Film-Tech Forum ARCHIVE: QSC 1400 hmmm

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Posted by Steve Kraus (Member # 476) on 07-29-2004, 07:42 AM:

A bit of hum on one channel only of a QSC 1400. Doesn't appear to be in the source only output so it's gotta be the amp itself humming. What do you think, power supply cap(s) going bad? Are there different P/S's for either side?

Posted by Ken Layton (Member # 133) on 07-29-2004, 09:32 AM:

Take the top cover off the amplifier. You'll probably find one or more of the filter caps have splooged all over the place.

Posted by Steve Kraus (Member # 476) on 07-30-2004, 08:15 PM:

Nope, no caps blown or otherwise noticeably bad. I was hoping I would find one warmer than the others. The hum is faint but it's there and I think it's less on cold start and increases when warm.

Posted by Tony Bandiera Jr (Member # 2365) on 07-30-2004, 11:22 PM:

Steve:
According to the Series One service manual, it may just be an internal adjustment (They call it a "Hum-Null Trim") or failure of some related parts, or even mismatched VALUES of the caps. (Would explain why it changes with temperature.)

Please contact me and I can send you a copy of the manual sections on the 1400.

And yes, there are separate power supplies for each side.

(Edited to add P/S info.)

Posted by Mark Gulbrandsen (Member # 72) on 07-31-2004, 03:45 PM:

It should go back to QSC or an authorized QSC repair station for repair and possible updates to the bias circuitry. I have had a rash of QSC 1400 and USA 900 failures lately and its really getting a bit tiresome. All of our 1400 and 900 loaners have been out just about solid all year being foreworded from one theatre to another while theirs are repaired at the factory.
Hey it is a dinosaur by todays standards and they are just getting old!!

Mark @ CLACO

Posted by Steve Kraus (Member # 476) on 07-31-2004, 07:19 PM:

Yeah I found useful info on the QSC site. This is my LS/RS amp. Hum is faint but I don't want any at all. Mark, I may throw in your old QSC 1200 while I play with the 1400.

Posted by Mark Gulbrandsen (Member # 72) on 07-31-2004, 09:57 PM:

Hummmm.....OK, but it only has about half the power and its also very old.

Mark

Posted by Bobby Henderson (Member # 840) on 08-01-2004, 12:24 AM:

I always tend to think someting has lost its ground if I hear a hum. But then I don't deal with movie theater P.A. system amplifiers. I'm used to farting around with my electric guitars and tube amp head. When I hear hum there it is usually one of two things. Either I have a power tube going bad some place or the freaking ground wire soldered to the jack in my Kramer Stagemaster custom has come loose again.

Posted by Steve Kraus (Member # 476) on 08-01-2004, 12:51 AM:

Obviously I could only sub the smaller amp if it still makes level but it seems likely since the input gains on the 1400 are only half-way up. Then I can investigate repair or adjustment. I may try to round up another 1400 too. If caps are going bad I'm sure the others in the rack aren't far behind. Then I'd have to think about replacing them all but in the meantime it would be good to have a spare to keep handy so a matched set can be maintained.

Posted by Mark Gulbrandsen (Member # 72) on 08-01-2004, 07:42 AM:

Steve,

Since many of them are failing would you please post that link to the useful QSC info that you found. I know that many others will be needing it.

Thanks,
Mark

Posted by Gordon McLeod (Member # 33) on 08-01-2004, 08:56 AM:

One some of them it was the ground on the input card that was causeing low level hum
dissasemble and clean its contacts also one version of the USA serries had issues on the lock ring around the 1/4 jack that was used to tie part of the circuit to ground

Posted by Steve Kraus (Member # 476) on 08-01-2004, 10:01 AM:

QSC Support Forum

QSC Schematics

It's actually kinda limited; they want money for the real manual. It's unfortunate when a manufacturer feels the need to make .pdf downloads a profit center.

Posted by Steve Kraus (Member # 476) on 08-01-2004, 04:27 PM:

OK...put in the 1200 for now. Level set using the amp's controls only (didn't touch the 500). The 1400 was at 0dB; the 1200 achieves level at not quite +1dB. The controls are marked differently and I wonder if they really are the same and they just changed the way they are marked. In that case it may be more instructive to say the 1400 was at 12 o'clock; the 1200 at 3 o'clock. I routed some mp3 music through these channels (LS and RS) letting it cook good and loud. Much more of a workout than most films I would think so I should be good for the time being. The 1400 hum wasn't that audible over a film but I'd rather deal with it and correct it.

Posted by Jason M Miller (Member # 2597) on 08-02-2004, 09:07 PM:

I am kind of new to this but I have had a year or 2 doing sound renforcment for a church so I could be wrong but here it goes. Check the gain on the amp, it might have been turned up, also check all your grounds including any power supplies or power conditioners you may be using, there could be a ground hum, check the frequency of the hum if it is around 50Hz to 60 Hz. Try also turning off any light dimmers. Make sure your audio cabels are not near any high current mains. Try to ground your amp rack

Posted by Steve Kraus (Member # 476) on 08-02-2004, 09:25 PM:

Anything is possible but doubtful since nothing has changed and I haven't been in the amp rack in a long, long time. The 1200 doesn't have the hum and when I get a chance I'll hook up the 1400 elsewhere to make sure that it does before messing with it. I see these things come up for sale used (obviously) and aren't that expensive so I'll probably pick up one or two. Then I can really look into it at my leisure.

I'm gonna bet that some caps while not exactly failing have simply changed values enough from heat and age that whatever that null point thing is is probably out of balance. May indeed be a simple adjustment. Or I have to recap the beast.

Posted by Mark Gulbrandsen (Member # 72) on 08-02-2004, 10:13 PM:

QSC does have available re-cap kits that have all you need to refurbish one. The kits are pretty inexpensive. Thay also have semiconductor kits for doing updates to older amps. You can get alot better performance with the newer semi's installed in an older a QSC. But if you buy the caps locally be sure to get 105 deg c. rated caps of the same value so they last. You'll probably have a hard time finding the PC mount main caps that'll fit in Chicago. The 1400's can run pretty warm when enclosed in a rack and cap life can be short so the 105c. rating is important. I just re-capped a USA 850 today and soldered a poor solder joint on the fan slow speed resistor. It was not making connection at all and the fan would only run at high speed when the sink got to 55 deg c. which is pretty dam hot. This amp suffered along this way for a long time, probably since it left the factory.

Mark @ CLACO

Posted by Tony Bandiera Jr (Member # 2365) on 08-02-2004, 11:50 PM:

Steve:

Manual is on its way.
[beer]

Mark has the best idea, get the cap kit from QSC. As explained in the manual, the VALUE of the caps MUST be matched closely or problems will pop up.