A12 Hi freq problem

[mbrebes]

As you remeber, after discussing the high frequency ringing on the output, it was decided that it was the SC25 opamp and you sent me another one. Installed it and the same thing! Started doing some troubleshooting and found that the phase switch seemed to affect the ringing. Saw where it was in the schematic and decided to remove the output transformer by removing the L1/L2 jumpers. The ringing went away. It's starting to look like the transformer is bad. I am going to pull the phase switch and the J5 jumper, when I can, to see it anything there (that I can't see) could be causing the failure. Any other suggestions.
Looks like that first opamp might be okay.

[tpryan]

That's interesting, because I couldn't duplicate your symptom with the SC25 you sent back. Let us know what you find, we have plenty of transformers.

[mbrebes]

Checked the phase switch and it is okay. Pulled termination jumper and no change. Open connection on input of transformer shows opamp okay. Open connection on output of transformer shows hi freq ringing still there. Looks like it has to be the transformer input coil since there is nothing else there. What do I do next? Return transformer?
This is definitely a strange one. Never seen anything quite like it before.

[tpryan]

I could exchange the transformer, or you could mail the whole board back. I haven't seen one exhibit ringing like you describe, but we have encountered a few shorted transformer primaries. I'm curious to see it. Your choice.

[mbrebes]

Since it will probably take the same amount of time, and because you seem as intrigued as I about the failure, I'll send you the complete board.

[mbrebes]

Circuit board is on its way.

[mbrebes]

Plug it in and see the oscillation yet?