Klipsch ProMedia 2.1 PC speaker repair - Page 1
Forum section: electronics / repair

author: nsd_c
author_rank: Contributor
timestamp: December 06, 2018, 02:07:32 am
content: Hi all,I have a set of Klipsch ProMedia 2.1 speakers on which some of the driving elements of one of the “HF” amplifier boards blew. Schematics are available onlinehere, thanks to Thompson Dale. (There are two “HF amplifier” boards; each drives one of the left and right satellite speakers. A separate “LF amplifier” drives the subwoofer.)I “repaired” the HF amplifier board by making an entirely new PCB (a copy of the original board with all new parts) and the system now works, but it looks like one of the satellite speaker mid-range drivers took a hit when the output transistors blew. I’ve isolated the fault to the driver by swapping left/right satellite speakers and even transplanting the suspected bad mid-range driver into the proper-functioning satellite speaker cabinet (to rule out the cross-over between the tweeter and mid-range driver).The “bad” driver technically works, but makes an awful buzzing noise at upper harmonics of the driving frequency.I supplied a tone to the system and recorded the sound from each speaker separately (using a dummy load in place of the other speaker). I've attached a plot showing a comparison between FFTs of each speaker playing the same 500 Hz tone.Note all the harmonics that appear on the bad speaker that don’t appear on the good speaker.The distortion between the good and bad speakers is plainly visible when looking at the recorded audio waveforms themselves (see attached).I tried this test with a range of sub-kilohertz frequencies and the results were all similar. I also tested the tweeters in the same fashion using a 10 kHz source signal as well, and they both looked fine.Anyone have any idea what would cause this in the bad speaker?I suspect that the bad speaker may have been subjected to +26 VDC for however long when one of the output transistors failed (if it shorted to the rail). The bad speaker measures ~5.2 ohms @ DC and the good speaker measures ~5.3 ohms accounting for lead resistance. (So about the same.) When I play a tone through the bad speaker, it's like the driver is rattling (as if it's physically damaged in some way), but it looks fine to the eye.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 06, 2018, 03:37:04 am
content: It's probable a few turns of the voice coil have gotten so hot their glue failed and they have separated from the former. Now that they are loose and free to move around what you hear is them rubbing against the magnet under drive.How did you make your FFT plots?

author: nsd_c
author_rank: Contributor
timestamp: December 06, 2018, 04:00:10 am
content: Quote from: cvanc on December 06, 2018, 03:37:04 amThat's interesting... I didn't know that this was a possible failure mode.[Edit #2]: If I push on the dust cap of the good driver, I can't hear or feel any rubbing action; it moves very smoothly. If I try to do the same on the damaged driver, I can feel and faintly hear rubbing. I'm going to say that you're probably very correct! Thanks for the info!I have ordered a replacement speaker from Klipsch (they wouldn't sell me just the driver). About $37-38 USD shipped IIRC; $26 for the driver, ~$8 for shipping, plus tax. When I get it, I want to make sure that the driver impedances/efficiency haven't changed (so that the left and right sound similar enough). If they're a good match, then the old driver is I guess trash, and so I could cut thesuspensionsurround/spider [edit: use the right words] up to take a look at the voice coil, I suppose.[also edit]: Is there a solvent that works well to dissolve the adhesives used on the surround/spider? I have a specialty solvent which softens super glue as well as garden variety solvents like mineral spirits, acetone, isopropyl, paint thinner... Would any of those be of use?Quote from: cvanc on December 06, 2018, 03:37:04 amI opened the .wav file inAudacityand chose 'Plot Spectrum...' from the 'Analyze' menu. I used the default window function (Hanning) and 'size' parameter (512), then exported the FFT of each .wav to separate text files, which I then plotted in gnuplot with the following:Code:set terminal png size 1200,400set output 'midrange_satellite_speaker_500Hz_comparison.png'set label 1 '500 Hz' at first 500,5 left rotate by 90set label 2 '1000 Hz' at first 1000,-30 left rotate by 90set label 3 '1500 Hz' at first 1500,-7 left rotate by 90set label 4 '2000 Hz' at first 2000,-31 left rotate by 90set label 5 '2500 Hz' at first 2500,-11 left rotate by 90set label 6 '3000 Hz' at first 3000,-42 left rotate by 90set label 7 '3500 Hz' at first 3500,-30 left rotate by 90good='good_spectrum.txt'bad='bad_spectrum.txt'set xlabel 'Frequency (Hz)'set ylabel 'Power (dB)'# easy and convenient way of skipping the first line of the file# (on which Audacity sticks column headers as text)set key autotitle columnheaderset yrange [-100:20]set xtics 2000 # major tic every 2 kHzset mxtics 4 # 4 minor tics for every major ticset grid xtics mxtics ytics # Grid lines on x tics, x minor tics, y ticsplot [0:22000] \good w l title 'Good driver', \bad w l title 'Bad driver'

author: nsd_c
author_rank: Contributor
timestamp: December 07, 2018, 06:53:15 am
content: Update: I watched/read a number of different videos/articles on speaker repair. The general guidance seemed to be that a voice coil rub could just be the voice coil being slightly off-center, which reportedlycan sometimes befixable with a variety of fixes, such as by removing the dust cap, shimming around the inside perimeter of the voice coil, then using acetone to soften the spider so that it hopefully changes shape enough that the rub goes away. (If I understood correctly...)I decided to try removing the dust cap (as this was supposedly possible to do in non-destructive fashion using a solvent). I was successfully able to use acetone to soften the adhesive around the perimeter of the dust cap enough that I could use a very thin/sharp (new) X-acto blade to separate the dust cap without damaging it or the cone. What I then saw was not encouraging: the aluminum former onto which the voice coil was wound had "ripples" in it, as if it were heated to the point that it deformed (). (I don't think it should ever look like this...right???)However, in the process I accidentally spilled acetone on the foam surround (), which instantly deformed it. Between the voice coil looking trashed and the surround now also trashed, I cared much less about trying to salvage the speaker. I cut the surround away but tried to salvage the cone and spider by using more acetone to remove the spider from the basket, but the acetone seemed to have no effect. I then damaged the cone () somehow to the point that it, too, was trash--probably because it was soaked with acetone it was soft, and I pushed too hard on it accidentally while trying to apply acetone to the spider.So, with now every component of the speaker trashed except the basket, I gave up and cut away the spider to allow me to just pull the cone/spider/voice coil assembly out.cvanc turned out to be correct. The voice coil was over-heated: the enamel-coated wiring can be seen in the attached photos to be burned looking (though somehow it did not short), and kind of expanded such that the wires are no longer parallel.Still, an interesting autopsy! Also, I at least have the basket and the dust cap! /sLinks:Speaker Failure Analysis, by Phil Allison and Rod ElliottSpeaker Rebuilding in 12 Easy StepsProSoundWeb forum thread, "Proper solvents/adhesives for reconing"Classic Speaker Pages forum thread, "Glue removal tips"AudioKarma.org forum thread, "solvents for removing old foam"