Just Big and Lousy

Today’s— uh, victim— JBL LSR2325P active studio monitor. It’s a nice sounding biamplified monitor with an active crossover system and suspicious “Imagine” brand capacitors. Hmmmmm. 😉

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Our music producer came to me with this loudspeaker he uses to play his creations for our news director, among other things, because it was crackling and popping ferociously when the input gain knob was touched. I found the input gain knob loose on the rear panel and guessed I’d also find cracked solder joints. But where?

Input gain control is below the inverted plastic bathtub under that board. So how do you remove this plastic bathtub? Desolder the shitty thermoplastic power switch—- which will melt and eject its metal parts. WTF??!!

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You can see the switch on the panel here – it’s a snap in flange mount – the only way to get around doing this would be to cut away the plastic flange and back it out, I guess. The tub it’s in is sealed so this wouldn’t create an air leak. But still— AARGH!!! Also, WHY THERMOPLASTIC? I have a problem with this. See, if the switch starts warming up, the plastic will soften, removing pressure from the contacts, creating more heat. Eventually the fault will only clear when the switch either totally loses contact or the thermoplastic erupts into flames.

Proper electronic assemblies use thermoSETTING resins. Glass reinforced polyesters/epoxies are nice. These are resins that set either when two parts (a resin and a hardener) are mixed, or enough heat+pressure are applied to kick over a curing reaction. This reaction is a one way process and the resulting product WILL NOT MELT and soften. It may eventually be flammable, but most thermosetting resins, especially glass fibre filled ones, have a very good track record of self extinguishing.

Phenolics are very common in solderable connectors. You can always tell when you’re dealing with a phenolic resin because it will not soften and allow the connector to deform with extended heating during soldering. These resins are often colored teal blue/green, or a tan color on Amphenol products. Ever wondered what the name “Amphenol” is about? 😉

Wikipedia article on thermosetting polymers

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I don’t even want to think too hard about what that AC power inlet fixture is made of, all things considered.

The header pins leading to both the boards inside this tub were also graced with total shit-grade soldering and I reworked them. There’s one board below with the three jacks and one board above with the amplitude pad and the HF/LF trim filters. I resoldered the input pot and tightened the nut around it with some Loctite purple on it. In theory, I probably should have used blue, but I can’t find the blue, and red is right out of the question. Whatever works, right? I’ve had just as good luck with things like this using nail polish on them.

 

That’s the fate that befalls any nail polish I buy that looked GREAT in the store but when I put it upon my claws it turned out all watery looking or otherwise unsatisfying. (“NYC Color”, this means you. Well– some of their shades. Some of their newer ones are actually formulated with, well, color, in them.)

After this– I can’t wait for my assistant here to show up so I can run my fingers through his thermal insulation and hear him make silly happy squeaky meows.

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How to achieve silly smiles… 🐱

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