Archive for December 19th, 2012
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:
- 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:
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:
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…