[AMPS] Burnt resistors

[Rich Measures]

>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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