D16/D17 and high swr (kpa100)

[Kirk Brown-2]

After six months of operating, my k2/100 shows high swr/hi curr when
connected to the dummy load. Running in battery/qrp mode, everything is
fine, so I suspected something in the PA. I checked R26/R27 and they showed
about 25k ohms each which seems a bit low (the alignment step calls for 43k
ohms). I looked at the schematic and decided to test D16 and D17. D16 shows
0.057 volts both ways and D17 shows about 0.3 volts. Is D16 shot?
Thanks,

Kirk, KA2P
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[Don Wilhelm-4]

Kirk,

Replace both D16 and D17 before touching the adjustment of R26 and R27 -
the resistance readings will be different with good diodes (about double
what they read now), and your wattmeter calibration should remain the
same as it was before.

If you want to sacrifice low power wattmeter accuracy, and also want a
more substantial diode than the 1N5711, you can use 1N4148 diodes.

For the future, disconnect the antenna when not in use.  The most likely
cause of those diode failures is a static surge.  Even in the winter
months, snow/rain/wind static can build up a substantial charge even in
the absence of lightning.

73,
Don W3FPR

Kirk Brown wrote:
> After six months of operating, my k2/100 shows high swr/hi curr when
> connected to the dummy load. Running in battery/qrp mode, everything is
> fine, so I suspected something in the PA. I checked R26/R27 and they showed
> about 25k ohms each which seems a bit low (the alignment step calls for 43k
> ohms). I looked at the schematic and decided to test D16 and D17. D16 shows
> 0.057 volts both ways and D17 shows about 0.3 volts. Is D16 shot?
> Thanks,
>
>  

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[David Cutter]

Some radios seem more susceptible than others.  When a newly licensed friend
got arcing and shocks from a Z match feeding a low 210ft doublet, I fitted
2k2 carbon resistors from each feeder to ground.

David
G3UNA

> For the future, disconnect the antenna when not in use.  The most likely
> cause of those diode failures is a static surge.  Even in the winter
> months, snow/rain/wind static can build up a substantial charge even in
> the absence of lightning.
>
> 73,
> Don W3FPR
>
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