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Thread: Ampeg AX-44C Hum

  1. #1
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    Ampeg AX-44C Hum

    Amp has a hum. Sounds exactly like an open input jack. As if the tip of the Jack is not making good contact with the shorting mechanism. I can't seem to track it down and it's rather mind boggling because turning the Ch A level volume DOWN to zero makes it worse. In fact, when on Ch A, lots of hum at zero volume, little to no hum above zero.

    I think this is a tricky grounding issue. I don't know.

    When on Ch B the hum is louder, and the level knob on Ch A still has the same effect. However it causes hum at zero AND when at 11.

    10v bipolar opamp supply is good. The hum is the same with and without the 12AX7 installed.

    I replaced IC2 because the hum is in both channels and ground pin7 kills the hum. Didn't help.
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  2. #2
    Lifetime Member Enzo's Avatar
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    10v supply is "good"? So zero ripple on each? How about the 15v supplies, how much ripple on each?

    You have a bunch of JFETs that serve channel switching related mute and enable functions. My suspicions point to them. Note the B and A volume controls each have one. That might explain why the other channel control still has effect.

    Also, my general practice is to go down the row of ICs. I look for any DC offset at each output pin.
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  3. #3
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    Ok thanks.

    I never have any luck measuring ripple with my meter. Either that or I've never actually had ripple as an issue.

    There was 1.5v offset on U2 pin 7 before replacing it. 350mv AFTER replacing it. Is that still too much?

  4. #4
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    It's 390mv offset on pin 7. +16mv on 5 and +17mv on 6. No ripple...says my meter.

  5. #5
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    Rails are +10.28 and -10.33 on opamps. Having my phone anywhere near this amp causes lots of interference. Really seems like something isn't ground or a connection is unterminated.

    Inserting a plug into the insert kills the hum.

    +/-15v are actually 17v.

    Enzo how do you test jfets? Just like a bipolar?
    Last edited by lowell; 02-13-2018 at 11:59 PM.

  6. #6
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    Quote Originally Posted by lowell View Post
    Rails are +10.28 and -10.33 on opamps. Having my phone anywhere near this amp causes lots of interference. Really seems like something isn't ground or a connection is unterminated.

    Inserting a plug into the insert kills the hum.

    Enzo how do you test jfets?
    With the amp out of the cabinet or in the cabinet is the interference?
    out of the cabinet you may be missing some shielding.
    nosaj

  7. #7
    Lifetime Member Enzo's Avatar
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    Phones are terribly noisy, keep them away from your amps.

    1.5v offset is off to me, the few millivolts after is OK to me.

    If your meter is unsure, use your scope, that is what it is for.

    I don't see ripple readings on the 15v rails.

    JFETs? I understand them as a low resistance path with a separate gate to turn them OFF. So unpowered, I expect something like maybe 50-200 ohms source to drain. You can measure for that in the circuit. If the gate shorts to the S-D path, any control voltage on that gate can get onto the signal path, so I look for voltages on the source or drain that don;t belong. When the gate voltage is present, they should turn off and be essentially open.
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  8. #8
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    Very odd to me that the schematic rotates 1.5vdc on pin7 as TP6. Odd furthermore that before replacing the opamp... it WAS 1.5vdc but is now 390mv on both channels.

    Ripple on main 17v rails is 1v p-p.

    Grounding the inputs to U2 doesn't help. Ground output pin 1 helps alot.

    So I'm pretty stuck. The source seems to be inside the opamp... which has been replaced.

  9. #9
    Lifetime Member Enzo's Avatar
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    Apologies, if the drawing says 1.5v, then it might be correct. But I see nothing in the schematic to suggest there would be any offset.

    What does grounding TP4 or 5 do?
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  10. #10
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    Grounding TP 4/5 has no effect on the hum.

    Just a reminder that the hum is there on Ch A with volume level at 0 but far less throughout all of the sweep. And also very little hum even up on 11. That's on Ch A.

    On Ch B the Ch A pot has the same effect on hum EXCEPT there is alot of hum at 0 AND 11... very little throughout most of the sweep.

    This is with every other pot on the entire amp set to 0.

  11. #11
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    Ok...ok...ok. I shoulda been suspicious of this from the get go. There is a wire from the PCB input Jack ground pad to a wire nut that is screwed to the chassis along with the pcb. Removing that chassis connection kills the hum! And the amp soumds great. Whoever was in here last... and there was someone in here that did a not so bang up job replacing filter caps, added this unnecessary connection causing either a ground loop or shared current with a different ground (common) circuit. The schematic has a chassis common, signal common, and dirty common.

    Amazing what one displacement of a common connection will do!
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  12. #12
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    And yes... still 390mv on U2 pin7. Both channels.

    Amp sounds awesome.

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