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Old 16th March 2013, 10:09 PM   #31
ultra is offline ultra  Greece
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yes i did
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Old 16th March 2013, 10:24 PM   #32
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There isn't much in the amplifier that can cause a hum. That's typically caused by something connected to the mains. Are you sure that your signal source is not producing the hum?
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Old 16th March 2013, 10:32 PM   #33
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I get the same noise even when the input RCAs are shorted to ground.
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Old 16th March 2013, 10:36 PM   #34
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Is the hum a 50/60Hz hum?

Have you tried a different signal source?
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Old 16th March 2013, 10:46 PM   #35
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no, its a higher frequency noise.
it sounds like you hold a microphone and the wind is blowing.
shouldnt the amp be dead silent when i ground the RCAs?

i will try with a different source though
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Old 19th March 2013, 09:28 PM   #36
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nothing changes with a different source.
i replaced all 8 op amps in the preamp section but i still dont get any output, only the loud hum.
Here is what i get at U300

pin 1 1.6
pin 2 0.01
pin 3 0.06
pin 4 -8.88
pin 5 1.54
pin 6 1.54
pin 7 1.54
pin 8 6.00

if i lift R300 and i inject signal to one end, i do get some signal on the output, but the hum is still loud.
However i noticed that the output swings between -2V to 0VDC.
First pic is what the signal looks like at Pin7 of U301
and the second one at pin2 of U401 (it is actually before and after R309).
Exactly the same applies to pin 7 of U302 and pin 2 of U400.
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Old 19th March 2013, 09:48 PM   #37
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Anyone feeling generous enough to send me a copy of the schematic to follow with?

linear8002@hotmail.com

Thanks!
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Old 19th March 2013, 09:58 PM   #38
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Received, thank you.
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Old 19th March 2013, 10:14 PM   #39
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Did you ever find the break in the feedback circuit? Because you said you checked traces, BUT the BDOUT is tapped BEFORE 'L400' so if you hardwired the connection on THAT side of the choke and there is still a break in the choke that output will never couple with the other phase which has it's own respective ties to the feedback system.

Perry is right, you can't ignore that break. I would check L400 to make sure NONE of those wires are snapped near the PCB, i've seen that on a few of these. The breaks like to hide sometimes under the glue too.
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Old 19th March 2013, 10:22 PM   #40
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yes i have found the break, repaired it and the feedback now is as it should be.
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