Film-Tech Forum ARCHIVE: intermittent sound dropout

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Posted by Martin McCaffery (Member # 37) on 01-21-2012, 12:33 PM:

I've been trying to solve this problem for over a year. Every time I think it is fixed, it starts again a month or so later.

The center channel drops the highs, giving voices the sound of coming through a movie telephone filter.

Things we done that have "fixed" it in the past:
Replaced LED's in reader
Replaced Kelmar power supply for both machines.
Replaced the Ashely X1 crossovers on all the amps.
Last fall I had two service calls (one to track down the bad crossovers and one to reset the whole A/B chain).
In addition we've pulled out and reseated the cards in the Dolby 650 (and cleaned it out). We checked the connections in all of the amps.
The LCD display on the 650 shows no dropout of the center channel, but you can hear it on the monitor, which should eliminate a bad speaker (though we checked those, too).
Sound was good in November, we closed in December, and the sound was fine for the first week in January. I heard quick bits of it for the first show last week and then was out of town, so couldn't follow up. Last night, new show and it was horrible (thankfully, a subtitled film).
The dropout doesn't usually last more than 30 secs. Sometimes it just happens once a film. Last night it was on and off all through the film.
Does it in Digital,SR and Mono.

What's left? Dolby motherboard?

Posted by Louis Bornwasser (Member # 3063) on 01-22-2012, 12:00 AM:

Definatly look at the crossover again. If you still have the older XR-1 crossovers, not the newer ones with no controls, I am certain this is it. You could operate the very small switches on the crossover and then leave them set where they started. Same with the pushbuttons.

It is a sad fact that our last project just before closing was to redesign the Ashly crossover due to unreliability in the theatres. Drops right in and is preset for your model JBL speaker. We made 500 of these and then closed.

It is possible that an amp channel is bad, but not likely, since they rarely cure themselves.

Currently on vacation in Vienna, back Tuesday. Louis

Posted by Ron Lacheur (Member # 1126) on 01-22-2012, 12:33 AM:

Dirty gain pot on the offending channel?

Posted by Marco Giustini (Member # 4544) on 01-22-2012, 05:51 AM:

Hi there,

If I am not mistaken you have not tried replacing the amplifier, yet. If you haven't got a spare one why not swapping, say, the centre one with the left one, to see if the issue follows the amp or the channel?
You may also want to find a way to monitor the amp output, does your monitor accept amp return lines?
Louis, could it be an overheating issue?

Posted by Jim Cassedy (Member # 4115) on 01-22-2012, 10:01 AM:

It sounds like I've got almost the same intermittant problem,
only on my system the LOWS of the center channel drop out,
making it sound like a "movie telephone filter".

The set up sounds similar to yours. Ashly amps & CP-650.
It's very occasional and unpredictable, which makes it
tricky to troubleshoot.

The input pots & switches on the Ashlys notoriously unreliable.
"Exercise" them a bit & see if the problem goes away.

I haven't tried swapping my amps yet either, but I probably will
next time the sound drops out. Last time it happened was around
Thanksgiving week. It dropped out just as I was leaving the
booth, but by the time I got back inside, it had 'fixed' itself.
It's been behaving well since then but I still don't trust it.

Also, on my system, if the problem is going to happen, it almost
always only dropped out during the trailers or the first reel of
the feature. I never had it go out during the middle of the film.
That first lead me to beleive it's perhaps a temperature related
issue, but sometimes I come in early to do work and the amps
are on for several hours before the first show, which you'd
think would give them time to 'warm up' & stablize by showtime.

So no matter what, it seems to be happening randomly.

Very vexing! Sometimes I wish it would just catostrophically
fail. It would be much easier to troubleshoot!

Posted by Scott Norwood (Member # 30) on 01-22-2012, 10:17 AM:

You say that this is audible on the booth monitor. Is that with the monitor set to the "amp" position or the "processor" position? Or both?

You are not using the Dolby crossover card, correct?

Does the problem occur with the processor in bypass mode?

Posted by Martin McCaffery (Member # 37) on 01-22-2012, 10:59 AM:

Louis:
What's the best way to contact you when you get back?

Posted by Louis Bornwasser (Member # 3063) on 01-22-2012, 11:09 AM:

Cell phone works; fax does still for a little while.

