[R-390] Electronic question

[Bob Camp]

Hi,

This gets into the "strange but true" section of electronic components.
Carbon comp resistors behave in an odd way at RF. You would expect them to
get inductive or capacitive as frequency goes up but they don't. Oddly
enough they simply get lower in value as frequency rises up to a couple
hundred MHz. The data I have seen was from the 1940's and applied to values
above a couple of hundred ohms.

It's been about 30 years since I read the paper but as I recall the drop off
was at a different rate for different sized resistors. The net effect -
replace carbon comps with carbon comps if there is RF on them. If it's a DC
circuit then go for something that is less humidity sensitive.

    Take Care!

        Bob Camp
        KB8TQ



----- Original Message -----
From: "Kenneth Crips" <w7itc@hotmail.com>
To: <r-390@mailman.qth.net>
Sent: Saturday, August 31, 2002 9:26 PM
Subject: [R-390] Electronic question


> A question, I smoked four resistors in My TS520-SE.  My fault, I did an
> idiot job tuning it.  The resistors are on a circuit board controlling the
> grids, plates and modulation of the 6146B finals. no real harm, just have
to
> dig through the resistor stash and install another set. The resistors in
> question are all 4000 ohms 1/4 watt carbons. The four are installed in
> parallel sets, the rule as I understand it is
> resistors in parallel are always half the lowest resistance, in this case
it
> should be about 2000 ohms.  Why do it this way. Why not just
> use a single 2000 ohm resistor. The only reason I can think of is
> it is a size thing. The area where this circuit board is located is
> on the bottom below the finals and there isn't much room. What say
> you out there oh great Guru's of the Order of St Videcon.
>
>                                Ken
>
>
>
>
>
> Kenneth A. Crips W7ITC, Cheyenne, Wyoming
> Home of Chew Crew II, The Beezie the Beez-Bomb,
> Chic' MOB (mother of Beezie), and Cowboy Joe
> the Cheyenne Chewboy and the pard' of the yard.
>
>
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