[Elecraft] KPA100 diodes blown...

[Don Wilhelm]

Brett,

I do not know how many other radios have wattmeters directly at the 
antenna jack, so I really can't compare others.

You can place a high value resistor (1k or greater - that is 20 times 
the 50 ohm impedance) across the antenna and see if that helps.  An RF 
Choke could also be used for the same purpose.  RFC10 (100 uHy) was 
removed from inside the KPA100 because it can couple to parts of the 
base K2 and cause out of band oscillations.  You could add an RF choke 
of 100 uHy or a little greater across the antenna external to the KPA100 
if you want the protection it afforded back again.

I am confused because you state that you disconnect the antennas when 
not in use, but then you refer to the 3 turn winding in your homebrew 
receiver.  The only certain cure for damaged diodes is to fully 
disconnect the antenna connection when the K2 is not in use - I 
certainly hope you do not operate during periods of thunderstorm 
activity, sine that is a dangerous situation unless you have extensive 
(and expensive) lightning protection (equal to that installed at a 
broadcast station) installed in your ham station - a simple ground will 
not perform that function.  I have never had a diode failure in my 
KPA100, but I disconnect the antennas and connect a dummy load directly 
to the K2 when it is not in use.

The 1N5711 diodes are a bit on the fragile side, but they do provide the 
best voltage response vs. frequency curve.  If you want hardier 
wattmeter diodes, you could replace them with 1N4148 diodes.  The 
accuracy of the wattmeter may suffer by 5 to 10% (and that may be band 
dependent), and the readings below 3 watts may not be as accurate and 
with the 1N5711s.  Since you have to replace the diodes anyway, the 
choice is yours to make - just be prepared to accept the consequences I 
have outlined if you decide to use the 1N4148s, or if you choose the 
1N5711s, understand that they are more sensitive to static discharges 
and the take the necessary precautions so they are not damaged.

73,
Don W3FPR

Brett Gazdzinski wrote:
> I turned my K2 on after is sat a while, and hit the tune button to tune the 
> antenna, and got crazy indications, D16 and D17 in the KPA100 are blown.
>
> Is there nothing that can be done about these diodes blowing so easy?
> I need to order a set from Elecraft, but I leave all the other radios hooked 
> up and nothing blows in them...
>
> I disconnect the antenna's, and select the dummy load, plus I usually have 
> the homebrew receiver selected which has a 3 turn coil as an antenna input, 
> which should look like a short to any voltage, but enough voltage must get 
> through everything to blow the diodes in the K2...
>
> How are other radios protected against static buildup on the antenna?
>
> Brett
> N2DTS
>   
>