[R-390] Gain drift

[Bob kb8tq]

Hi

MFP is not a magic material. It will slow down moisture (and air) it will by no means totally block either one. 
There are plenty of examples of carbon comp resistors heading off for who knows where value wise in 
military / MFP coated gear.

They *do* have odd properties at HF which make replacing them with other resistors a bit tricky. Rather 
that going capacitive as frequency goes up, they stay resistive. Their value drops, but the device stays
resistive ( as opposed to an R parallel C combo …). When used as a Q lowering device this subtle difference 
does come into play. 

Bob

> On Oct 21, 2018, at 5:32 PM, Jacques Fortin <jacques.f at videotron.ca> wrote:
> 
> Hi Ross,
> 
> As the internals of the R-390A IF transformers have been MFP coated (at least in the ones I have) I do not believe that the ambient oxygen had a chance to go thru the carbon composition resistors cases used within.
> If the MFP coating is still intact, the resistors should still have the same value than when they were manufactured.
> And they are Allen-Bradley parts, known to resist "aging" better than similar parts from other manufacturers.
> But... in case of doubt, CAREFULLY remove the original resistors and replace those by the variant of your choice.
> 
> Audiophiles pretend that the carbon composition resistors are more noisy than other types of construction, and it is true to a point. 
> When DC current passes thru those, the path followed by the electrons within is not always the same, causing a type of "partition noise" at the microvolt level.
> The carbon film and metal film variants, on which less "mass" of resistive material is involved, creates less of this "partition noise" for the same current value going thru.
> 
> For the "inductive effect" of the spiraling of the carbon/metal film resistors tracks, some testing is in order, especially at very high frequencies.
> However, the tests I already performed on low-value 1W parts proved that the dominant factor is the inductance of the connecting wires, and way much than the resistive element within.
> I do not believe that bad behavior can be created by using replacement carbon/metal film resistors within a R-390A.
> 
> 73, Jacques, VE2JFE
> 
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