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Shielded against 60Hz speaker cable
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Hextejas:

--- Quote from: Cliff Matthews on March 16, 2019, 03:52:57 pm ---You must have a volt-meter, since you've already rebuilt the amp 4 times?
1) Power-on both amps (minimum volume and nothing connected to inputs) measure and report DC and AC differences between both amps output terminals.

--- End quote ---
Please be aware that this is new to me so I might be doing things the wrong way. For task 1 I measured the voltages with the probes on the +(Red)  and -(Black) of the amp outputs which are what are connected to the speakers.
So:
Left DC =0
Left AC=0
Right DC=.003
Right AC=0


--- Quote from: Cliff Matthews on March 16, 2019, 03:52:57 pm ---2) Report any DC voltage present (however small) at the output terminals on each amp.

--- End quote ---
I am assuming that you want the voltages measured with a reference to chassis ground.
So:
All are 000 with the exception of the Right DC=.005


--- Quote from: Cliff Matthews on March 16, 2019, 03:52:57 pm ---3) Power-off both amps, disconnect from mains and wait a few minutes for amp PSU caps to discharge. Report resistance between positive and negative output terminals to chassis ground on each amp.

--- End quote ---
All readings are ohms
Left --- = 0k
Left +++ = 41.3k
Right --- = 0k
Right +++ = 41.7k


--- Quote from: Cliff Matthews on March 16, 2019, 03:52:57 pm ---4) Still disconnected from mains, report resistance from the chassis of each amp to its wall plug connector (to see if open).

--- End quote ---
No connections are open and they report a small ohm reading.

These readings might give you a clue but they are meaningless to me. However I wonder where the small DC voltage on the right channel is coming from.

I removed the sub woofer from the chain and it made no difference. I routed the cable many different ways and was able to get the hum to a minimum level though I hear a strange tic, tic, tic sound like the sound at the end of a record being played. There is also a faint signal from a radio station. Weird !
The problem I have with the present routing of the cable has to do with an unhappy spouse. Admittedly the routing is ugly.
Cliff Matthews:
All looks OK, but the presence of a UPS I did not know about.. some UPS's run a stepped square-wave through the output transformer even when idling, so this could be the source. Why it's even never near an audio system I cannot guess.
Hextejas:
Well, I saw that one of the input RCA plugs was slightly loose so I decided to fix it. Then I discovered that the grounding wire was not making contact with the chassis,  a poor wiring job. When I corrected that, the 60Hz hum pretty much disappeared..
So, I owe a hearty thanks to all that helped me and sorry for the shoddy job.
NiHaoMike:

--- Quote from: Hextejas on March 17, 2019, 03:26:04 pm ---Separate question. The UPS has a heavy battery in it. Can the battery contribute to the hum problem ?

--- End quote ---
Not the battery itself but I used to have a Belkin UPS that used a pulse charging circuit that really likes to induce noise into nearby analog audio amplifiers. It worked by shorting the low voltage side of the transformer for a brief moment and then letting the leakage inductance work as a boost converter. It doesn't help that it was a $10 or so special with a plastic case...
DimitriP:

--- Quote ---Then I discovered that the grounding wire was not making contact with the chassis,
--- End quote ---

It could've been a lot worse: Somone could have posted a part number for shielded speaker cables.
Be glad no one did :)
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