Help With 60Hz Hum PLEASE

Putting my previous speculation in more general terms, I suspect basically that the grounding approaches in the two components, possibly in combination with some condition-related issue (such as a leaky line filter capacitor), somehow caused an interaction between the two components resulting in small (and perhaps expectable) amounts of AC leakage to chassis in one of the components (perhaps occurring via stray capacitances in a power transformer or via a leaky line filter capacitor) to be seen by the amp as signal.

That was my thought several posts ago. But then if that was the case Joe would have heard a 120Hz hum wouldn’t he have? And connecting the signals grounds of the Adcom to the chassis of the Marantz would not have stopped the hum, imo.

I think your response in your previous post is more likely the reason for the hum/buzz problem and the fix Joe came up with. The only thing I might add is we don’t know if Joe tried every separate channel of the Adcom to see if every one had the hum when connected to the Marantz.

Joe bought the Adcom used in a non working condition. If I remember Joe said the whole Adcom amp was dead. He had the amp repaired and the repair tech told Joe the center channel was the problem. Joe remembers the tech said he replaced the bridge rectifier in the power supply of the center channel.

We really don’t know the full extent of the damage to the center channel. We do know all of the 5 channels of the amp are fed by a common power transformer. Just a guess the center channel fault event caused the primary of the power transformer to overload and caused the overload protection of the amp to open.

I would like to know if any of the thermistors were damaged or blown open by the center channel fault event.

Al said:
Although I’ll add that if the modules in the Adcom conform to the photo you supplied (as opposed to the schematic), as we said earlier it would appear that the Adcom’s analog ground (which is common with the ground sleeves of the RCA connectors) is connected to XLR pin 1 through a thermistor. And if that is the case then the solution Joe arrived at resulted in a reduction of the impedance between the Adcom’s analog ground and the chassis of the Marantz Pre/Pro (which you had indicated is common with the signal ground of the Marantz), from whatever impedance the thermistor has during normal operation to essentially zero. The introduction of that more direct connection between the signal grounds of the two units would seem consistent with a reduction or elimination of hum, that might have been caused by small amounts of AC leakage to chassis in either component.

So I guess the only way to find out for sure is for Joe to pull the Adcom back out of the rack and check for the thermistor in position R47 on each of the 5 channels and check if they show conductivity.

LOL, good luck with that happening anytime soon.

Cheers,

Jim