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Are you using a jax light exciter lamp red led for your sound source. If so these are notorious for picking up huming...

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Here's something that has happened to me twice, in two locations, during the last week.

I was getting complaints of a "crackling" sound in the speakers. Theatre personnel were telling me that they must have a blown speaker or something. I replied, "It can't be...", because in the setups they had it was highly unlikely. When I got there and listened to it, there was a sound like an old, scratchy vinyl record comming from the speakers.

I must have worked on that projector for five or six hours (overnight) aligning and realigning the A-Chain and checking all the system components. After a while I gave up and called for help. Here is what I was told...

IF your projector drive belts (Simplex PR-1050) rub against the pulley flanges they will become frayed along the edges. As those frayed parts come whizzing past, the sound head will actually pick this up and it can be heard in your speakers during the quiet parts of the films.

I changed the belts and double checked their alignment. The problem went away! I just had the same problem in another theatre. (identical eauipment) First thing I did was to change the belts. Wouldn't you know it... Problem solved!

Not only that, if you're not careful, the vibration from the projector motor running can be heard if you turn the volume control up high enough.

Now Bowles has fixed this problem somewhat, as I have not noticed it being as bad as of late. Either that or they are always playing digital! But during the first few years, it was far worse than an exciter bulb would have sounded.

Like I said, I thought it was an out of line A-Chain at first. (More than one print had the same symptoms, thereby ruling out scratched film.) The film path was running perfectly. There were no blown bearings or anything like that either.

I checked to make sure there wasn't something in the processsor... swapped out all cards. I swapped out he Jaxlite preamp. I tested for bad grounds, etc. Nothing worked.

Then I noticed that the thing made the sound even while there wasn't film in the projector. If I pulled the exciter lamp (Now converted to the Jaxlite unit) out of the projector but kept the exciter power ON (thus keeping the preamp running)the sound was still there.

I hooked the scope up to it (right at the USL's preamp inputs) and I could see the little spikes in the signal. I must have sat there staring at it for more than an hour, motor running with no film and no exciter lamp. I couldn't figure out for the life of me what the F*** was causing it.

When I called for help I was told that Jaxlites are notorious for this as well as some other things. It's one of those things where drive belts are one of the LAST things you would think to check. When the theatre manager asked me what I did to fix it she just said, "What the....?"

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I like the idea of the Jaxlite, but with that preamplifier (which has something like a +15 dB gain) it picks up all kinds of junky interference.

Wanna' good demonstration on why you should keep the lights off in the booth when you have Jaxlites? Wait till you have an empty house. On the last show of the night, for instance. Open the sound head door and stand about 3 feet from the projector. Hold a Bic lighter about waist high and flick it two or three times. That puppy will pick it up and it'll sound like gunshots going off!

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Also be sure that the motor frame is properly grounded. Not all electricians run a 3rd wire for ground but it should be done to meet NEC. This is especially true on older RCA and Simplex sound reproducers that have a small wire jumper from the motor frame to the cradle.
As far as JAX lights go I would stay away from them completely. There is really no advantage to them at all and the extra preamp that is required in most installations only makes matters worse. Either go with true reverse scan or check out the new Swiss made LED exciter replacement from ICECO. Its cheaper than the JAX light and works a whole lot better.
Mark @ GTS

Here's another thing I found with Jaxlight...
I got a call about the sound being muffled. I "scoped it out" and the it was perfect. The lens was clean, too. The RTA showed a huge dip in the pink noise response at right around 8 KHz. I couldn't bring it back up by adjusting the preamp.

The solution I found was to move the cell a bit higher so that the slit image was falling on another part of the cell. The response came right back up.

It has been 1228 days since the last post.

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