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Old 4th August 2013, 12:00 PM   #1
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Question Fully Securing Air Core Inductors to Circuit Boards

I am on my second pair of crossovers for my Polk SR6500 car audio speaker set (a 6.5" mid driver and 7/8" ring radiator type tweeter per channel).

The first set was stolen along with other gear, a high speed chase occurred (ie lots of police), the criminal rolled the also stolen vehicle (but it wasn't mine thankfully), and my xovers were tossed about.

I did get them back. They functioned OK for about three months and then one started cutting out the tweeter intermittently and then the other one did too. This was likely a circuit board trace crack issue but that is not my concern at the moment as I do not intend to allow a theft to happen again.

I thought the true story would be interesting and fun to visualize as the convict did some barrel rolls, is in prison, and will be there for quite a while.

Anyway, I want to correct a problem with the crossover design and it is that the air core copper inductors will release from the circuit board (ie the bonding agent, whatever it is, fails after time).

So, as a frequent DIY home auto mechanic also, I was wondering if applying a bead of clear RTV where the copper inductor meets the circuit board would be OK? Most certainly, the RTV can handle the temperatures, heh (-75F to 400F). And, it will bond securely (I use the stuff for tons of difficult to "stick together" substances).

Lookup Permatex product #80050. The link is: Sealants : Permatex® Clear RTV Silicone Adhesive Sealant.

Let me know what you experts think. I attached a photo of what the xover looks like. Each xover has two air core inductors that I would like to fully lock down.

Thanks!
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File Type: jpg Polk_SR6500_Crossover_Unit.jpg (56.2 KB, 181 views)
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Old 4th August 2013, 01:01 PM   #2
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I use a small amount of (two pack) epoxy resin (glue) to hold them in place, with great success.
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Old 5th August 2013, 04:19 AM   #3
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Quote:
Originally Posted by JonSnell Electronic View Post
I use a small amount of (two pack) epoxy resin (glue) to hold them in place, with great success.
Thanks for the info.

I know the stuff you are talking about but do not have any on-hand. I do like to keep my garage stocked with just about everything but cannot think of an easily defensible reason for buying some, heh. "It is for the lug nuts so they won't fall off the car!"

I assume that by not objecting or warning to my use of RTV, RTV will work also?

Thanks again!
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Old 5th August 2013, 07:24 AM   #4
Johno is offline Johno  Australia
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Make sure you use a neutral cure RTV. Some of the more commonly used versions have a highly acidic cure, ie they give off acid fumes in the curing process. You will find this very corrosive to copper. Need to find the ones specifically intended for electronics assembly.

Same can be said for 2 part epoxy resins as well - similar cure phenomena.
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Old 5th August 2013, 11:52 AM   #5
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If you were building from scratch, I'd say "cable ties, threaded through holes in the board under the coils." For the ones you pictured, I'd use neutral cure RTV or hot-melt glue.
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Old 5th August 2013, 12:48 PM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by PrecisionAudio View Post
Lookup Permatex product #80050. The link is: Sealants : Permatex® Clear RTV Silicone Adhesive Sealant.
That one is of the acetic variety, it is not suitable. You should use a neutral one (alkoxy or methoxy)
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Old 5th August 2013, 02:24 PM   #7
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Never had problem with any RTV. It's very handy.
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Old 5th August 2013, 02:32 PM   #8
Mooly is online now Mooly  United Kingdom
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Glue gun everytime... pass me the glue gun, job done.
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Old 5th August 2013, 05:54 PM   #9
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Quote:
Originally Posted by Johno View Post
Make sure you use a neutral cure RTV. Some of the more commonly used versions have a highly acidic cure, ie they give off acid fumes in the curing process. You will find this very corrosive to copper. Need to find the ones specifically intended for electronics assembly.

Same can be said for 2 part epoxy resins as well - similar cure phenomena.
AHA! Excellent information (actually a warning, heh).

Yes, the RTV I use does strongly smell like vinegar (acetic acid) until it fully cures (around 24 hours). So, that is a "no go" and I now understand why. Thanks Johno! Someone else checked out my link to the Permatex RTV product and said that it was a "not-to-use" and I greatly appreciate that. Thanks Elvee and all other respondents too!

I will try and locate a product "specifically intended for electronics assembly" as recommended. Google "neutral cure RTV" to start.

In the meantime...

Any recommendations on a brand/"model" of safe-to-use glue/epoxy before I start digging around the internet and get to the point where a radio shack kid says, "naw, this stuff will work great" (without really knowing). No offense to RS but I've been told some pretty contradictory stuff in the past.

What have the experts here (all the people other than ME) found as a "non-acidic" adhesive?

I do have a hot glue gun and the "sticks" are this white translucent kind which is likely similar to ALL of the different sticks, right? I've only ever had this kind glue for this gun during all of these past years (heh). Well, the problem I've run into using this glue gun and the "sticks" is that the glue will stick (bind the surfaces) but it doesn't take much pressure (a fingernail only) to separate the glue bead from what it is supposed to be bonding... My experience anyway. Other than that, I love the hot glue gun idea!

It is very unlikely that I will be able to get yet another new set of these xovers because Polk stopped production of their model SR6500 a few years ago unfortunately and I really like their smooth sound.

So, I'd like to lock down the inductors, heh.

Also, on a different topic but perhaps within the same discussion group:

Does anyone offer a service (even a private home/garage service) where I could send to them one of my "old" xovers for expert analysis which would be the identification/measurement of all components and the creation of a trace schematic so IF these two fail, I could know the specs in order to build a couple? I do not have the know how or equipment to do so. The resistors and caps are labeled but the inductors, "glow fuse," and other items are not and measuring those, from what I have read, is beyond my capability... I do wish I knew how to as it would be fun.

I greatly appreciate all of the help on this. Thanks all.
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Old 5th August 2013, 06:46 PM   #10
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All you need is s PEAK Atlas LCR meter. You might need to desolder one leg on some parts to get a measurement, but it will quick, easy and you'll learn a lot.
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