EEVblog #1322 - JBL LSR308 Studio Monitor Speaker REPAIR

[EEVblog]

Repairing a JBL LSR308 studio monitor speaker.
Digital troubleshooting in a speaker? Yep, that's a thing!
How to systematically track down a fault and *know* that part has failed.

00:00 - Introduction
01:55 - Inside
04:42 - The chips
07:05 - Voltage checks
07:58 - Safety protection
08:18 - Oscilloscope time
10:29 - ADC checks
11:59 - Signal injection testing
13:08 - Power amp DSP debugging
15:35 - External load checks
19:21 - Suspect identified
20:05 - Rework Desoldering
24:28 - Component replacement soldering
27:01 - Winner Winner Chicken Dinner



Full 55min video:

[Tek_TDS220]

There is no adhesive between the copper traces and the board.  Boards are made by pressing copper sheet to layers of prepreg in a heated press.  The epoxy in the prepreg adheres to the copper as the epoxy cures.

Informative video as usual.  I prefer the shorter format, but I'll check out the longer version.

[langwadt]

I've cut the pins with a scalpel but looking at it so close up the width of the cut does seem to put stress on the pads 

another method I've heard of is to thread a wire under all the pins and pull on that to to lift each pin in order while melting the solder

[EEVblog]

There is no adhesive between the copper traces and the board.  Boards are made by pressing copper sheet to layers of prepreg in a heated press.  The epoxy in the prepreg adheres to the copper as the epoxy cures.

Yes, the epoxy is the glue.

[WattsThat]

As Dave was speculating on the ‘350 pin polarities, power rail levels and then scoping the led signal line, was i the only viewer screaming at my screen “Compare it to working unit!!!”. Holy good unit Batman, use the resources you’ve got.

As for the cutting the package leads to release the body of the chip, I will attack that with one of two methods, depending upon working clearances. If there’s room, my preferred method is to cut the pins with an Dremel abrasive cut-off wheel. If that doesn’t work, use a pair of micro fine tip dykes or cuticle nippers which are great for dealing with fine pitch parts.

[EEVblog]

As Dave was speculating on the ‘350 pin polarities, power rail levels and then scoping the led signal line, was i the only viewer screaming at my screen “Compare it to working unit!!!”. Holy good unit Batman, use the resources you’ve got.

I did actually do that in the end and confirmed the polarity.
Some people may not have the luxury of a working unit, and a good repair/debugging comes from not assuming you have that luxury.

[CChin254]

I did an IC Identification based on the images uploaded to Flickr.

[thm_w]

Surprisingly, the speaker is actually grounded, and not just connected to negative output. IMO its nice to see, as so much old audio stuff had ungrounded metal casing and just did not seem safe to me..

[VK3DRB]

Coincidentally, I just debugged a Studio Precision ASP speaker unit for someone. These are expensive high-end speakers.

Symptom: Distorted squawk sound at power up, volume low.

Design: Has four LM3886's PA chips. One drives the tweeter, other three drive in parallel (current sharing) the main speaker. Schematic found online... http://www.tangible-technology.com/schematics/Event/SERVICE%20MANUAL_ASP6_ASP8.pdf

Debug Technique: Heatsink gets very hot very quickly without audio input. Suspect one of the LM3886's have failed. The split power supply rails looks clean and at the correct levels on an oscilloscope. No signal at the amplifiers' input. Speakers' resistances were OK. Pulled back each LM3886 off the heatsink (don't even need to remove the PBCA) to see which one gets hottest quickest when powered on for a few seconds. There is no electrical effect in not heatsinking these devices briefly. The tweeter's LM3886 barely got warm. One of the main speaker LM3886's chips got very hot; the two others got very warm. Powered off, removed the hot LM3886 by bending it repeatedly. Powered up again and the two other LM3886's did not get warm. When re-attached to the heatsink, providing an audio source produced good sound; proving the other LM3886's were not damaged. Capacitors etc around the dud LM3886 are fine.

Conclusion: The LM3886 culprit was found and a new chip has been ordered. An enjoyable and easy debug. Always helps to have a schematic diagram!

Footnotes: What is weird is these speaker units worked fine, but were left in storage at room temperature for 2 years after which one speaker unit failed immediately upon power up. The power supply design does not lend itself to efficiency and is a little cheap 'n cheerful IMO.

[CChin254]

I'm honestly surprised that these high end speakers don't use high quality audio capacitor judging from their Bill of Materials.

[EEVblog]

Surprisingly, the speaker is actually grounded, and not just connected to negative output. IMO its nice to see, as so much old audio stuff had ungrounded metal casing and just did not seem safe to me..

Yes, noticed that on a replay.

[VK3DRB]

Why didn't you order the chip variant that had the thermal pad at the top? It is possible the original design was just for that but the parts with the thermal pad at the top were not available, so the manufacturer used one with the pad at the bottom. A quick check would tell you if that pad is ground or not.

As for replacing the chip, I would have tried a pre-heater and hot air at a low airflow to remove the chip, but your Exacto knife is a good technique but does not work too well if the copper has mask over the area underneath, although you could scrape it off and then try thermal transfer.
 
As for the pin registration, it is the house painter's dilemma - is near enough good enough, or do we risk making it worse by trying to make it perfect?

My fracturing technique worked OK for the LM3886's on the speakers I just worked on because the pads were quite large and at one side of the module and it was easy to stress the pins' weak spot and the junction of the package.

Winner, winner, chicken dinner - a great feeling when one has debugged a problem and got it working!

[bsfeechannel]

I'm a big fan of repair and restoration videos. They include teardown, detective work, research and building techniques.

They always tell a story.

[EEVblog]

Why didn't you order the chip variant that had the thermal pad at the top?

Because it's good practice to use the same part and same thermal technique as designed. If I used the other type then I would have had to explain my reasoning to a hundred commenters asking why I did that instead of using the "proper" part.

[EEVblog]

I'm a big fan of repair and restoration videos. They include teardown, detective work, research and building techniques.

I know people like repair videos, but this one didn't seem very popular. I guess people saw it was a speaker and just went 

[Clear as mud]

Hmm, interesting.  I haven't done much repair work, and usually if I get to the point that I suspect a chip is bad, I just give up, assuming that whatever killed that one may have killed others too.  But you had some pretty good arguments for the other chips still being good, based on observations and measurements.  I liked that.  I may try going a little further next time I run across something that seems to have a bad chip in it.