>Rich says:
>
>>To find out if it was a parasite, measure the value of the VHF
>>parasitic >suppressor resistors. Much increased ohms with no outward
>>sign of damage >indicates VHF energy payed a visit.
>
>This leads to a question: Why is there no outward sign of damage? Is it
>a pulse type failure, where the energy is large enough to alter/destroy
>the carbon comp, but nor enough to cause heating of the case?
so it seems. Most VHF parasitics appear to be quite brief.
>
>Is it something to do with current concentration being different indifferent
>parts of the resistor?
I doubt that the current could vary. It is a series circuit -- i.e., the
carbon granules are in series with each other.
>Is the effect the same on all types of resistor?
No - only in carbon-comp. resistors.
>It just seems an 'unusual' failure mode - but there may well be a very
>good explanation.
The resistor is virtually shorted out by a short length of copper
bus-wire, whose inductance is typically 0.09uH. HF energy is not capable
of developing much voltage drop across the suppressor inductor. VHF
energy is.
Rich...
R. L. Measures, 805-386-3734, AG6K
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