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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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