Tubes Asylum

Posted by Triode_Kingdom (A) on March 09, 2015 at 11:15:53

In Reply to: RE: Dynaco ST70 OPT failed secondary? posted by Tom Bavis on March 09, 2015 at 06:34:39

"it will be VERY difficult to discern if part of a parallel winding is open, unless you have a four-terminal ohmmeter."

This can be accomplished with less than $5 in parts if the OP has a DVM. A LM317 set as a 100mA current source can be used to create a known current through the resistance to be tested. A DVM can then be used to measure the voltage across the resistance *inside* the applied current. If the DVM has a 1.999V scale, it's possible to measure down to 0.01 ohm. In the example of the 4 ohm output you mentioned, I assume each winding has a DCR of 0.76 Ohm. That's easily measurable using this technique. I know this is a bit of trouble, but it's a heck of a lot cheaper than buying an OPT unnecessarily.



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• Simple Low Ohms Measurement   (Open in New Window)