2

Re: KPA100 100% Duty Cycle - Operating Temperature

In reply to this post by Don Wilhelm

Don,

You were right on the money with your diagnosis.

I have the Elecraft W1 wattmeter. At 10 watts or below, the "14 watt" range
LED was illuminated. As I rotated the Power knob past 10 watts to just
about 12 watts, the "14 watt" LED went out and the "140 watt" LED was then
illuminated!

So, I've got to replace D16 and D17. Any hints on that replacement job will
be appreciated. I found
an old forum posting from someone who apparently had a difficult time
disassembling the KPA100 to get to the two diodes.

Meanwhile, I've to go back and find your e-mail (I saved it somewhere) on
bleeding off static
charges on the antenna to protect D16 and D17.

Thanks!

- Bruce K1FFX

-----
Bruce Rosen
K1FFX
K2/100 6982 KSB2 KAT100-1

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Don Wilhelm

Re: KPA100 100% Duty Cycle - Operating Temperature

Bruce,

To do the job properly, you will have to remove the board from the heatsink.
Remove the shield and the standoffs - note that the 2 standoffs toward
the front are shorter.
Then remove the 6 screws holding the PA transistor and Q3 and Q4.
There are 3 more screws on the rear panel to remove.
You can then remove the board from the heatsink. You may want to
unsolder the wires from the speaker, but that is your choice.

Locate D16 and D17 near the SO-239 jack. Remove them and replace with
new 1N5711 diodes.
If you do not have good desoldering equipment, don't fret - if you can't
unsolder the diodes one end at a time, crush the diodes and remove the
leads one at a time. Clean up with solder wick and if solder remains in
the holes, heat the pad and push it out with a wooden toothpick.
Normally, just replacing the diodes is sufficient, but you can check
your work by powering only the base K2 with the external wattmeter/dummy
load (you can do that before mounting the KPA100 board onto the
heatsink). If you have proper power control in the low power range, all
is well. If not, there may be another problem with the KPA100 wattmeter
circuit.

BTW, examine the KPA100 board. If it has blue toroid cores (other than
the Low Pass Filter), then it is the most recent. If instead you see a
red toroid core at L16 and L15, it would be a good time to upgrade it
with KPA100UPKT.
Also examine the shield - if it has a notch over the SO-239 that is only
3/16 inch or less, it is the new one, and you should also have a shield
over the speaker magnet. If both those conditions are not true, add the
KPA100SHLDKT.

73,
Don W3FPR

On 11/5/2017 1:37 PM, K1FFX wrote:

> Don,
>
> You were right on the money with your diagnosis.
>
> I have the Elecraft W1 wattmeter. At 10 watts or below, the "14 watt" range
> LED was illuminated. As I rotated the Power knob past 10 watts to just
> about 12 watts, the "14 watt" LED went out and the "140 watt" LED was then
> illuminated!
>
> So, I've got to replace D16 and D17. Any hints on that replacement job will
> be appreciated. I found
> an old forum posting from someone who apparently had a difficult time
> disassembling the KPA100 to get to the two diodes.

aj4tf

Re: KPA100 100% Duty Cycle - Operating Temperature

In reply to this post by K1FFX-2

Bruce,
I had the same problem a while ago, but instead of just replacing the
diodes, I installed some machined IC socket pins in the PWB (i.e. pins from
a Mill-Max IC socket), then put the diodes in those. That way, if they
have to be replaced again, it's a "no-solder" job that can be performed in
the field with some needle-nose pliers.

David AJ4TF
K2, K3S

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