Film-Tech Forum ARCHIVE: Jaxlight hijinks

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Posted by Randy Stankey (Member # 64) on 06-12-2001, 08:00 PM:

I was reading the other thread about sound problems and Darryl said...

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

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.

Just wondering if anybody else has any stories to tell like this.

Posted by Joe Redifer (Member # 3) on 06-12-2001, 08:32 PM:

A theater out here called "Colorado Cinema Holdings Limited Liability Corporation Bowles Crossing 12" has Cinemeccanicas in the booth. They have the reverse scan LEDs but nothing like a Jaxlite (Jaxlites aren't even reverse scan!). They were the first projector I had ever heard with reverse scan. This theater opened as a Mann Theater. For the first couple of years, every time analog sound would play it would sound like a scratchy record. In fact, here is a recording made from that theater (tapping directly into the cinema processor) of the Hollywood Pictures logo played in analog. Click here to get the 13 second MP3 This MP3 is no exaggeration!

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.

Posted by Randy Stankey (Member # 64) on 06-12-2001, 09:17 PM:

That's very similar to the sound my projectors made. The only difference it had more of a pulsing, "Woosh...Woosh" quality.

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....?"

Posted by Barry Hans (Member # 477) on 06-12-2001, 11:42 PM:

I did a Jaxlight install at a theatre which had a hum in the system and grounding the soundhead/ preamp worked, I thought, untill the manager came up and said he just stuck his head in the auditorium and heard a hum. Turned out the humm he heard was one of those little floresent tittle boards above the auditorium entrance!

Posted by Randy Stankey (Member # 64) on 06-13-2001, 01:21 AM:

I hear ya', man!
I have one theatre that has removed all of their incandescent lights from the hallways and theatre entrances and replaced them with fluorescent 'capsule lights'. Every time I would walk into one house I'd hear this hum. It would go away as I got closer to the speakers, making it very hard to "find". It took me two or three visits to finally realize that it was from one of those capsules!

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!

That's how sensitive they are!

Posted by Darryl Spicer (Member # 711) on 06-13-2001, 03:41 AM:

Randy,

A lot of noise can be eliminated if you ground the system in two places. When we installed the jaxlights here in town we first used those dc adaptors that you use for calculators. We then grounded the negative conection to the plug in electrical socket. on the cfs system. we also ran a ground from the ground on the line amp to a screw on the sound head frame. This helped a lot. Yes you do have to make sure that the belts are lined up perfectly and that you get rid of any belts that may be cracked. What is interesting is that we would not have been using the jaxlights had componet engineering had there simpler reverse scan reader ready for sale. I have a prototype of this scanner in one of my auditoriums here. Also another thing to watch for and I have had this happen. Make sure on the simplex five star sound head you check and tighten the solar cell wiring on the connection strip. I have had thes come loose and produce the same sounds you have described.

Posted by Gordon McLeod (Member # 33) on 06-13-2001, 10:10 AM:

I have found the best solution is to use the cinema processor as the power source for the preamp and the Jax light itseld powered off the excitor supply.
The use of the processor as the ps for the preamp and also as the hub of the star ground eliminates a lot of the grief.
Also the cell itself can be microphonic as well. In one location I had to mount a rubber gasket to isolate the cell bracket from the case

Posted by Mark Gulbrandsen (Member # 72) on 06-13-2001, 10:15 AM:

This thread should have been called JAX Light Hijacks....Cause any dealer that sells one is doing their customer a big disservice....

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

Posted by Randy Stankey (Member # 64) on 06-13-2001, 08:53 PM:

I much prefer the CE reverse scan kit. It's just about as easy to convert to the reverse scan as the Jaxlight. Furthermore, it's 10 times easier to align the A-chain and you get 10 times better results.

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.

When I called to ask for advice I was told the hypothesis is that for some reason the cell is somehow "burning out" because of the wavelength of the LED or something like that.

Posted by System Notices (Member # 2357) on 10-23-2004, 11:01 PM:

It has been 1228 days since the last post.

Posted by Daryl C. W. O'Shea (Member # 1303) on 10-23-2004, 11:01 PM:

I guess this is as good a place as any...

Are LED replacements available for Jax Lights? What do these cost? Or can regular "replacement Dolby LEDs" be used?

What's a typical lifespan of Jax Light LEDs? I find the advertised 10,000 hours somewhat hard to believe.

I'm not for buying them, I just have to put both sides (cost) down on paper.

Posted by Don Anderson (Member # 397) on 10-24-2004, 10:44 PM:

Replacement LEDs are available from UltraStereo for about 80 bucks. No sodering (sp), they just slip right in. Replacing the LED ended our Humming problem. JaxLights sux, but when you had a supercheap company running a theatre, that's what they put in. The other theatre in town is still using exciter lamps, I just gave them a box of 6 about 4 weeks ago.