[R-390] Resistors

[Bob kb8tq]

Hi

The “problem” with carbon comps at RF is that they don’t really go inductive or capacitive. The 
physics of the resistor is such that they stay resistive, but drop in value. The effect has been 
known at least since the 1940’s. It certainly was well known by the time the R-390 was designed. 
The effect is similar to stray capacitance (since internal capacitance is what causes it). Large
values are impacted at lower frequencies compared to small values. 

Since carbon comps for “fancy stuff” pretty much died out by the 1970’s, digging into them also
died out. This sort of stuff has been “nerd trivia” for a *long* time. I just happened to have a prof
in school (back in the 70’s)  who was a resistor nerd ….  You would have to dig into the IRE 
proceedings (1947 maybe?) to find the papers.

Bob


> On Sep 25, 2017, at 1:19 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Stan wrote:
> 
>> I was wondering what the group recommends for
>> replacements for the carbon resistors?  I've found
>> a few in the AF deck that are out of tolerance and
>> wondered if I should go with the Ohmite OY series,
>> metal film or ?
> 
> No doubt you will receive a flurry of replies saying that metal film resistors cannot be used at RF because of their high inductance.  That is by and large a false myth.  I have measured thousands of metal film resistors, both through-hole and surface-mount, with values from fractional ohms to a few hundred megohms, and found that most can be used well into the GHz range with no problem (the exceptions are some very high-value resistors, in the tens to hundreds of megohms range).
> 
> Zack Lau (now W1VT), ARRL's Senior Lab Engineer for many years now, wrote on the subject long ago in "Lab Notes" (QST):
> 
> "Q: How does the inductance of metal-film
> resistors compare with that of carbon
> composition resistors?
> 
> A: The metal-film resistors made today
> seem to be quite low in inductance, and
> are comparable to carbon types. I've used
> them well into the VHF range with little
> difficulty. However, these should not be
> confused with wirewound resistors, which
> are probably too inductive even in the
> MF/HF spectrum."
> 
> <http://www.arrl.org/files/file/Technology/tis/info/pdf/9208066.pdf>
> 
> That does not necessarily mean you should replace all carbon comp resistors in a 390/390A with metal film resistors.  Carbon comps are among the most rugged resistors available, and metal films are more fragile with respect to damage from overload (i.e., overcurrent and over-dissipation, particularly surge currents).  This is not a huge issue in most 390/390A circuits, and even where there is a potential risk (power supply decoupling resistors and other high-current, high-dissipation locations), it can be mitigated by using MFs with twice the dissipation rating of the CCs being replaced.
> 
> Best regards,
> 
> Charles
> 
> 
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