Greetings all.
I've been restoring a NAD T753 as another learning project. Initially, I had a power issue regarding the relays. There was a diode that was covered in the RTV/Hot glue stuff they were using at the time which was getting shorted to adjacent components that I fixed. After this, I noticed an intermittent hum/buzz reminiscent of a ground loop hum, which is independent of volume changes. I changed the cable to a non-grounded one and removed all peripherals, but the hum still persisted. Eventually I got the idea to disconnect one of the four fans underneath the heatsink. The hum stopped. I tried disconnecting a different fan, it stopped again. so it seems disconnecting any one of the four will cause the hum to reappear. As i'm still learning, i'd like some input as to how to hunt the cause. I'm thinking either the capacitors 875/874 would be something to check first, though I doubt both would be bad at the same time and there are no visible signs. Next, considering the fans are always on, I don't think this should be and maybe the thermistor is causing an issue. Normally these thermistors would only cause fan activation as temperature increases, but would this component being faulty cause the hum? Somehow I don't see this happening as if it was just locked in the on position, it would have a higher resistance to ground and not be able to cause a loop between two ground points. What are your thoughts on where to look next? I will be disassembling again to check the thermistor anyway to see why the fans are always running, but other than that i'm unsure how to test.
I've been restoring a NAD T753 as another learning project. Initially, I had a power issue regarding the relays. There was a diode that was covered in the RTV/Hot glue stuff they were using at the time which was getting shorted to adjacent components that I fixed. After this, I noticed an intermittent hum/buzz reminiscent of a ground loop hum, which is independent of volume changes. I changed the cable to a non-grounded one and removed all peripherals, but the hum still persisted. Eventually I got the idea to disconnect one of the four fans underneath the heatsink. The hum stopped. I tried disconnecting a different fan, it stopped again. so it seems disconnecting any one of the four will cause the hum to reappear. As i'm still learning, i'd like some input as to how to hunt the cause. I'm thinking either the capacitors 875/874 would be something to check first, though I doubt both would be bad at the same time and there are no visible signs. Next, considering the fans are always on, I don't think this should be and maybe the thermistor is causing an issue. Normally these thermistors would only cause fan activation as temperature increases, but would this component being faulty cause the hum? Somehow I don't see this happening as if it was just locked in the on position, it would have a higher resistance to ground and not be able to cause a loop between two ground points. What are your thoughts on where to look next? I will be disassembling again to check the thermistor anyway to see why the fans are always running, but other than that i'm unsure how to test.
