Ancient Harman-Kardon PC Speaker Re-Capping

HK Powered Speakers - front view

HK Powered Speakers – front view

Suddenly a resonant thwup-thwup-thwup-thwup fills the house, but no helicopters fill the skies; in fact, most of the noise seems to be inside the house and … it’s coming from the shop. We look at each other and dash toward the basement door, knowing perfectly well that this is the part of the movie where the audience chants “Don’t open the door! Don’t open the door!

Come to find out that it’s the pair of old Harman-Kardon powered speakers attached to the PC attached to the Thing-O-Matic; the PC is off, but I left the speakers turned on. Quick diagnostics: turning the volume down doesn’t reduce the motorboating, pulling the audio cable out of the PC doesn’t change anything, the only cure is to turn them off.

Under normal circumstances, they’re pretty good-sounding speakers, at least to my deflicted ears, although I have my doubts about the effectiveness of that reflex port. I plugged in a pair of unpowered speakers as subwoofers down near the floor, just because they were lying around; a pair of 75 mm drivers does not a subwoofer make, fer shure.

Pop quiz: what’s wrong?

Need a hint? Looky here:

HK Powered Speakers - wall wart

HK Powered Speakers – wall wart

Disassembly:

  • The front cloth grille has four snap mount posts, two secured by hot-melt glue blobs: pry harder than you think necessary
  • Two screws near the top of the bezel thus revealed hold it to the back
  • The bottom two screws holding the driver frame in place also hold the bezel to the back
  • Remove two screws from the grooves in the bottom of the back
  • Amazingly, the driver has two different size quick-disconnect tabs; the neatly polarized wires slide right off

Cut the audio cable just behind the back panel, then push the two-piece cable clamp outward from the inside:

HK Powered Speakers - cable grommet

HK Powered Speakers – cable grommet

The bottom of the circuit board shows considerable attention to detail. Note the excellent single-point ground at the negative terminal of the big filter capacitor:

HK Powered Speakers - PCB foil side

HK Powered Speakers – PCB foil side

And, of course, that’s the problem: most of the electrolytic capacitors were dried out. My ESR tester reported the big filter cap (downstream of the bridge rectifiers) as Open and several of the smaller caps were around 10 Ω. Replacing them with similarly sized caps from the heap solved the problem.

It should be good for another decade or two…

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  1. #1 by jim oslislo on 2012-12-19 - 08:21

    I am curious about testing capacitors in and out of circuit. I was under the impression that an electrolytic cap removed from it’s circuit could seem perfectly good under the applied voltage of a tester and yet fail when subjected to the higher voltage in use. Is it possible to test a cap without removing it and without having access to the circuit diagram?

    • #2 by Ed on 2012-12-19 - 09:50

      test a cap without removing it

      Absolutely!

      A good electrolytic cap’s equivalent series resistance (ESR) is usually a fraction of an ohm, but a dried-out old cap will have an ESR anywhere from a few ohms to infinity. The capacitance remains pretty much unchanged, which means you can go through all the labor of unsoldering it, only to find that it still measures 100 uF and seems “perfectly good”… even though it’s dead.

      I built an Arduino-based ESR tester and wrote it up for Circuit Cellar in October 2009 (files from their FTP site). It rams a square-wave test signal through the capacitor from a low-impedance source and measures the resulting voltage across a sampling resistor. The voltage is so low that it doesn’t turn on any junctions and the impedances are so low that the circuitry around the capacitor looks like an open circuit.

      You can find DIY analog ESR testers on the Web (like that one); I used an Arduino so I could talk about magnetics and curve fitting and suchlike… [grin]

      I use it a lot on old equipment!

    • #3 by madbodger on 2012-12-19 - 10:15

      You’re right on two counts: there are some situations where you can’t fully test a capacitor in-circuit, and many capacitor testers don’t test with full operating voltage. In this case, neither of these applied – it was obvious that the capacitors were bad with the first test, so more testing was unnecessary.

      ESR testing like this is a (somewhat) recent concept, so many old articles and books don’t mention it, and just refer to capacitance and leakage tests, both of which are trickier in-circuit.

      • #4 by Ed on 2012-12-19 - 12:04

        obvious that the capacitors were bad with the first test

        Heck, the ESR test just confirmed the diagnosis: anything that old has bad caps!

        After I extracted the caps from the PCB, the capacitance meter reported pretty nearly the right values…

  2. #5 by hexley ball on 2012-12-19 - 12:11

    We use the same model speakers on my wife’s computer, and they have a very bad case of RFI susceptibility. Even when the computer is off, the speakers (always powered up, of course) emit a whine if someone is using a mixer in the kitchen. And don’t even ask what happens when I fire up the HF transceiver…

    Once upon a time, i opened them up and started adding bypass caps. Got the noise down by 20 dB or so, but never completely killed it.

    Have you heard something similar? Or maybe my unit is, uh, special :-)

    • #6 by Ed on 2012-12-19 - 12:52

      Have you heard something similar?

      I just tried an HT and can report that a 146.52 MHz carrier @ 5W from about three feet causes a bizarre growling. The sound varies strongly with antenna orientation and distance, so I’d say the power amp in those speakers might be useful as broadband RF detector…

  3. #7 by Red County Pete on 2012-12-19 - 12:29

    My 20″ Princeton monitor has the problems associated with bad caps–built circa 2004, and the symptoms are right. The partly disassembled monitor and a kit of replacement caps is waiting in the workroom in the barn for that round tuit. Part of that is cleaning up the rubbish from a couple of electronics salvage projects.

    Assuming the Princeton recapping works, I’m going to take a close look at reviving my 1991 Mac Classic II. I have some synthesizers I’d like to hook up again, and an interface that doesn’t play well with modern equipment.

    • #8 by Ed on 2012-12-19 - 12:53

      and an interface that doesn’t play well with modern equipment

      A simple matter of software, assuming you could get the connectors to fit. [wince]

    • #9 by William on 2012-12-19 - 14:58

      “Assuming the Princeton recapping works, I’m going to take a close look at reviving my 1991 Mac Classic II.”

      I used to have to tweak the 5V power supply pot a smidge (they left a hole open in the shield for this, yay engineers that care) every 2-3 years on my aunt’s Mac Classic. It would drift just enough out of spec to make the system go wonky. Maybe your Classic II just needs a slight twist of a pot?

      • #10 by Red County Pete on 2012-12-19 - 16:44

        Re interface: It uses the Gottverdamnt AppleTalk interface. I’ve seen an article on shifting the interface (a Mark of the Unicorn Midi Timepiece, aka MOTU MTP) to USB (the MTP runs at 100kB), but that means any files on the Mac are lost. I’m a lousy keyboard player, and I’d like the computer help. Lack of budget takes non-free solutions off the table, and the OS/free solutions ain’t set up for the MTP. I’ve thought about the programming, but life is too short to get up that learning curve, and MOTU isn’t very helpful in giving out information on the MTP.

        Re Mac: the Classic and the Classic II are two rather different beasts. The C II was deliberately crippled (10M ram, max), with the 5V supply being intertwined with the flyback supply for the CRT. Current info on the web says it’s likely capacitors, and at 21 years old, it makes sense. (It booted once a year ago, but never fully loaded, then gave a funky stripe
        pattern on the CRT on all subsequent power ups.)