On 7/19/11 2:48 PM, Mark Ketchell wrote:
> Okay, I'll ask too...
>
> The 20 meter section of my ICE 419b failed - infinite SWR.
>
> I removed cover from relay and polished contacts, eliminating it as the
> possible failure point.
>
> Inductors - still same size and shape, can't be them.
>
> Pulled the 3 caps and took to elec supply store. All 3 checked good on
> their tester (had them check twice). Took to friend with HP meter. The two
> shunt caps (.001 @ 2kv) each checked fine - the 270 pf (+/- 5%) checked as
> 251 pf - about 7% off. Would have thought it was close enough to find a
> resonate point, but it would not.
Frequency goes as 1/sqrt(LC) so a 7% error in C is about a 3% error in
frequency. 3% at 14MHz is about 500kHz..
That is enough to screw up a filter.
>
> I simply replaced all three and it works just like new.
>
> Why would the caps check as okay, but be absolutely worthless in the
> circuit? IF it is the series cap (270pf at 1kv) that fails, then why
> doesn't it ever seem to take the .05 @ 200v series cap right at the antenna
> port?
Voltages and currents inside a filter can be substantially higher
(resonant rise) than you'd expect from the steady state power flow. If
the Q is 100, then there's 100 times as much energy circulating inside
the resonant elements as flowing through.. 10 times the voltage/ 10
times the current.
If not the series cap, how does a 100 watt radio generate enough
> voltage to cause a 2kv cap to fail? There are resonate auto-switched
> antenna on each band, so it "shouldn't" see high SWR.
>
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