Film-Tech Forum ARCHIVE: Hum/buzz when SH-1000 door is closed

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Posted by David E. Nedrow (Member # 4981) on 12-10-2011, 08:07 PM:

We're still limping along with a seen-better-days USL JS 195, sans SR/A card.*

Thankfully, virtually everything we show is DTS, so we usually don't run into any trouble.

Every once in a while though, we get a film with no DTS option and have to run the analog track.

In addition to the usual problems with the analog chain we experience, a new one popped up this time. We started MARGIN CALL Friday and during the first show, there was a pretty good low level sustained hum, along with a higher pitched buzz, that started as soon as the queue activated stereo.

I had no idea what was going on. I checked everything and switched first to mono, then to bypass, which at least made it less disconcerting for the audience (hey, passes for everyone).

After going to bypass, noting that the problem was still there, I opened the door of the SH-1000 to make sure everything looked correct. I verified that the film was latched over the sound head, etc. I had pushed on the latch to make sure it was tight, and checked the monitor for the noise. It was gone. Huh, something must have been loose.

I then closed the door and, while walking past the monitor, realized that the hum was back.

Hmm, could it be the door? Yes it could. With the door open, no hum. Door closed, hum.

My thought is that something is at the outer limit of a tolerance, and the pressure of the door "stop" against the sound reader assembly is pushing it beyond and into audible land.

It's a BACP RSTR-2000 reader and LED, but, at some point in the past, thick adhesive was packed around the locking screws that hold the reader. I'm not sure if I'll be able to do anything there.

So, what should I look at first in re: adjustments?

I'm pretty sure I have all the audio test films I need for analog.

One other note. I've never been able to get the correct level out of this reader. It's output is well below Dolby ref.

-David

Posted by Scott Norwood (Member # 30) on 12-10-2011, 08:17 PM:

My bet is that some light leakage from the port glass reflection or the shutter area of the projector is being reflected into the soundhead, which would cause a 48Hz hum (assuming a 2-bladed shutter). Usually, this happens when the door is open, not closed, however.

As for the level, you may need to loosen the three screws the hold the LED part of the reader and rotate it slightly while running cat. 69T. There is some play there and you will see when the output peaks by looking at the meters on the processor. When you find the output peak, tighten the screws and re-set Dolby level. If this does not solve your problem, you can cut one or both of the links on the circuit board (which pulls straight out of the "lens" assembly of the reader) to increase the current to the LED (and reduce its life by a corresponding amount).

Posted by Martin McCaffery (Member # 37) on 12-10-2011, 08:17 PM:

Do you have a bright light near the soundhead? Stray light bouncing around the reader when the door opens is the first thing I'd check.

Posted by Darryl Spicer (Member # 711) on 12-10-2011, 08:55 PM:

quote: David E. Nedrow
I then closed the door and, while walking past the monitor, realized that the hum was back.

Leave the door open. [Smile]

Posted by David E. Nedrow (Member # 4981) on 12-11-2011, 01:35 AM:

Gee, thanks Daryll. I knew there had to be someone who couldn't resist. [Wink]

"Doctor, Doctor, it hurts when I do this."
"Well, don't do that. "

In all seriousness though, I did check for reflections, etc. No port windows, no lights on in the booth.

The 48Hz shutter hum sounds promising. I suppose the light could be reflecting off the inside of the sound head inspection window. I'll put some black paper tape on the inside of the window and check for hum (door closed).

Posted by Ian Parfrey (Member # 5122) on 12-11-2011, 05:35 AM:

Easy stuff first.

Is the hum 48 or 60hz?

The answer to this will automatically eliminate one half of your (if applicable) stray light problem.

48Hz? - Then it has to be post shutter light bounce.

60Hz? - then it is either reflected flourescent light OR a possible LED power supply issue. The latter seems unlikely as you mentioned is goes when the door is opened.

See if you can nail down the frequency and take it from there.

Cheers

Posted by Kenneth Wuepper (Member # 1174) on 12-11-2011, 06:45 AM:

Then there is also the possible pair of frequencies derived from the scanner centering.
Too far in and you hear the DTS time code. (Like a vacuum cleaner in the lobby) or the sprocket holes (106 Hz).

Since the closing of the door in this model sound head moves the tension roller closer to the sound drum, you could have a loose lateral guide setting and that will cause either of the noises mentioned depending on the scanning error.

