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RWD
`Olu`olu

USA
850 Posts

Posted - 12/29/2008 :  09:07:46 AM  Show Profile
I played at a rehabilitation center(older folks)this last Sunday and the PA was nearly unusable. It hums badly and I want to help fix it. It is an older built-in system with several small speakers on the ceiling surrounding the open meeting area (atrium).

The hum resembles the noise you would get when you touch a plugged in cable but not as loud. The pastor asked to use my PA to get through the service (I play through a powered PA mixer with one JBL PA speaker).

I know their system has been humming for more that a month now and
I would like to get suggestions on how diagnose and fix the problem. Those folks should not have to listen to that horrible noise during service. Help would be appreciated.
Thanks in advance...

Bob

Lawrence
Ha`aha`a

USA
1597 Posts

Posted - 12/29/2008 :  11:45:15 AM  Show Profile
The most likely cause of Hum in a PA that is present even when the inputs are disconnected and the main fader is turned down is failing main power supply capacitor(s). This can be temporarily verified by attaching equal or larger valued capacitor(s) across the existing one(s). It is best to have the repair manual (with schematic) to work from and order the new caps which will replace the old ones. Order the repair manual from the PA manufacturer, find the part number and order the parts, remove the old ones,then solder and/or screw the new ones in.

If you do not know how to order and/or read the repair manual and schematic and also spot the main power supply caps and mount and solder the new ones in place, then you probably should not attempt the repair.

P.S. Replacement parts are usually sold for 3 to 5 times the original cost (an arm and a leg), expect to spend $100 for a couple of these caps for a 100-200 Watt PA.


Mahope Kākou...
...El Lorenzo de Ondas Sonoras

Edited by - Lawrence on 12/29/2008 3:11:40 PM
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RWD
`Olu`olu

USA
850 Posts

Posted - 12/30/2008 :  02:41:16 AM  Show Profile
Ok, now I know that the power supply could be the problem so I will keep that in mind.
I will probably not be messing around with the capacitors though. I have never shook hands with one that I actually liked.

Bob
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RJS
Ha`aha`a

1635 Posts

Posted - 12/30/2008 :  06:52:26 AM  Show Profile
One of the senior places I go to spent always 2x what they would pay for a new system to fix their old one - and it went out again afterless than a year.
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Lawrence
Ha`aha`a

USA
1597 Posts

Posted - 12/30/2008 :  07:32:12 AM  Show Profile
quote:
I have never shook hands with one that I actually liked.
Yeah... especially if it is charged up at 50 to 200 volts!! (in this case) Caps in old-style (tube) TV setS can have charges up to 30,000 volts and, let me tell you, that really hurts.

If you are lucky, and if there is an electronic surplus store nearby, and if you have the old caps already removed, you may be able to buy a replacement set for as little as $10. (Lots of if's, but this is what I have done in the past.) However, I also spent $100 on a couple of caps for my Sony AV receiver, replaced them, threw away the old ones, then had the hum come back in less than 30 days. Turned out it was a loose grounding screw on the chassis (it did not have a lock washer so it came loose again due to thermodynamic stress). I really embarrassed myself to myself to put a lock washer on when I found the real problem after having wasted $100 bucks and several hours of time.

Often these days it is cheaper and better to buy a new PA than attempt to repair an old one, especially if the old one is more than 10 years old.

Good luck...


Mahope Kākou...
...El Lorenzo de Ondas Sonoras

Edited by - Lawrence on 12/30/2008 07:43:42 AM
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Earl
`Olu`olu

USA
508 Posts

Posted - 12/31/2008 :  08:05:32 AM  Show Profile  Visit Earl's Homepage
I noticed the phrase "old system". Another clue is that the problem appeared fairly suddenly and is ongoing. That would indicate a failing component like a power supply. They could replace the power amp or receiver (whichever is in place).

Unless you are an electronics hobbyist and able to get into the guts of the device and replace parts, you probably should not fool around inside the case. This also presumes that your time spent on the repair has little or no value too. Would you rather spend a day with a soldering iron, or learn a new slack key piece?

PA systems do have a finite working life. Electronics get abused, parts age, and even the speakers wear out. I recently worked on a hockey arena with a 22 year old professionally built system, and it was beginning to fail routinely.

A good rule of thumb with most electronics like a home stereo or a commercial grade PA system is: when the repair cost is more than half of the replacement cost, it's time to go for new. It will save you $$$ -- and frustration -- in the long run.
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wcerto
Ahonui

USA
5052 Posts

Posted - 01/03/2009 :  12:44:45 AM  Show Profile
Always clean your flux off after you solder. Alcohol on a cotton swab works swell. Yes, I am nutzoid about that. Ask Paul.

Me ke aloha
Malama pono,
Wanda
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Lawrence
Ha`aha`a

USA
1597 Posts

Posted - 01/03/2009 :  11:54:09 AM  Show Profile
now WANDA!... B E H A V E!!

You should not go around telling people to flux off!



Mahope Kākou...
...El Lorenzo de Ondas Sonoras
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rendesvous1840
Ha`aha`a

USA
1055 Posts

Posted - 01/06/2009 :  1:43:02 PM  Show Profile
She's incorrigible, since she had that "High Reliability Soldering" class several years ago. She critques every solder joint she sees, and makes me show her all my projects.
Paul

"A master banjo player isn't the person who can pick the most notes.It's the person who can touch the most hearts." Patrick Costello
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