[Elecraft] KPA100 100% Duty Cycle - Operating Temperature

[Don Wilhelm]

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.