Re: Resistor in series

Tom Lee

Hi Jeff,

You are correct that, depending on value, metal film resistors can have significant inductance (relative to the carbon comp version). In this particular circuit, though, you are looking at 7.5 kilohms of resistance in series with a negligible inductive reactance (I'd guess an inductance well below 100nH, so an inductive reactance of tens of ohms or so at the top end of the scope's bandwidth). So here, the choice is driven by power and capacitance (and maybe aesthetics). Inductance of the resistor is totally irrelevant.

I confess to a general prejudice against carbon comps (they're horribly noisy under bias and don't age gracefully), but they will work here, of course. Just pre-measure the resistance before installation. You might find that even NOS resistors have drifted significantly upward in value just sitting on a shelf.

-- Cheers,

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 11/25/2020 20:09, Jeff Dutky wrote:
So I feel like a superstitious/nostalgic idiot, but I had second thoughts about replacing the out-of-spec resistor pair with modern metal film resistors. I went to eBay and bought a dozen (allegedly) genuine Allen Bradley carbon composition 7.5 K Ohm 1/2 Watt resistors for what is probably a completely unjustifiable price. I also looked to see if anyone was selling FDH2161 diodes, but no luck on that score.

My logic, such as it is, is pretty simple: a carbon composition resistor is going to have basically zero inductance, and I wasn't sure that a metal film resistor would NOT have some inductance (my understanding is that the film resistors have a spiral cut through the film to increase the effective length of the resistive element; that should create some significant inductance, right?). My illogic is that I will be replacing the existing resistors with cosmetically identical parts (yes, in a place where nobody will ever see them, but I will know, and that will be enough. A person should take pride in their work even if nobody else ever sees it).

Anyhow, is this pure foolishness, or is this foolishness justifiable?

Also, I went over the parts of the board that looked to have been reworked and cleaned things up with a cotton swab and IPA. It looks a lot better now, and I was also able to determine that there were NOT any solder bridges shorting things that shouldn't be shorted.

-- Jeff Dutky

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