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I have a Music Man HD130 212 combo (with reverb and tremolo) on the bench with a weird problem. The transistor driver stage has a nasty ultrasonic oscillation. It happens whether the output stage is in standby or fully on -- no difference. So it's not coming from the NFB line or from the power tubes, it's contained within the driver stage. Everything before the master volume is fine, totally clean.
If I pull the opamp OR the non-inverting-side transistor (Q2), the oscillation stops. Q2's output is in-phase with the input to the opamp, so it makes sense that that's the source of the oscillation. But there's no solder bridge or any leakage path that I can find.
Obviously something blew here before, because the two TO220 transistors were replaced with NTE291s and the 2-channel opamp was replaced with an NTE778A. I'm not thrilled about there being NTE stuff in here, but all three components test ok. I tried a few other opamps and the results were the same. I don't have a compatible TO220 NPN on hand here to try in place of these two NTEs.
All electrolytic caps in the amp are new, replaced before I saw the amp. All resistors in the stage measure within spec.
Is this just the NTE curse or should I be looking somewhere else? I don't want to order some 2N6488s and wait five days just to find out that it doesn't help (though I should probably get some anyway).
If I pull the opamp OR the non-inverting-side transistor (Q2), the oscillation stops. Q2's output is in-phase with the input to the opamp, so it makes sense that that's the source of the oscillation. But there's no solder bridge or any leakage path that I can find.
Obviously something blew here before, because the two TO220 transistors were replaced with NTE291s and the 2-channel opamp was replaced with an NTE778A. I'm not thrilled about there being NTE stuff in here, but all three components test ok. I tried a few other opamps and the results were the same. I don't have a compatible TO220 NPN on hand here to try in place of these two NTEs.
All electrolytic caps in the amp are new, replaced before I saw the amp. All resistors in the stage measure within spec.
Is this just the NTE curse or should I be looking somewhere else? I don't want to order some 2N6488s and wait five days just to find out that it doesn't help (though I should probably get some anyway).
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