I have rebuilt a Threshold 4000. Everything has been changed on the front end and output boards. Basically, all that is original is the transformer, bridge switch, wiring and the meter board. Bias has been set. Reproduces music fine (good numbers when tested with HP 8903)
I'm getting a buzz (60 hz) which gets worse as I wiggle or move around the ground wires in the amp. It's the left channel but right will have some buzz too unless I remove the fuses from the left channel.
-I've checked all of the components on the left FEB, except resistors.
-Compared a working 4000 with the same circuitry design to make sure I didn't install something incorrectly.
-Tested with RCA shorting plugs, buzz gets better but still there.
Thinking it has something to do with a ground or a ground loop. I've checked all the grounds, resoldered ground connections, checked to make sure all grounds match those on the working amp. All looks good.
I'm about to start switching boards with the working amp to see if I can isolate the issue, but, before I have 2 amps in pieces, I thought I'd ask for some help!
Anyone have any ideas??
Thanks,
Eric M.
I'm getting a buzz (60 hz) which gets worse as I wiggle or move around the ground wires in the amp. It's the left channel but right will have some buzz too unless I remove the fuses from the left channel.
-I've checked all of the components on the left FEB, except resistors.
-Compared a working 4000 with the same circuitry design to make sure I didn't install something incorrectly.
-Tested with RCA shorting plugs, buzz gets better but still there.
Thinking it has something to do with a ground or a ground loop. I've checked all the grounds, resoldered ground connections, checked to make sure all grounds match those on the working amp. All looks good.
I'm about to start switching boards with the working amp to see if I can isolate the issue, but, before I have 2 amps in pieces, I thought I'd ask for some help!
Anyone have any ideas??
Thanks,
Eric M.
Hi, sounds like an earth loop as you say....I think people are going to want pictures to SEE what is going on 
Can you expand on this bit?......a bit 😉
Is it totally silent if you remove fuses from just the left channel?
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Yes, let me add some detail:
-If the fuses are out of the left channel, the right does not buzz, it works fine.
-If fuses are in the left, both channel buzz but the left is louder. (it's loud, like "I screwed up somewhere loud")
-The buzz sounds like 60 cycle hum (from what I recall from my Junior High electronics class) but measures 120 Hz with the 8903.
-If I wiggle the ground wires (especially the 2 that go to the left front end board) the buzz will bet louder of quieter. I can actually "strum" them like a guitar string and the buzzing will sound like a bass being played (does that make any sense?)
Thanks,
Eric
It goes to the main ground lug which is under the ground plate over the filter caps. I moved this ground lug to the top of the plate so I could better confirm the grounds were good and tight.
Black wires from RCA go to ground lug (probably what you are calling the Star at the caps).
These wires (not sure why there are 2?, I can't find a schematic), the screen of the input wire and the black wire from the RCA skt to the star are forming a loop.
I've also wondered why there are 2 ground wires going to the FEB board, but it's that way on all my 4000's and 400a's. I know if I remove the ground on the shielded (screened) cable, the buzz is even louder. I can try removing the 2 ground wires to the FEB but am concerned they are there for a reason and not a mistake.
The screen mustn't be removed. The wire from the RCA to gnd that Zen Mod noticed is the one to disconnect, from both skts
Ah, OK, that makes more sense than what I was thinking. I'll give that a try and post up.
Thanks,
Eric
I think you will find this useful, it's by member Bonsai http://hifisonix.com/wordpress/wp-c...re-up-a-Power-Amplifier_Updated-Autosaved.pdf
You're welcome. Proper grounding is a fascinating subject, with many variables and not always easy to solve......
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