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AA Crossover severe heat problem


Dave A

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Working on some AA crossovers today and I run across something I have not seen before. Where the two coil wires cross the Zener bracket they are literally melted onto the bracket. The Zeners and the bracket show no signs of severe heat though I have no idea what heat level would be required to melt this old insulation. So crossover guru's, what say you about this?DSC_0001.thumb.JPG.1822bacd0364831752c1659667f278ab.JPG

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9 minutes ago, jimjimbo said:

Dave, in all of the AA's that I have rehabbed I have never seen that.  Following.

Me neither.

8 minutes ago, henry4841 said:

Doubtful the circuit would ever generate enough heat to do that. My guess stored in some place that was extremely hot at one time. 

Well that was my initial thought too as there are signs of wax melt. However why the wire insulation melt just on the Zener bracket and not the whole wire? Otherwise this board is pristine and shows no sign of general extreme heat.

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10 hours ago, Dave A said:

Working on some AA crossovers today and I run across something I have not seen before. Where the two coil wires cross the Zener bracket they are literally melted onto the bracket. The Zeners and the bracket show no signs of severe heat though I have no idea what heat level would be required to melt this old insulation. So crossover guru's, what say you about this?

 

Since the Zener, Zener (RED)wires and Bracket show no sign of heat damage I suspect the Woofer Inductor (which I believe these melted wires belong to) of the AA was overheated by an amplifier output shorting out throwing DC rail voltage onto the Woofer/Inductor circuit. Do you know if the woofer was blown in the speaker at some point. I suppose another option was the speaker/network was used at sustained high level SPL/Power levels over time.  I would be concerned that the inductor could have damaged windings which could shift the crossover function off spec.

 

 

miketn

 

 

 

 

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And the zeners are OK? Correct? As in functional. Just my $0.02

2 hours ago, mikebse2a3 said:

 I would be concerned that the inductor could have damaged windings which could shift the crossover function off spec.

Me too. If there was enough current flowing to do that to the lead wires from an inductor it could easily have melted the enamel insulation inside the windings of the inductor itself.

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First check the amp for oscillation. It probably is oscillation at a frequency you cannot hear.

 

The Zeners are doing their job and clamping the voltage to 5.1v, depending on the amplifier power they can get very hot very fast if it's screaming away at full power. Say for a 50 watt amp they will need to clamp off  28v peak to just 5.1v, they will get hot. Now imagine if it's a 300 watt amp oscillating at full power.

 

Edit: The network will knock the 28v peak input (for a 50 watt amp) down to around 18v by the time it sees the zeners but you get the picture. The diodes will pass anything above 5.1v and dissipate it into heat.

 

Mike also has a good theory but it only looks to be melted where it was in close proximity to the zeners mount/heatsink. If the amp was passing DC it should be in protection or blow a fuse on the amp. If there is not protection or the fuse isn't blowing for whatever reason the woofer would be making very loud noise and the entire circuit for the woofer including inductor would be melted. The black wires look closer in proximity to the zeners, the red wires are barely touching the very bottom of bracket possibly far enough away from the zeners for heat to dissipated enough to not melt the PVC jacket. The black wire passes directly along the bottom of the zeners where it would be at it's hottest part of the bracket.

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Well the crossovers arrived and are now fixed. It would not have taken much heat to melt those wire covers. Bigger then normal diameter and melted with any heat even close by. Hard to even get heat shrink stuff to work without seeing that stuff start to melt. Never saw this junk before and hope to never see them again.

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Dave never said the woofers were fried. That was speculation based on just the picture. I guess Dave was sent the picture by the customer first, and when the crossovers showed up it was just the damaged insulation?

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All the drivers were fine according to the customer. The picture I used was taken here but the problem was so odd I posted on it.

On 8/17/2022 at 12:10 PM, captainbeefheart said:

Mike also has a good theory but it only looks to be melted where it was in close proximity to the zeners mount/heatsink. If the amp was passing DC it should be in protection or blow a fuse on the amp. If there is not protection or the fuse isn't blowing for whatever reason the woofer would be making very loud noise and the entire circuit for the woofer including inductor would be melted. The black wires look closer in proximity to the zeners, the red wires are barely touching the very bottom of bracket possibly far enough away from the zeners for heat to dissipated enough to not melt the PVC jacket. The black wire passes directly along the bottom of the zeners where it would be at it's hottest part of the bracket.

Zeners are in the tweeter circuit. Red wires took soldering heat just fine but those black insulated wires melted like crazy.

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53 minutes ago, Dave A said:

Zeners are in the tweeter circuit. Red wires took soldering heat just fine but those black insulated wires melted like crazy.

 

Yes I'm very familiar with the circuit. The tweeters can only handle a couple watts which is 4Vrms, or 5.6v peak. The diodes start to conduct around 5.1v clamping the voltage to that maximum amplitude thus protecting the tweeter from seeing too high an amplitude and fry it.

 

If I were in your shoes I would strongly advise your customer to have his amplifier inspected by a professional, or at least someone that can hook it up to varying loads (some capacitive loading get's marginally stable amps to show themselves) and see if it's stable.

 

The picture is clear to me that the wire only melted when in contact with the zener bracket/heatsink. This tells me the heatsink is what got hot and it doesn't take a lot to melt the old PVC insulation. The black wires were touching the heatsink in very close proximity to the diodes themselves where it would be hottest. Down below where it has good contact with the wooden board could have dissipated off enough heat to not damage the red wires that look to also be touching but to no extent as much as the black wire. It's not uncommon for amps to become unstable and oscillate at a high frequency we cannot hear, I typically find them between 30kHz-60kHz. The amp could have been putting out full power at that frequency which the zener's would clamp and dissipate off the extra power so it doesn't go into the tweeter.

 

DC from the amp would have caused an awful noise through the woofers and most likely destroy the thin magnet wire of the voice coil rather quickly, if the voice coil held up long enough to melt the wiring it would show in the low pass inductor because it's magnet wire is fairly thin and probably damage easily if the voice coil held up longer. Typically the voice coil burns up pretty fast thought saving the internal components and wiring of loudspeaker.

 

I'm not a betting man but I'd certainly put my money on high frequency oscillation caused from the amplifier that damaged the wires from zeners getting hot from doing their job and protecting the tweeter.

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