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Thread: Loud hum 100Hz

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    Loud hum 100Hz

    Hi, first time poster here, and actually had to search the difference in size between Neptune and Jupiter.

    So, I have this Engl Fireball 60 (e625) from the second hand market which is emitting a loud (like real loud) 100Hz hum. There is some guitar sound coming through however it's very bad sounding.
    I made a short video so you'd have an idea what's going on:

    https://photos.app.goo.gl/pYgBEjWKJXsxcsor9

    I swapped all preamp/power tubes for new ones and tried the fxloop send to another poweramp, the hum is still there but far less noticable, and the signal is ok.
    The fireball is diode rectified, and since the hum frequency is 100Hz I suppose there's no problem there.

    I ordered some new filtercaps, but I doubt the problem lays there, I did a little charge/discharge test with a MM, and when the amp is in service, there is 380volts over them.

    So I'm not sure what to do next. Ground loop perhaps?

    The scheme of the fireball:
    https://el34world.com/charts/Schemat...l_fireball.pdf

    Cheers

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    Last edited by InDecember; 06-02-2019 at 08:01 PM.

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    Stray Cap DrGonz78's Avatar
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    Well if it is 100hz hum I would check for AC ripple, suggesting a leaky mains filter cap. I would think a ground loop would be a 50hz hum. In the US we have different mains voltage so it is 120hz DC hum and 60hz AC hum.

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    Quote Originally Posted by DrGonz78 View Post
    Well if it is 100hz hum I would check for AC ripple, suggesting a leaky mains filter cap. I would think a ground loop would be a 50hz hum. In the US we have different mains voltage so it is 120hz DC hum and 60hz AC hum.
    Thanks, yes you're right about the 50hz ground loop. You mind sharing how and where I can measure AC ripple? I know a thing or two about electricity in general, but my amp expertise is zero. Also, I just have a digital multimeter here.

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    i suggest to change all filter caps anyway,they were not so good back then and they are already old,plus you don't want to change them one at a time in a pcb board made like that.
    Shotgun method,its not very expensive and will last much more,buy F&T.

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    Supporting Member Jazz P Bass's Avatar
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    "You mind sharing how and where I can measure AC ripple".
    You have already demonstrated that you can safely measure the DC voltage on the power supply nodes.
    So do the same except set your meter to read Volts AC.

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    Ok, so measuring AC voltage over the filtercaps? And it should read 0, if not they are faulty, is that correct?

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    Hi. We're having this same conversation over at TGP with the OP, BTW, FYI.

    No, AC ripple at the reservoir (first) filter cap will not be zero--you can expect up to 10vAC and still have normal operation.

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    Last edited by xtian; 06-02-2019 at 06:37 PM.

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    Quote Originally Posted by xtian View Post
    Hi. We're having this same conversation over at TAG with the OP, BTW, FYI.

    No, AC ripple at the reservoir (first) filter cap will not be zero--you can expect up to 10vDC and still have normal operation.
    You meant to say 10vAC?

    Thanks for elaborating on this forum too.

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    Senior Member Pedro Vecino's Avatar
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    Quote Originally Posted by InDecember View Post
    I ordered some new filtercaps, but I doubt the problem lays there, I did a little charge/discharge test with a MM, and when the amp is in service, there is 280volt over them.
    I dīont know if I have understood it well, but the voltage in the first and second filter capacitor should be between 425 and 440 volts approximately.
    Visually check if some of the surface mounted* ones have any brown crystallized trace on the positive terminal.

    (*) In other models two of them are mounted on the underside of the board.

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    Quote Originally Posted by Pedro Vecino View Post
    I dīont know if I have understood it well, but the voltage in the first and second filter capacitor should be between 425 and 440 volts approximately.
    Visually check if some of the surface mounted ones have any brown crystallized trace on the positive terminal.
    Oh sorry, that's a typo. I measured 380v not 280.

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    Yes, AC.

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    Old Timer J M Fahey's Avatar
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    I would troubleshoot a little before random replacing parts.

    1) I see power tubes glow bright purple when you switch amp ON, suspect some nasty LED illuminating them.
    Checked schematic and there they are :LD2 and LD3 . Oh well.
    Which makes me suspect your video shows the amp in the full tilt Ultra Gain mode.

    a) link/upload the User manual, this amplifier is funky.

    b) set amp to "normal", not "Ultra", all controls on zero, test and upload video.
    I want to check whether hum responds to controls or not

    c) pull Phase Inverter V4 and repeat tests ... any change in hum/buzz?

    Please post results before proceeding further.

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    Juan Manuel Fahey

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    Hi, yes the powertubes are illuminated by 2 leds. (They illuminate when power is on, its independent of channelchoice).

    I must say I did test it on both channels (clean and ultra gain) and there is no difference in the intensity or volume of the hum. Gain control (or any other) isnt affecting it either. I also tested with pulling v3 v4 v5 and v6 and again no change. The only thing that makes the hum even louder is increasing one of the 2 master volumes (I just went a quarter of the way up, don't dare to go higher).

    A link to the manual:
    https://englamps.de/wp-content/uploa...5_Fireball.pdf

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    Last edited by InDecember; 06-02-2019 at 07:55 PM.

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    Ok so I'm measuring 800 ACvolts over the first filtercap. I'm sure I'm doing something wrong here, or is this an indicator the cap went bad?

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    Probably.

    Had a Fender Deluxe Reverb reissue with a bad filter cap yesterday.

    Symptoms are as you mentioned: hum, lower DC voltage than normal and over 100volts AC at first filter node.

    Your filter is probably bad.

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    Old Timer olddawg's Avatar
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    A picture of the board would be nice. Frequently the filter cap leads become disconnected from the pads on the board after years of vibration. I would resolder everything in the power supply area first. A floating filter cap displays the same symptoms as a bad one.

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    Last edited by olddawg; 06-03-2019 at 04:00 AM.

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    Don't forget the joker g1's Avatar
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    I think your meter can not measure AC when DC is present (inexpensive meters often can't). You would need a cap in series with the positive probe, something like a .1uF 600V.
    But I also agree with the others that the main filter cap is probably bad. The DC voltage is too low like Pedro said, even 380V there is too low.

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    Old Timer J M Fahey's Avatar
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    Like G1 said, your meter can not measure AC if mixed with DC so you can not measure ripple (you still can measure mains and transformer windings though)
    So "800VAC" means nothing in this case.

    Please repeate 2 tests:

    1)
    c) pull Phase Inverter V4 and repeat tests ... any change in hum/buzz?
    2) with V4 pulled, measure VDC at +V , which is the positive end of the first filter cap.
    I am guessing itīs open/unsoldered/pad or track cracked/"it is not there", but guesses are checked by measurements.

    Hint: if a cap is open, a meter set to DC reads about the RMS voltage fed there by the transformer while if the cap is doing its duty it will read about the peak voltage.

    The second is about 1.4 times the first.

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    Juan Manuel Fahey

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    The new filtercaps came in today, I installed them and the hum is gone. This was prolly a standard repairjob so sorry if I made it look like a massive problem, it's because of my lack of knowledge of these amps. Anyway, I learned alot, things to keep in mind for the next time, thx for helping me troubleshoot guys. Cheers

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    Old Timer J M Fahey's Avatar
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    Cool.
    So you were straight missing your first power filter.
    That explains both the massive (unfiltered) hum and the 30% lower "DC Voltage" ... although it was not real DC but unfiltered full wave rectified AC ... not the same.

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