----- Original Message -----
From: "Colin Lamb" <k7fm@teleport.com>
To: "rick darwicki" <n6pe@yahoo.com>; "AMPS" <amps@contesting.com>
Sent: Wednesday, December 28, 2011 3:18 PM
Subject: Re: [Amps] Alpha low drive mod resistors
> "Doing a low drive mod on Alpha 76CA. Saw five 470 ohm 2W composition
> carbon as recommended, hard to find. Will modern carbon film or metal film
> resistors work or are the inductive enough to screw things up?"
>
> Carbon comp resistors are not good long term choices. They change
> resistance just sitting on the shelf (years), absorb moisture and more
> importantly change resistance when they are run near their rating. Many
> years ago, I wanted to use some 2 watts resistors in a commercial product
> and made tests to see if I could "age" them to become more stable. In
> open air, I took 2 watt resistors and ran them at 2 watts for 1 minute,
> then cooled them down. The resistance changed permanently. So, I ran the
> same test again, with the resistor changing to a new value after it had
> cooled down. I kept running the test and the resistor kept changing
> value, although at some point it started going in the opposite direction.
>
> Many of the resistors made today have the resistance on the surface and
> material is removed in a spiral manner, so they can have inductance.
> However, it is often very slight and will vary depending upon value. The
> manufacturer will have the information available.
>
> In the value you need for the Alpha, you can make up the required assembly
> then check it on something like the MFJ vhf bridge. See if the resistance
> changes with frequency. A network analyzer or more sophisticated device
> is better, but may not be necessary.
>
> 73, Colin K7FM
>
Ohmite OY series are far superior and available at Mouser.
Carl
KM1H
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