Posted by Louis Bornwasser (Member # 3063) on 01-22-2012, 11:10 AM:

Cell phone works; fax does still for a little while.Louis

Posted by Brad Miller (Member # 2) on 01-22-2012, 11:49 AM:

I'm with Ron on this one. Seen it happen a lot once Ashley amplifiers get some age to them. I would recommend you turn off all amplifiers except center and with something fairly constant audio playing (maybe a loop of a trailer) jiggle the gain pots on the back of the amplifier and see if you can make it drop. If so, that's your problem.

Posted by Martin McCaffery (Member # 37) on 01-22-2012, 03:48 PM:

Spent about a half an hour playing with the levels. Couldn't force it to go out, so maybe dirty pot is the immediate problem.

And, as with Jim, it was the low freq going out, not the high (so much for my musical skills). Lowering the low freq perfectly replicated the problem.

I'll just have to keep an ear out for it happening again.

Posted by Dominic Espinosa (Member # 2122) on 01-22-2012, 03:58 PM:

Ironically I too had a center channel dropping out highs rather than lows intermittently a while back.
Thought it was the xover but it turned out to be the horn driver.

Nevertheless I'm curious as mentioned above: when it's dropped out do you hear full signal from the monitor listening to the amps or processor or both?

Posted by Eric Robinson (Member # 2966) on 01-22-2012, 10:35 PM:

Sounds like assumptions are being made before this important question is first answered.

quote: Scott Norwood
You say that this is audible on the booth monitor. Is that with the monitor set to the "amp" position or the "processor" position? Or both?

Posted by Monte L Fullmer (Member # 2797) on 01-22-2012, 11:40 PM:

..swap the diaphrams the HF horn drivers between L and C...windings on those diaphrams can open up when they want to.

Posted by Dave Macaulay (Member # 813) on 01-23-2012, 11:43 AM:

You're assuming that the monitor has a "processor" function, and that it's actually wired up. But with amplifier crossovers (my least favorite kind) the processor monitor won't tell you much in this case. I haven't come across a processor without crossovers that managed to filter all the highs or lows out of a full range signal, although it is possible.
This is most likely the Ashley crossover module has failed, they do. The volume control pot is also known for failure and noise (cheap unsealed potentiometers that are never moved all eventually become unreliable, and high quality sealed pots are expensive).

Posted by Martin McCaffery (Member # 37) on 01-23-2012, 01:18 PM:

quote: Dominic Espinosa
Nevertheless I'm curious as mentioned above: when it's dropped out do you hear full signal from the monitor listening to the amps or processor or both?

I believe the monitor comes off of the amp (Louis can tell you if I'm wrong). When the sound drops out, you hear the dropout on the monitor.

Posted by Tony Bandiera Jr (Member # 2365) on 01-23-2012, 07:42 PM:

quote: Martin McCaffery
I believe the monitor comes off of the amp (Louis can tell you if I'm wrong). When the sound drops out, you hear the dropout on the monitor.

That makes it 99.9999999999999999999999% probability it is the amp or crossover. [Big Grin]

Posted by Ron Lacheur (Member # 1126) on 01-23-2012, 09:38 PM:

I'm still thinking it's the gain pot on the amplifier. Do you get static when you rotate it?

Put the processor in mute and crank your booth monitor to max ( hopefully they aren't set specifically for SPL ). Cycle the pots back and forth. If you hear any static, that's most likely the issue.

I used to do this with QSC MX amps and it was quite effective in figuring out which ones needed repairs.

Posted by Eric Robinson (Member # 2966) on 01-23-2012, 11:12 PM:

OK. I'm probably going to get roasted for this but,

quote: Tony Bandiera Jr
That makes it 99.9999999999999999999999% probability it is the amp or crossover.

Assuming the monitor is tied to the amp outputs only, a high freq drop which originates from the processor outputs would create a high freq drop on the monitor also.

Not arguing the point, I may have it totally bassackwards somehow.

Posted by Louis Bornwasser (Member # 3063) on 01-24-2012, 09:34 AM:

1. The monitor is across the amplifier output.

2. The crossover is part of the amps. Ashly XR-1 crossover is a problem, but the amp is sterling. (3 db down at 100K)

Yes, we all agree that the bad pots/switches are almost certainly the problem. That is why we redesigned the crossover. louis