"Tar" filled crossovers

[voice of the theater]

Last year I recapped the crossovers on my Heathkit AS 101's and they now sound phenomenal (Altec Valencia drivers in Heathkit Cabinets with Heathkit crossovers). I also own a pair of Altec Valencia 846A (16 ohm Valencias from the late 60's with the green drivers). I've been told that inside the 846 A crossover "can" (it looks like a tin can) all the crossover components are covered with a tar/asphalt type goo. From what I understand, many of the older Altec crossovers had this type of tar/goo inside them encrusting all the components. My question is: what is the purpose of this goo? Is the theory that if you thoroughly encrust all the crossover components inside the "can" with this goo that this coating shields the components from dust and oxygen (oxidation?)? Wondering if it's similar to why the La Brea Tar Pits preserve/protect animal and plant remains? I have noticed that the crossover pots on my (over 40 year old) 846A Valencia speakers are whisper quiet--you can rotate them while listening to white noise (or music) and they don't have even a trace of static while being rotated and they don't cut in or out like other crossover pots that I've owned in the past. That got me wondering--does the tar/asphalt type coating keep dust and oxidation to a minimum inside the pots/crossovers? Or is the fact that 40 plus year old pots are still whisper quiet (and I've never cleaned them) just due to the fact that they really good quality pots (which I doubt--I don't think they are any better than other pots from the same era)? Just wondering why the need for the GOOP? Thanks in advance for any answers regarding this--Inquiring minds want to know......

[bowtie427ss]

Just wondering why the need for the GOOP?

My guess is that vibration isolation, and patent infringement prevention were at or near the top of the list.

Might also have been done to prevent user serviceability/promote business for authorized repair persons.

I'm sure it served multiple purposes much akin to the epoxy in a CDI ignition module. Most fail as a result of a 30 cent cap going bad, the epoxy ensures that you'll spend 100-500 bucks to replace the entire module which probably contains 5 bucks worth of components. I call it "marketneering".

[bfish]

Encapsulating components or circuits in tar (or other pourable hardening materials) is called potting. It hearkens to the pre-PCB days of point-to-point wiring, and was done to fix the component(s) in place to protect them, and especially the solder-joint connections from failure to vibration and shock. In the case of crossovers that are either mounted in a speaker enclosure or at least on one, they're definitely subject to vibration. It was the equivalent of the modern technique (called staking) of securing components to a PCB with zip ties, hot glue, etc.

Added
In another life I tested radar and fuzing sub-assemblies for a defense contractor. After testing, nearly all those assemblies were potted to ensure their function in high shock/vibration environments. As Bowtie mentioned, CDI ignition modules are also potted even today. Imagine how long a printed circuit board would last fastened to your engine if they weren't.

Added later
In addition to potting and staking, there's also a newer technique called conformal coating that uses a layer of transparent thermosetting resin to hold components rigidly in place. This method offers a degree of serviceability.

[Old Guy]

My guess is that vibration isolation, and patent infringement prevention were at or near the top of the list.

Might also have been done to prevent user serviceability/promote business for authorized repair persons.

I'm sure it served multiple purposes much akin to the epoxy in a CDI ignition module. Most fail as a result of a 30 cent cap going bad, the epoxy ensures that you'll spend 100-500 bucks to replace the entire module which probably contains 5 bucks worth of components. I call it "marketneering".

Military electronics are often "potted" in epoxy.

Ups reliability, but item then becomes unserviceable.

[robertbartsch]

...Frankly, I've swapped out the old caps in several very old crossovers and I cannot say the systems now sound better.....

That said, you probably want to know if the tar in the crossovers can be safely removed so that you can swap caps. In this regard, I thought I saw some posts on this on the other Lansing cite and, as I remember, it involved heating the components in a very low oven - say 200F for a while.

...anyway, I do remember reading this thread and I think it even had pictures, so the subject of cap removal in tar-filled crossover units has been discussed.

[voice of the theater]

Thanks everyone for the replies--I guess potting works since my 40 + year old Valencia crossovers still sound/work great. As far as removing the tar to re-cap the crossovers, they sound great now so I'm going to leave them as is. The fact that both Valencias sound identical to each other also makes me want to leave them alone. My AS 101's sounded different from each other--that was a major motivation for the overhaul. One sounded a little brighter/crisper than the other until I installed all new capacitors and now they sound identical to each other--both have nice crisp highs, smooth mids, and nice deep lows (with a little EQ "magic"). I'm afraid that if I bake the Valencia crossovers and pry the cans open and try to remove/replace components I'm in danger of accidentally ruining one or both crossovers. I have trouble baking a frozen pizza to perfection I still wonder if potting the crossovers in tar has the (unintentional?) effect of protecting the inner components from dust and oxidation besides the intentional effect of protecting them from vibrations etc.....

[robertbartsch]

I'm restoring a set of Velencias "Bs" now and the biggest improvement you can make is swapping the HF diaphragms with new GPA units.

....had a pair of Heathkit 101s for +30 years.....loved them too!