8mm Forum: Optical Sound Hum

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Topic: Optical Sound Hum

Paul Adsett
Film God

Posts: 5003
From: USA
Registered: Jun 2003

posted June 26, 2003 02:45 PM

I have a problem with my Elmo GS1200. When I play back films which have optical sound tracks I get a very high level of background hum, which is very annoying.The hum is presnt both through the projector speakers and the AUX OUT connections. I do not get significant hum when playing back films with magnetic tracks. Does anybody have any idea how I should go about trouble shooting this problem? It's not a big deal since 99% of my film collection is on the usual magnetic tracks, but I would like to fix the optical playback so I can show an occasional film which has optical tracks.

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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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Kevin Faulkner
Film God

Posts: 4071
From: Essex UK
Registered: Jun 2003

posted June 26, 2003 05:58 PM

Hi Paul,
I have no hum on my GS with optical playback. Does the hum change pitch if you drop the speed to 18FPS. If it does then it would suggest that the optics are out of alignment and are also picking up light from the actual piture frame. Is the output from your optical sound very low and muffled as this would also be a good indication of the same problem. It might be worth trying a new lamp.
Another possibilty is that you have AC current getting through to the lamp itself. This has to be a very smooth DC current. A problem in the regulator smoothing circuit would give rise to this sort of problem. A way to check this is to disconnect the wire at the bottom of the lamp holder and then run the lamp on a 3 volt battery supply. It's probably easier to remove the lamp supply plug under the main circuit board and then run a supply into the plug at that point. Dont forget that the chassis of the lamp holder is your neg terminal.
Hope that gives you some pointers, Kevin.

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GS1200 Xenon with Elmo 1.0...great combo along with a 16-CL Xenon for that super bright white light.

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Paul Adsett
Film God

Posts: 5003
From: USA
Registered: Jun 2003

posted June 26, 2003 06:34 PM

Thanks Kevin, as always, for your excellent suggestions. I will get to work implementing them, and see if I can narrow down the problem on my GS. [Smile]

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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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John Clancy
Phenomenal Film Handler

Posts: 1954
From: Cornwall
Registered: Jun 2003

posted June 27, 2003 02:17 AM

I believe the optical sound cell can also be positionally adjusted. I've never had to do it on a GS but constantly have to adjust the cell on my 35mm machine otherwise uneven stereo level and hum result.

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British Film Collectors Convention home page www.bfcc.biz. The site is for the whole of the film collecting hobby and not just the BFCC.

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Kevin Faulkner
Film God

Posts: 4071
From: Essex UK
Registered: Jun 2003

posted June 27, 2003 04:08 AM

Hi John,
Yes you are right but it shouldnt make a great differance unless its badly out. The cell is oversized on the Elmo machines so it relies on the optics to make sure that only the slit of light is focussed onto the cell.
I have just read the service manual and Elmo tell you to attach the cell to the machine and then everything else is aligned from there. I dont think the cell is the problem in this case. As I said earlier I think that either the optics are out of alignment or there is residual AC getting through to the lamp.
I had low output on my machine for a while and it turned out to be the lamp itself because as I rocked it arround in it's holder the volume level changed quite dramatically. It still had its original lamp in the machine but I bought a new one and then noticed that the filament was slightly askew in the old lamp. When I put the new lamp in the volume level probably trippled. I have also made an electrical mod to the circuit on my machine which boosts the level for optical sound which then means the voloume control doesnt have to be turned up so high and therefore keeps the residual hiss levels down to a minimum. I have found over the years that most Elmo optical machines play back optical films with a much lower level comparred to the Magnetic track. This as the manual says is intentional because of the varying level of optical tracks but I think it was a bit overkill on the GS.
Must stop rambling, Kevin.

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GS1200 Xenon with Elmo 1.0...great combo along with a 16-CL Xenon for that super bright white light.

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Paul Adsett
Film God

Posts: 5003
From: USA
Registered: Jun 2003

posted June 27, 2003 07:24 AM

I followed Kevin's advice and determined the following:
1. There is no hum at all if I disconnect the exciter lamp. It is only present if the exciter lamp is lit.

2. Hum is still present with the projector running in the optical mode, even with no film going through the projector. No hum in magnetic mode.

3. Sound quality (ignoring the hum issue) seems to be pretty good in optical playback, although I have to crank up the volume significantly higher than for magnetic mode.

Sounds like there may be voltage ripple on the exciter lamp circuit, as Kevin suspected. I have no idea where to go from here, but am wondering if hanging a capacitor across the exciter lamp lead (to ground) would help smooth out the ripple without having to go into trouble shooting the GS exciter lamp power supply circuit. Might putting in a new exciter lamp help the situation?

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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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Kevin Faulkner
Film God

Posts: 4071
From: Essex UK
Registered: Jun 2003

posted June 27, 2003 06:57 PM

I dont think the lamp will be causing this problem. As I said in an earlier post the GS does naturally have a low output from optical soundtracks.
I still think your best bet would be to feed the lamp from a couple of 1.5v cells in series to check this out.
If you remove the back of your GS you will see a small brown plug and socket under the main circuit board on the LHS. If you disconect the plug you could then supply the lamp from an external source.
The GS does have a capacitor effectively accross the lamp and its 100uF at 16Volts. I would however suspect that the hum is on the 12 volt supply which supplies the lamp regulator. If this is the case then it should be sorted as it also supplies the machines logic circuits which would also suffer eventually.
Paul you can mail me direct if you wish so that we can go into this in more depth. What I can tell you is that the GS does not normally suffer from hum in optical playback. Maybe you could find a someone with an oscilascope who could check at the lamp base to see if you have residual ac ripple present.
Kevin. [Frown]

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GS1200 Xenon with Elmo 1.0...great combo along with a 16-CL Xenon for that super bright white light.

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Barry Johnson
Master Film Handler

Posts: 358
From: United Kingdom
Registered: Jul 2003

posted July 19, 2003 05:24 AM

Just a long shot this,but have you examined under a magnifying glass the actual soundtrack? Hum is caused by an ineffective guard band either side.or just one side of,the optical track.On occasions this anomaly is wide enough to get scanned by the beam causing hum.
This used to be a common problem on early 16mm sound films.
Now if this is happening on all your optical films then critical realignment of the beam may be in order.This will not be easy as in 35mm we use a 'buzz-track' test film to align the beam if it becomes necessary.Very rare now of course due to digital sound option on the film.
Hope this may help you to nail the cause.

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Standard8 rules!!

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