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Topic: Eumig 807D hum

Steve Castle
Junior
Posts: 6
From: Sydney, NSW, Australia
Registered: Jan 2015

posted January 12, 2015 02:11 PM

Hi all,

I am new here. I have recently dragged out my old 807D to start digitising my old film as a retirement project. I have had this projector since new. It has the dreaded hum in the audio, which some say can be resolved by moving the coils. However, if those coils have never been moved since new, why would they affect hum? Isn't it more likely that the power supply filter capacitors, C10 and C15 have dried out?

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Steve Castle
Junior
Posts: 6
From: Sydney, NSW, Australia
Registered: Jan 2015

posted January 16, 2015 11:46 PM

I have replaced the power supply filter caps and there was no change to the hum. Is there anyone out there who can direct me where to look? I don't think it is a grounding issue as it I am not connecting any external amplifiers or recording units.

Regards,

Steve

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted January 17, 2015 05:34 AM

Hi Steve,

If your problem was a song we all liked, we'd call it "an oldie, but a goodie" because it's a pretty classic Eumig 800 series complaint. Over the years it's gotten a lot of air time here: shame you can't dance to it.

My gut tells me the root cause of the problem is the fact that they hung a decent sized power transformer pretty close to the sound head. I'm sure that aluminum wall in between provides some shielding, but maybe not quite enough.

These machines are not spectacular where grounding goes either. There have even been stories of people getting slight shocks by touching the case. You don't have to connect up to the outside world to make a ground loop, you can have a dandy one inside the same device too. If I hadn't grown to accept a little hum as my Eumig's endearing quirk, I'd probably start digging here. (I did firm up chassis ground for safety and also to connect up to external circuits more gracefully.)

I tried replacing my electrolytics too: same result. It is something reasonable to try, after all: these caps are getting old enough to have kids in college! At the time I was working for a power supply manufacturer, so it almost HAD to be supply ripple. (When all you have is a hammer, everything starts to look like a nail...)

I think the hum buck coils are kind of a Band-Aid to try to deal with the magnetic fields around the head. Maybe they really do help, but aren't a total solution to the problem.

I became curious how electric guitars work recently and did some reading about them: seems they use hum buck coils too for the same reason. Of course this isn't much help to you, but it is interesting (...at least to me!).

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All I ask is a wide screen and a projector to light her by...

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Brian Fretwell
Phenomenal Film Handler

Posts: 1785
From: London, UK
Registered: Jun 2014

posted January 17, 2015 10:13 AM

If the 800 series is basically the dame as the 700 one the motor and lamp transformer are on the same former increasing the magnetic field round it, so nothing cancels out. The aluminium might shield the heads from electric fields if earthed well but not magnetic ones, unfortunately unless there was a circuit to take power out of them. ie as a Faraday screen.

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted January 17, 2015 10:38 AM

The Eumig 800s have this nice enclosed head which is probably meant to shield the pickup from magnetic fields, but you notice the Elmo heads are completely open. If I remember right there are no hum bucking coils in Elmos either. However the Elmo transformer is mounted further away so maybe the field isn't such an issue.

Maybe this is the price you pay for a nice compact projector. The Eumig 800s are significantly smaller than an equivalent Elmo, but the more you squish things together, the more they interact.

If they did a super-8 projector today the power supply could be a switcher. Any radiated field frequency would be well above the audio range and you could mount the heads on top of it as far as hum is concerned.

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All I ask is a wide screen and a projector to light her by...

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Paul Adsett
Film God

Posts: 5003
From: USA
Registered: Jun 2003

posted January 17, 2015 02:03 PM

Steve, the GS1200 has two hum bucking coils fairly close to the mag heads, and they need to be postioned correctly to fully cancel out hum.

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The best of all worlds- 8mm, super 8mm, 9.5mm, and HD Digital Projection,
Elmo GS1200 f1.0 2-blade
Eumig S938 Stereo f1.0 Ektar
Panasonic PT-AE4000U digital pj

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted January 17, 2015 02:57 PM

I guess they applied them as needed: ST-1200HD doesn't have them, ST-800 doesn't either.

Is there anything nearby in the GS that would produce a strong magnetic field?

I bet they had people at the factories that could tune these two to the minute just by the sound of the hum when they plugged in, and nobody will ever know the art like they did.

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All I ask is a wide screen and a projector to light her by...

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted January 17, 2015 05:41 PM

What I'm not getting about these is how they work.

Here's how I imagine it:

You have a sound head picking up the recorded sound on the track (signal), but unfortunately also getting the hum from the stray magnetic fields (noise).

So you put another winding out in the same space to pick up a sample of the noise of equal magnitude as the one at the head and you wire it in series but opposite polarity so the two noise pickups are equal and opposite and they cancel, leaving only the desired signal.

-seems simple enough.

So then why are there two coils? Are each trying to pick up half the hum?

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All I ask is a wide screen and a projector to light her by...

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