I tested, and the polarity is correct. Based on computer's UPS indicates, the ground wire has voltage present. My AC test pen also confirms that. I think the ground is just open, because I regularly touch my computer case and never got shock.
Even though, it is not related to my hum problem. My receiver and the st-70 are all 2 prong plug.
That sounds safe... How did you determine the "polarity" is correct with no ground reference at the outlet?
I did some test with different audio source with my dynaco st-70.
1. Costco Yamaha 7.1 audio receiver, HUM
2. Laptop with AC adapter plugged in to the wall, NO HUM.
3. Bluetooth receiver with USB power adapter plugged in to the wall, NO HUM.
Here is my observation.
When it hums, it is somehow ground looped without grounding any of my device. All of my devices used are just 2 prong plug. How? Here is my hypothesis. The big power transformer like the one in dynaco st-70 has some stray capacitance across primary and secondary. Thus, they can ground loop through the neutral and live rail in side the wall.
1. Costco Yamaha 7.1 audio receiver, HUM
2. Laptop with AC adapter plugged in to the wall, NO HUM.
3. Bluetooth receiver with USB power adapter plugged in to the wall, NO HUM.
Here is my observation.
When it hums, it is somehow ground looped without grounding any of my device. All of my devices used are just 2 prong plug. How? Here is my hypothesis. The big power transformer like the one in dynaco st-70 has some stray capacitance across primary and secondary. Thus, they can ground loop through the neutral and live rail in side the wall.
I stick my ac tester pen in the longer slot from the wall socket. It does not light up. It will light up in the shorter slot and the ground hole.
This sounds like the wall socket is incorrectly wired. Find and open the breaker for that outlet and observe the socket wiring by colors. If you can't see the error, it may be at another socket (typically in the same room, but nearby). Do not use this socket until the error is corrected.
Generally, you can trust the $10 plug-in three-light testers to tell you the truth, so one is a great investment.
Be safe, and all good fortune,
Chris
They will not detect an N/G swap. Or a bootleg ground.
i.e.: they will tell you an outlet is wrong, but can not prove it is right. ENTIRELY worth the $9. I have several. But if I need to know, I have an expert trace the wiring from the street to the listening room.
John at another forum worked on a smarter tester.
I want to report back that the hum is fixed.
It was a ground loop, through a "amplifier switch" box. I overlooked this box, which was really stupid mistake by me.
I use a "switch" box to connect my st-70 and the AV receiver to the same pair of speakers. However, the box shares the same ground among all the connections, I guess, to save the cost. I swapped out the box. The hum is fixed.
It was a ground loop, through a "amplifier switch" box. I overlooked this box, which was really stupid mistake by me.
I use a "switch" box to connect my st-70 and the AV receiver to the same pair of speakers. However, the box shares the same ground among all the connections, I guess, to save the cost. I swapped out the box. The hum is fixed.
