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Old 2nd April 2004, 01:26 PM   #1
dougpke is offline dougpke  United States
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Default Distortion after warmup

We have a pair of Peavey DTH S4 PA speakers. Recently, the one cabinet has developed a buckling sound in the woofers, and the horn cuts in and out also. The speaker functions fine until it has ran for awhile (warmed up). Have eliminated the drivers as a cause of the problem, and have narrowed it down to the crossover assembly. The assembly has a 4PDT full range/biamped switch, but the rest of the crossover is pretty much standard design. The solder connections are all good. Any suggestions as to which type of component (cap or inductor) would be more likely to have a heat related failure? Or any other suggestions where to look.....?
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Old 2nd April 2004, 04:06 PM   #2
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Default Re: Distortion after warmup

Quote:
Originally posted by dougpke
Any suggestions as to which type of component (cap or inductor) would be more likely to have a heat related failure? :
First guess is the caps. They can fail two ways: Either a disconnection of circuit(no sound) or a short (full frequency delivered to the tweeter)

The only failure I'm familiar with (not actually seen this) in a coil is the magnet coating somehow becomes worn and creates a short (full frequency delivered to the woofer)

I can't comment on the switch but I would also try by-passing it to test.

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Old 2nd April 2004, 04:40 PM   #3
markp is offline markp  United States
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The switch is most likelt the problem. As it heats up the metal parts expand a lose contact with each other then cool and make contact again then heat, cool, heat, cool....
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Old 2nd April 2004, 05:09 PM   #4
dougpke is offline dougpke  United States
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The switch is never used (it's locked in the full range position), but both the low range and the high range circuits pass through the contacts of that switch more than once. There is no noticable damage to any of the components (signs of heat), so perhaps I will jumper over the switches contacts, reassemble and try it again. Due to the size of the cabinets, I hope the switch is the problem. I have to reassemble and transport these cabinets back to the club, and then haul the replacements back home each time I'm wrong
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Old 2nd April 2004, 06:01 PM   #5
markp is offline markp  United States
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Yep, bypass the switch with hardwire. Even though it is not switched the contacts can still heat up due to the current running through them. Since you dont use the swoitch it's no big deal to bypass it.
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Old 2nd April 2004, 07:14 PM   #6
Svante is offline Svante  Sweden
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I'd also bypass the switch as a first option. The symptom you describe sounds just like a bad contact.
Keep us posted on wether if this was the problem!
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Old 5th April 2004, 01:33 PM   #7
dougpke is offline dougpke  United States
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Thumbs down Back to the Drawing Board

Removed the switch and installed buss wire jumpers on the PC board. While there was no distortion from the woofers the horn started cutting out after it had warmed up. Pretty much leaves two caps in series, with a resistor in parallel with one of them, and an open air coil across the leads leading to the high end driver. Whatcha think, the caps next?.....
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Old 6th April 2004, 01:47 PM   #8
markp is offline markp  United States
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Nope, it has to be the resistor. When they heat up the resistive element expands and contracts on cooling making it crack with time. It will measure good when cold then be an open when it heats. Take a meter to it when the problem starts.
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Old 6th April 2004, 03:25 PM   #9
dougpke is offline dougpke  United States
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But the resistor is only in parallel with the first cap. Wouldn't the signal still be reaching the driver? I was under the impression that the resistor was only there for adjustment of some sort.
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Old 6th April 2004, 03:40 PM   #10
markp is offline markp  United States
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Yes but the cross over freq will change dramatically causing some serious distortion. I've never had a cap cut in-and-out on me but I've had tons of resistors go.
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