The adjusting and locking of this scanner is essentially a technician related adjustment requiring the use of test film and additional test equipment to have a properly done job.

I would recommend also that you simply leave the door open until you can have a technician do a proper alignment of the scanner.

KEN

Posted by Dave Macaulay (Member # 813) on 12-11-2011, 10:01 AM:

I would remove the little rod that prevents the door from closing when the lay-on roller is not latched. That's the only mechanical thing that could do aything with the soundhead action, shifting the film so the DTS track, frame lines, or sprockets are picked up.
If it still buzzes, you probably have an optical issue - light is getting into the reader. The BACP reader is pretty good at rejecting strange light sources but it can get interference.
Fluorescent lights are a source of 120Hz buzz and light bouncing off the shutter is 48Hz. A 60Hz buzz is almost always a grounding issue. Make sure the soundhead case is properly grounded and that the optical reader output cables have the shields grounded at the processor end only.

Posted by Louis Bornwasser (Member # 3063) on 12-11-2011, 11:10 AM:

I like Dave's rod idea. Especially true if door sags. Louis

Posted by Sam D. Chavez (Member # 1841) on 12-11-2011, 12:08 PM:

I have not seen an "interlock" rod on the door of the Simplex soundhead in many years but it does sound like this door has one of these rods.

It can be removed one way or another.

Posted by Tony Bandiera Jr (Member # 2365) on 12-11-2011, 11:46 PM:

Yep, Dave hit on the most likely culprit.

BUT, as Scott and Ken hit upon, you MUST first get the reader and LED aligned properly before you go any further..and it is NOT a "do-it-yourself" project, you must have all the proper test loops and a 'scope AND an RTA to do it right..otherwise you're just pissin into the wind.

Posted by Kenneth Wuepper (Member # 1174) on 12-12-2011, 08:18 AM:

Regarding the rod on the door of an SH-1000 sound reader:
There is a possibility that an operator could leave the pad roller disengaged and start a reel without sound. The SH-1000 heads had that little fail safe by making it impossible to close the door when the pad roller was not properly engaged.

I have seen the paint worn thin on the top of the doors of these units where the last act of threading was to run your hand across the top of the door to be sure the roller was engaged and the door was closed properly.

Posted by Robert Koch (Member # 3728) on 12-12-2011, 01:31 PM:

The reason all those rods are missing is that the door would1`nt stay closed with the pressure roller latched at the end of the nite so to avoid flat spots on the felt roller you removed the damned rod.

Posted by Monte L Fullmer (Member # 2797) on 12-13-2011, 12:23 AM:

..why at the end of the night, I leave all gates, traps and the door open - after I gave a good scrubdown in a booth that had the XL DeLuxe equipment..

Posted by Robert Koch (Member # 3728) on 12-13-2011, 02:06 PM:

I tried to correct my post yesterday, but failed. Should have said door wouldnt stay closed with pressure roller UNLATCHED, and this was the reason to remove the rod.

Posted by David E. Nedrow (Member # 4981) on 01-06-2012, 07:50 PM:

Sorry for the lag, but here's an update.

Adjusting the LED and pickup lens made no difference to the hum. So, another careful reading of the Simplex back-matter (Fields Notes). One suggestion was to connect a wire from ground on the sound processor to ground on the pickup assembly.

I alligator'd a wire between the two and that fixed the hum that started last month.

I'm installing conduit and new wiring for everything, so I'll take the time to separate high/low voltage lines, speaker wires, etc. I also want to get all of the grounds running to the panel. The current setup relies on the metal in some conduit as a common ground, but some of the wiring actually runs to another panel.

The other odd thing that happened at the same time the hum started, was that I now have ground loop noise in our video projection. You know what I mean, a very slowly rolling dark band moving up the screen, repeating ad-nauseum.

I still haven't tracked down what changed, but clearly something did.

I have some other questions re: the levels from the analog head, but I'll post that under separate cover.

-David

Posted by Bruce Hansen (Member # 281) on 01-06-2012, 08:13 PM:

The "bars" rolling through your video are hum. Has someone been doing electrilcal work ar your place, or has some new equipment, that draws a good amount of current been pluged in? It almost sounds like a ground and a neutral have been connected together somewhere - that would cause hum all over the place, or one leg of your power has been loaded more than the others, that imbalance will also cause hum.