Seventh Circle N72: odd distortion on scope

[soapfoot]

Primary resistance is the same as a known good unit, nominally if not exactly.

Reducing signal generator output did indeed make the distortion go away.

 

[swpaskett]

So, that sounds like right transformer built on wrong core or defective core. Never seen that, but that's nothing new. I am still curious about what you had to do to the transformer. Nothing, in my experience, you could have done would have made a transformer behave this way, but then I have never seen a transformer behave like this. If you opened the case (which looks like it can be done) as part of your rework I would have ethical challenges asking for a warranty replacement, but if all you did was on the outside of the can, either 7th Circle or Carnhill owe you a replacement, IMO.
They don't tell us the core material. I have seen weird things with damaged ferrites (cracks or chips) but those cores are not used for audio, SFAIK.

 

[soapfoot]

All I did was desolder it from a PCB.

PCB had through-plated holes, so it was a little stubborn. I had to apply a bit more heat than I’d like, a few more times than I’d have liked, and had to apply gentle mechanical force a few more times than I may have preferred.

nothing more than that, though.

We’re going to see what Carnhill thinks is going on.

 

[moamps]

I think you should be 100% sure that your measurement method is correct before contacting the manufacturer. In your place, I would set the generator to 100mV RMS, 400Hz, confirm it by measuring with DMM, additionally confirm the perfect sine wave shape on the primary with an oscilloscope, set the input pot of the scope to cal !!! (how do you know what voltage you are measuring if the pot is not at the cal position?), then measure the voltage at the secondary, it should be twice as high, and confirm it with an oscilloscope.

 

[soapfoot]

Im

I think you should be 100% sure that your measurement method is correct before contacting the manufacturer. In your place, I would set the generator to 100mV RMS, 400Hz, confirm it by measuring with DMM, additionally confirm the perfect sine wave shape on the primary with an oscilloscope, set the input pot of the scope to cal !!! (how do you know what voltage you are measuring if the pot is not at the cal position?), then measure the voltage at the secondary, it should be twice as high, and confirm it with an oscilloscope.

Fair enough. I’m sending the faulty unit back to SCA, and they can do their own tests before contacting manufacturer.

They’ve been fantastic throughout this troubleshooting exercise

 

[soapfoot]

I have solved this little mystery. Rather predictably, it's mostly operator error.

Turns out the very-inexpensive pocket-sized signal generator I was using was showing a pretty serious DC offset of a few volts.

Once that issue was corrected, everything looks perfectly normal.

 

[JohnRoberts]

Its always the last thing you check....

JR