Source for replacement resisters

[Davegmc]

Hello: I am attempting to repair a 1970s vintage Micronta analog multitester. (I don't know why, something to do I guess). I believe I need a 8.5 ohm resister (colors are gray, green, black, silver, and red). I un-soldered the blown resister and it was printed 8.5 ohms on the circuit board. But I can't find any sources for a resister like this. Can anyone point me to a source? Thank you very much!

Dave

 

[Bluejets]

You could use 2 metal film resistors.

8.2 ohm and 0.3 ohm in series

or if low wattage a preset pot and adjust to required level.

https://www.jameco.com/z/64Y100-329...et-Potentiometer-25-Turn-1-2-Watt_240338.html

 

[(*steve*)]

Digikey as 16.9Ω 1% resistors. You can place two of these in parallel to get 8.45Ω.

You can also use this site to calculate resistors that can be placed in series and parallel to get a specific value.

That tells you that a 16.5Ω and a 17.4Ω resistor in parallel will yield 8.469Ω, a value about 0.4% low. With 1% resistors, you are still likely to have a value within close to 1% of the required value. Once you allow for resistance in switches and the vagaries of reading an analog meter, that's probably close enough.

 

[(*steve*)]

If you want higher accuracy, start with an 8.66Ω resistor, measure it's actual resistance accurately and calculate the required parallel resistance to bring it down to 8.5Ω

The benefit of this method is that the tolerance of the much higher value parallel resistor will have relatively little effect on the total resistance.

As an example, suppose the actual resistance was 8.635Ω. This requires a 543.7Ω resistor in series. Let's assume we have a 549Ω 1% resistor (this is an E96 value). It could be between about 543Ω and 555Ω. The worst value would be 555Ω, and even at this value the total resistance would be 8.5027Ω, a 0.03% error, a difference likely to be swamped by lead resistance and the resistance of the solder joint!

Let's assume that all we had was a 5% 560Ω resistor. At it's maximum likely value of 588Ω, the combined resistance would be 8.5100Ω, still only a tad over 0.1% high.