[R-390] Resistors

[Glenn Little WB4UIV]

Check any "new" carbon composition resistors for value before using them.
Carbon composition resistors are made of carbon powder and a composite
binding agent.
This binding agent might be hydroscopic (all that I have seen was).
If there are cracks or porous flaws in the encapsulation, the binding
agent will absorb moisture and expand.
The value of the resistor was determined by the amount of carbon in the
binding agent and the compression created by the encapsulation.
If the binding agent swells, the carbon powder will be less compressed
and the resistor value goes up.
You might be able to restore the resistor to its original value (for a
finite period of time) by heating it to drive off the moisture.
This is the failure mode as I understand it.
I am open to corrections.
All carbon composition resistors that I measure are high in value.
Some brands higher than others.
Fortunately, the resistor value is not extremely critical in our older
receivers.
There are ceramic composition resistors that may work.
We used these in surge suppressors where we originally used carbon
composition resistors for their surge handling capabilities.
If we used carbon or metal film resistors in this application we had
issues with inductance and the inability of the thin film being able to
absorb and dissipate surges without opening.


73
Glenn
WB4UIV

On 9/24/2017 9:07 PM, Stan Gammons wrote:
> On 09/24/2017 07:38 PM, Bob kb8tq wrote:
>> Hi
>>
>> Metal films are fine in the audio, IF, and bias sections of the radio. 
>> If you get into a resistor that is in the RF path, use a carbon comp 
>> if that what was there originally. They are a bit weird in their RF 
>> performance and it’s hard to duplicate their characteristics.
>>
>> Bob
>>
>>
> 
> Hi Bob,
> 
> Ok.  Sounds like the Ohmite carbon comp are what I should use in RF if 
> any need replacing.
> 
> Thanks!
> 
> 
> 73
> 
> Stan
> KM4HQE
> 
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Glenn Little                ARRL Technical Specialist   QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv at arrl.net    AMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)  USSVI LM   NRA LM   SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
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