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Old 18th September 2005, 09:17 PM   #1
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Default QSC MX-1500a problems

There are alot of these amps in use in movie theatres in my servicing area and we are starting to see many failures of them. Some have had shorted bridge rectifiers and some just seem to drop a channel and the result is no output, but that channels circuit seems to be operating just fine. Most are about 10 years old and many of them use the built in QSC X-over card as most of the systems they're in are bi-amped. I'm getting tired of sending these back and forth to either QSC or the local service center and would prefer to start repairing them myself to effect a faster turn around and less service trips to remove end then install a loaner amp and then re-install the customers repaired amp and so on.

Has anyone out there that also services these amps seen any common problems with them... cap failure, output relays bad, solder problems, or certain parts of these amps that commonly give a headache? They are certainly beginning to cause some big headaches around here!

Thanks in advance for any suggestions on this......

Mark
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Old 19th September 2005, 05:32 AM   #2
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Hi,

Mr.GulBrandsen....please try this link

http://www.qscaudio.com/cgi-bin/ultimatebb.cgi

technical forum of QSC......

regards,
K a n w a r
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Old 19th September 2005, 05:37 AM   #3
Enzo is offline Enzo  United States
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Biggest problem is they weigh a ton.

I don't see them as often as I used to, but I was an authorized QSC service center. I need to see if they have my updated address come to think of it.

My experience with the larger QSCs is that in the twin board models, it is inevitably the one underneath that fails. Just my karma, nothing electronics about it.

What you need to do is get the service manual from QSC. MAke sure to ask for the complete model name, don't leave off the littel a at the end. The schematics and layout are there, plus a theory section and some good troubleshooting.

I have fixed my share of blown channels, they tend to be straightforward, shorted power xstrs and IRFZ44s. Now and then the little sm modules go bad. There is no documentation for them, they are replacement only, though you could probably identify bad parts on them and replace them without drawings.

I do replace bridges more than on other brands. Since these amps use flying rails they can be upsetting initially for you to intuit. But once you wrap your head around them they fall into place. The filter caps occasionally go bad.

The drivers fail more than I would expect. I forget on that one, but I think it is a TO220 pair 2SC3298/2SA1306 or something. There is one little TO220 driver for a whole output side, and there is a low ohms emitter resistor for each like 3.3ohm or 5.6ohm or so. Check for open.

These are commutating amps, so there are several power supplies and circuits to switch them in and out.

SOmetimes 15v zeners go. And don't expect the voltage on the 5532 to be a steady +/-15VDC. Those supplies have limits but they do track.

Now and then the chip fails, I think they are socketed anyway.

Get the book.
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Old 20th September 2005, 04:55 AM   #4
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Thanks for the tips on this amp! Being a commutating amp it is somewhat unusual and a little difficult to understand but I will study the service manual and get a grip on this design... we have alot of them in this area. QSC told me that complete modules are no longer available for this amp so I ordered a rebuild kit for it today and we'll see how it goes. I have to have it done by late tomorrow as I will be out of town the rest of the week.

The QSC service tech I talked to advised on the rebuild kit due to tha age of the amplifier... about 10 years. He said there are semicondictor upgrades that were done to these amps to make them more reliable and plus it includes all new electrolytics as well. Makes sense to me to go this route as good reliability in cinema sound is a must and the multiplex this amp is out of is a very busy plex!!

Mark
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Old 20th September 2005, 12:12 PM   #5
djk is offline djk
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The bridge rectifiers fail quite often in these, no heatsinking. The 15V zeners too, outputs and drivers.

Do not replace the NE5532 with any other type opamp, the protection scheme on the finals depends on the internal current limiting of this opamp!
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Old 20th September 2005, 02:21 PM   #6
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Thanks DJK!

Tne rebuild kit includes all new op amps so the peoper ones will get installed. I noticed alot of heat damage to the board near some power resistors and some resulting granuluar looking solder connections. I might re-solder those with silver bearing solder because of its higher heat characteristics.

Mark
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Old 20th September 2005, 10:26 PM   #7
djk is offline djk
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The lead/tin/silver solder sold for electronics use has the same melting point as normal electronic solder.

It is required for silver plated SMD devices and for gold due to tin migration issues.
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