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In Reply to: RE: Dynaco ST70 OPT failed secondary? posted by deafbykhorns on March 08, 2015 at 17:10:36
Would seem impossible since it's heavy gauge wire - the solder joints may be a weak link, though. If so, the other taps may be OK. I HAVE heard of primary failures - with a section having high resistance. Primary is two windings in parallel. Secondary may be as well - it will be VERY difficult to discern if part of a parallel winding is open, unless you have a four-terminal ohmmeter. The 4 Ohm winding, for instance, has a DCR of about 0.38 Ohms.
Follow Ups:
and got the same readings above. The transformer in question measured a DCR of .2 ohms at the 8 ohm and 4 ohm tap. I'm getting a signal at each screen tap of the EL34's on the scope.
The EL34 gets extremely hot in a very short amount of time which tells me something is shorted and pulling excessive current. I haven't isolated the transformer out of the circuit to test but primary might be partially shorted as well since I see the cap charging(tubes out) across the plate windings. It doesn't do this on the good transformer.
The owner of the amp attempted triode wiring and had the screens crossed up between the tubes. Would this damage the transformer?
"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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Buy Chinese. Bury freedom.
I normally use my ESR meter, but I can't find the damn thing.
I'm sure there's a way to Spice this circuit
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