I have a radio I believe may need a resistor replaced. Antique radio supply has all the values in this amp except the suspect one. It is a 3.9 ohm. The closest value to that they have is 3.3 or 4.7 ohm. Would either one of those work? What is the best way to check a resistor to see if it is bad?
John

I have a radio I believe may need a resistor replaced. Antique radio supply has all the values in this amp except the suspect one. It is a 3.9 ohm. The closest value to that they have is 3.3 or 4.7 ohm. Would either one of those work? What is the best way to check a resistor to see if it is bad?
John

with a ohm meter

The closest value to that they have is 3.3
that should be fine if its within 1percent as the original is probally 5 percent.btw you will have to clip one lead of the resistor in questionbefore you test it .

Actually 3.3 ohms is a little more than a 15% variation from a 3.9 ohm resistor.
A quick Google search shows that one of our sponsors, Parts Express, carries 1/2 & 1W flameproof and 5W wirewound 3.9 ohm resistors.

http://www.partsexpress.com/webpage.cfm?FILTER=3.9+ohm+resistor&FTR=3.9+ohm+resistor&search_type=main&WebPage_ID=3&x=11&y=11

oops sorry my maths a bit off tonight.

Most of those old resistors were 10% tolerance originally. Depending on the specific spot in the circuit, you may be able to vary this value pretty far without it mattering much. 3.9 ohms is awful low, whats it doing exactly? If you're not sure, maybe a schematic would be of assistance.

Also, unless the resistor is burned, or out of tolerance with the ohm meter, I wouldn't even bother with it. I actually use 20% as my personal guideline for replacement, and in many cases radios have been seen working with stuff way beyond 20%.