Repairing the Harmon Kardon HK570i Stereo

Sunday, July 1, 2007

For quite a long time my stereo has been sounding terrible. This was an expensive system when I bought it, back in 1982. But back then it sounded GREAT!

For most of the last two decades I've been caught up in career and family matters, and recently realized that I just haven't been listening to music, and part of the reason was that the only high fidelity sound I could find came from earphones. Sometimes you just don't want to be tethered down. I decided it was time to fix the stereo, or get rid of it. (Maybe both. After twenty five years an upgrade might not be out of order.)

I cleaned off the workbench, having just finished my mailbox project. I downloaded the schematic for the transceiver off the internet and studied it for a few days. Then I opened it up, hauled out the multimeter, and started measuring voltages. Of course, everything I measured seemed to match what was marked on the schematic, so that didn't reveal anything. I let it sit for about a week.

Back at it again, I knew there was a fool proof method. I'd inject a known signal into the input, and follow it through the amplifier with the oscilloscope until I saw it get changed. Then I'd know what area of the circuit needed repair. Out comes the equipment.

I started with the signal coming out of the amplifier by connecting the 'scope to one of the speaker outputs, and the second trace to the AUX input, where I was injecting the signal. It didn't quite make sense to me, since I was seeing the same undistorted signal on both traces. (Sorry it's so blurry; trust me, except for a tiny delay, the signals are obviously the same.)

Since I wasn't seeing the distortion, I went back and grabbed the speakers and hooked them up. Again, no distortion. I added the CD player and swapped out the function generator. Yup, still there. On BOTH channels! I pondered that result for awhile, and eventually realized that I was only hearing distortion from the low end of the spectrum. I swapped it back, then dialed the function generator down to lower frequencies. Sure enough, there was distortion at a few well selected frequencies.

But the oscilloscope still showed clear and equal traces.

All of this took me quite a few weeks to get through. I'd just dabble at it a few minutes at a time, once or twice a week. Finally, I couldn't ignore what the tests were telling me: there is nothing wrong with my stereo. The problem must be in the speakers. I know there's a cross-over network inside each of them, but I wasn't crazy about having to get the components out of the speaker cabinet. I finally resolved to work on it, and the first step was to remove the front fabric, which is made to be removed and just pops off.

I certainly wish I'd done that as the very first thing, since once I could see the woofers, the problem was obvious. There's a foam ring that used to hold the paper speaker cone to the edge of the speaker, but after twenty five years it has just disintigrated. Why was I hearing distortion on both channels? Because both woofers are wrecked! The speakers were the last element I would have suspected, but now that I've seen them, I know that the mystery has been solved. It didn't take me more than a couple of days to find a guy in town that could sell me replacement foam and the right kind of glue. This is a common problem, and it wasn't long before I had both speakers back in working order.