[TowerTalk] ICE 419B schematic

[Jim Lux]

On 7/19/11 3:15 PM, K1TTT wrote:
> Again, its probably not the voltage but the current that is the problem...
> See my last message.
>
> Also on mine the capacitors on 20m hadn't 'failed' they had shifted value...
> It was obvious to see using a minivna to sweep the filter that it was still
> there, it just wasn't resonating where it what supposed to any more.  In
> mine the 10m section had one of the shunt caps shorted, the others were ok,
> but I replaced them all anyway.  The 20m section had one of the shunt caps
> changed from 1000pf to 247pf on a handheld dmm, my mfj 269 reads it as 614pf
> at 2mhz but is highly frequency dependent where the good one is a stable
> 1040pf or so over a wide range.
>

Depending on the kind of capacitor.  A lot of film capacitors are 
essentially a bunch of smaller capacitors in parallel with what are 
effectively little "fuses" in series with each sub-capacitor.
The idea is that if the dielectric punctures or has a flaw, that segment 
will draw excessive current, blow the fuse, and the rest of the 
capacitor keeps on working.

Hammer the cap repetitively, and the capacitor value gradually decreases 
as the segments are "disconnected".

A similar mechanism is in play in ceramic doorknob caps.  The plates 
gradually demetallize, reducing the capacitance.