[Amps] Alpha low drive mod resistors

[Carl]

----- Original Message ----- 
From: "Colin Lamb" <k7fm at teleport.com>
To: "rick darwicki" <n6pe at yahoo.com>; "AMPS" <amps at 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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