SCI Pro One buzzing on output

[WhinyLittleRunt]

What are some troubleshooting steps to figuring out why the audio output of the Pro One is buzzing/humming really loud? It wasn't always doing it; it started happening fairly recently. Output jack ground problem maybe?

[cornutt]

Often, when any audio device has hum or buzzing in the output, it's caused by a filter cap in the power supply going bad. Put a scope on the power supply outputs and look for ripple. If you see some, have a close look at the filter caps. You may see one or more that are bulging or leaking; if so, those are obviously bad. Actually, given the age of this synth, I'd probably just change all of the caps anyway. The other thing that can cause ripple in a power supply is an open diode in the rectifier, but in my experience, the caps are more likely to fail than the diodes are.

[Solderman]

If it's a Rev 1.2, the transformer was moved from the PCB, but SCI stupidly attached the center tap to the chassis,(ie. to the safety ground pin on a 3 prong power outlet) so you end up with two common lines the minute you connect it to a mixer. I used a 2 prong adapter and the hum went away.

If it just recently started doing it, and any of the bypass caps surrounding the 4 main voltage source lines were faulty, it would play havoc with everything, not just the audio signal. I'd check your output cable first, then maybe make sure the power transformer wiring is secure.

[WhinyLittleRunt]

If it's a Rev 1.2, the transformer was moved from the PCB, but SCI stupidly attached the center tap to the chassis,(ie. to the safety ground pin on a 3 prong power outlet) so you end up with two common lines the minute you connect it to a mixer. I used a 2 prong adapter and the hum went away.

If it just recently started doing it, and any of the bypass caps surrounding the 4 main voltage source lines were faulty, it would play havoc with everything, not just the audio signal. I'd check your output cable first, then maybe make sure the power transformer wiring is secure.

Good advice. I can tell you that the power cable (assuming this is what you meant by "output cable") is frayed at the chassis. But yes, it just started doing this.

When you say you used a two-prong adapter, do you mean you wired in a replacement power cable that didn't have the ground plug? Should I be looking into this as a replacement for the existing cable that's in need of replacement anyway?

[Solderman]

I just meant your standard 3 to 2 prong adapter for the power outlet.

By output cable, I actually meant audio out. I've seen it before, where a bad audio cable caused ground hum, although it wasn't all that loud.
If your power cable is frayed, you should get a tech to fix this asap. Without seeing it, I can only assume the worst in terms of missing insulation, shorts, etc. It sounds like this is the most likely suspect of the ground hum.

[WhinyLittleRunt]

Update:

After all that, it turned out the channel on the amp was flaky Not even going to get into it because what should have been obvious just wasn't...

But after having a good look at the power cable it definitely should be replaced to avoid any more fraying.