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Topic: What Could Be Wrong With This Passive Crossover? |
b0b
From:
Cloverdale, CA, USA
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Posted 29 Mar 2016 1:15 pm |
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I picked up a pair of PA cabinets real cheap. They work fine at low volume, but when it reaches a certain volume, there's a loud distortion noise coming from one of them. I've confirmed that the speaker is good and replaced the horn driver with a brand new one. The problem persists, so it must be in the crossover.
Everything looks fine - no burn marks or bad solder joints. The glass tube part says "LIGHT" on the circuit board. The filament in it looks intact but it doesn't light up.
I've never worked on a passive crossover before. What could be wrong with this? Is there something I could troubleshoot with a multimeter? Need advice.
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Georg SĆørtun
From:
Mandal, Agder, Norway
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Posted 29 Mar 2016 2:25 pm
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Suspecting the primitive high frequency "peak protector" - the light bulb - to be broken.
I guess that bulb is supposed to warm up / light up during short peaks and thereby increase series resistance for the horn. If it doesn't work as intended the horn will distort and eventually blow at high levels. The filament may be partly or entirely "shortened" as a result of heat and vibration, so it doesn't heat up and change resistance as it is designed to. It may even have failed so it acts as a diode above certain levels, which certainly would explain the distorted sound.
There may be more to that circuit than I can read from the picture, but without a diagram that's my best guess. |
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Craig Baker
From:
Eatonton, Georgia, USA - R.I.P.
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Posted 29 Mar 2016 3:41 pm
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Bob,
Un-solder one lead from the lamp and check it with an Ohm meter. Beyond what Georg mentioned, the insulation of one of the capacitors may be breaking down creating a temporary short at high volume levels. The varnish on the magnet wire used in the inductor may also be breaking down. First, I would substitute known capacitors and see if that helps.
Craig
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Georg SĆørtun
From:
Mandal, Agder, Norway
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Posted 29 Mar 2016 4:01 pm
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| Need a very, very, sensitive ohm-meter to check the lightbulb and compare the reading with that on the other bulb in the speaker pair. Lightbulbs show seriously low ohm when cold, so won't be much difference to measure if the filament has shorted. It is only when the filament gets heated by the power flowing through it that its internal resistance rise - if it is OK. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 29 Mar 2016 4:11 pm
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Are you using the 1/4" connectors or the Neutrik ones? Crappy 1/4" connectors can lose their tension... on bass peaks you can get intermittent connection. Neutriks normally don't do this... just a thought.
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b0b
From:
Cloverdale, CA, USA
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Posted 29 Mar 2016 4:31 pm
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I pulled the light bulb out of its socket. It measures 2 ohms, so it's not open. Writing on it says 24v, 75w. 75 watts! I had no idea such things existed.
Gotta go to rehearsal. Project continues tomorrow.
_________________
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Georg SĆørtun
From:
Mandal, Agder, Norway
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Posted 29 Mar 2016 5:00 pm
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| Looks OK, and values are more or less as expected. Swapping those bulbs between the two filters and listen, is of course the easiest way to determine if there's anything wrong with one of them. I would also measure those bulbs in the sockets just to rule out bad connectors. |
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Tony Prior
From:
Charlotte NC
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Posted 30 Mar 2016 1:15 am
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well, are you sure they ever worked right ?
just a simple thought from a simple person ! 
Years back when I was working on a vacuum system for a 69 Corvette , which I could really never get to work correctly, a good friend who was a very savvy Corvette tech said to me.." You know these vacuum systems never really worked well even when they were new, right " ? He kinda ruined my day !
uhmmm.....
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Jerry Erickson
From:
Atlanta,IL 61723
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Posted 30 Mar 2016 6:00 am
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| Is it possible that the distortion you're hearing is the cabinet rattling? If you have a rubber mallet, try hitting the sides, top and bottom of the cabinet to see if there is a loose brace or if there is any delamination of the wood in the cabinet. I'm guessing that it's using plywood for the cabinet. |
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Tim Marcus
From:
San Francisco, CA
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Posted 30 Mar 2016 11:17 am
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that bulb is there for protection purposes and will only light up during overload.
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