This is topic BAH! HUM-BUCK!!! in forum 8mm Forum at 8mm Forum.

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Posted by Steve Klare (Member # 12) on June 13, 2016, 11:32 PM:
So, I tried to be a decent citizen tonight: I went through my Eumig 800 series end to end and cleaned up the film path.

Part of the procedure is to disassemble the sound head and clean it inside. I've always been impressed how clean these stay inside: good thing too, since access isn't easy and it's not something you'll do every day.

Where things went severely downhill is when it was all back together and I ran my sacrificial test film. The transport worked fine and it didn't leave a mark on it, but you wouldn't believe the hum!

I thought maybe the connector that plugs the head into the chassis was shot, but I buzzed out the connections with an ohmmeter and they were solid.

About an hour or so in I realized that since the hum buck coil hangs on the head by a copper strap and I'd removed it to disassemble the head, it was not out of the question I'd put it back on flipped over.

I flipped it, powered the machine up and the now the hum returned to Eumig Normal!

What's the difference? The stray magnetic field inside the machine passes through the coil and develops a hum voltage. This is connected opposite polarity to the head so the hum pick up by the head and the coil cancel out. If you flip the coil over, it's as if you flipped the Norths and Souths of the magnetic field passing through the coil and now the coil's voltage ADDS to the hum picked up by the head (possibly doubles the voltage, and quadruples the power!).

So I am willing to be the bad example if I can keep this goof from happening to somebody else. Who knows, maybe someone out there is saying "Hold on, that happened to ME!" and some hapless Eumig is about to be redeemed from the dead projector shelf!

-Go for it!

I'm going to bed now! (-had enough for one night!)
Posted by Osi Osgood (Member # 424) on June 14, 2016, 11:23 AM:
Wow Steve!

I would have never even thought of actually opening the sound heads themselves and cleaning them. I can only imagine how much sharper the sound would be if i did this, (and maybe that balance head for stereo would be that much better!).

Could you perchance, put up a new post (or on this one) as to HOW to disassemble sound heads for cleaning?
Posted by Steve Klare (Member # 12) on June 14, 2016, 11:33 AM:
I can't take credit!

This was Paul Adsett's contribution:

Eumig - How to Clean Sound head?

That enclosed head always bothered me because I believe what you don't know really CAN hurt you (why would we have CAT scans otherwise?), so I was happy to finally be able to get in there and make sure.

Last night? -a couple of specks of emulsion dust: nothing terrible.

-just put that coil back the way you found it!

When I flipped the coil back, the hum was much better than when it was backwards (-no miracle here!). Then I re-loosened the mounting screw and rotated it back and forth and heard the hum rise and fall and I picked out the lowest level: now it's pretty decent, not perfect, but more than good enough.

I'm actually surprised this hasn't happened to me before. Other than goofing up re-assembly I didn't do anything different. Before next time I'm going to put a dot of paint on the surface of the copper strap that should be facing out and snap a picture of it assembled properly.
Posted by Graham Ritchie (Member # 559) on June 15, 2016, 12:12 AM:

Been there many times in the past, even in my aviation days I sometimes had bits left over from a cabin refit....mmmmm now where did that come from? ah well its flying, so it must be right [Big Grin]

These days a digital camera is a must, I cant rely on memory its not recording like it used to [Frown]

When I removed the Bauer projector from the cinema I took heaps of photos of the wiring including how things were wired up to the amps etc, lots of close ups. Even with photos I was really surprised I actually got things going again
Posted by Steve Klare (Member # 12) on June 15, 2016, 08:22 AM:
-as much as I love a good 35mm slide, when you are doing stuff like taking a projector apart there is nothing like a series of good digital stills.

These things come apart so willingly (-sometimes...), but that comes back to haunt you when it's time to piece it back together!

I did a huge surgery on my ST-1200HD last year...when I finished there was one screw left in the parts bin. As far as I know it was a leftover from some other project, but next time I do something like this I'm going to make sure to empty the bin first!
Posted by Paul Adsett (Member # 25) on June 15, 2016, 10:41 AM:
Although hum on the GS1200 is nowhere near the level of the Eumig 800's, it does exist. For that purpose the GS has two hum bucking coils, one for each track, right below the sound head. You minimize hum by fiddling with their position, just like the Eumig's.
And they are effective.

Posted by Steve Klare (Member # 12) on June 15, 2016, 11:21 AM:
In certain ways this is a better hum rejection technique than what I do externally: I have a 60 Hz. notch filter in the line out to the amplifier.

The notch kills that 60Hz hum a lot deader than the hum buck coils ever could, but it completely misses the harmonics at 120 and 180Hz which the coils should pick up. There is also a "B" key (-about 61 Hz.) on the left side of the piano keyboard the notch would thoroughly destroy if a music track ever tried to pump it through. A hum bucker would pass this note right through since it actually makes a difference if it really is hum or not in how the circuit responds.

What's nice is between the hum bucking and the notch, it manages to reduce the Eumig hum down to at least a background level where you have to get within a few feet of the speakers to really notice it. Much further away it just blends into the ambient sound in the room.

The shame of it is hum bucking as we see it is so imprecise. I'd really like to see some swiveled mount with fine screw adjustments: something worthy of an astronomer (-or at least somebody doing a front end alignment!). Instead you get this piece of metal you need to beat into shape like some medieval sword!
Posted by Paul Adsett (Member # 25) on June 15, 2016, 06:43 PM:
I agree Steve, but I bet that the Eumig and Elmo designers (correctly IMO) thought such a nice feature just was not warranted for what is probably a one-time adjustment.
But Elmo should certainly have designed precision adjustment into the alignment of their lens holder mount. The GS and ST lens mount adjustment/alignment is truly a hit and miss proposition - about the worst optical mount design I have ever seen.
Posted by Steve Klare (Member # 12) on June 16, 2016, 08:08 AM:
What they could have done instead is fix the location of the coil, maybe an extra winding integral to the head, and have tuned amplifiers adjust the level of line, first and second harmonics fed into the main amp to cancel out the hum.

-the catch? You'd need different circuits and therefore projectors for 50 Hz and 60 Hz countries. You'd also need a skilled technician at the end of the production line to do the fine tuning since it would be by measurement and not by ear. (It would be an absolutely mind numbing job too...)

Can you imagine having that job?

"I tuned 350 hum buck coils today"
"Would you like to go see a movie tonight?"
"-as long as it's silent!"

True fact: Hum bucking technology is also used for electric guitars. It's part of what makes film collectors cool people! (-hopefully not a major part!)

Maybe an improved lens mount is an opportunity for Mr. Van Eck.

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