[R-390] Resistors

[Charles Steinmetz]

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