Woofers Just Stopped Working?

[sirstinky]

Hi everyone. I am a bit of a noob when it comes to speakers and audio stuff. I have an old Panasonic shelf system. It has a 5-CD changer, tape decks, and 2-way speakers (tweeter I think 3" and a woofer, I think 6") in enclosed cabs. 50W each and 6 ohms. The leads that run from the speakers to the unit aren't removable (unless you cut them). A few days ago, I took the sheet metal cover off the receiver to replace a return spring in the CD-changer door. That's no problem, I'm good with electronics and have done that before. This time however, when I turned the unit on, there was no sound. All the wires were connected correctly. I tried switching them around between the jacks on the unit, but that didn't work. I only connected the low side (woofers) and that didn't work. However, when I only connected the high side (tweeters), they worked on both channels. Only the woofers don't work. Before I had it open, everything worked fine, and it had been a tank ever since I've had it new.

Any ideas on what happened? Is there anything I can test for, like resistance, caps, voltage? I have a multimeter. I don't want to throw this out. I'm good with electronics, so repairs in that respect aren't an issue. Any help is appreciated!

Thanks
Nick

 

[rowdymoody]

I would make sure the voice coils are not blown by pressing in on the woofer with your fingers, if you feel or hear scratching, or if they don't have their springiness and feel like they are sticking, they are blown - this is probably not likely, but a possibility.

Since they worked before you tore the unit open, I would be leaning more towards something got messed up while you had it apart. I would open it back up and check any circuit boards for signs of damage, and make sure any cables are plugged in properly.

 

[sirstinky]

Thanks for the reply. I opened it back up and did a thorough check of all the PCBs and components. All the caps look good, all the solder joints (that I could see) look good. This is a very well built unit. Solid all through-hole with high quality caps and nice solder joints. There's no way for me to check the speakers for problems, but I tried the woofers in the high side jacks and there's sound coming out of them, but no sound from the low sides, so the speakers are fine. I am suspecting another issue somewhere else. Maybe a bad amplifier? I stuck the probes from my multimeter into the jacks while the unit was playing, but there's no voltage signal. Is that normal? I checked the resistance of both speakers and it worked out to 13 ohms. Is there anything else I can check?

Thanks a million.

 

[americanaudiophile]

If the speakers have separate leads for tweeter and woofer input then there are separate amps in the main unit. So it is possible to have a bad bass amp and still get treble.
The woofers are OK since you get sound when you connect them to the tweeter amp terminals.
Could be a bad ground connection or a blown fuse if it has any. Search for a service manual if you want to go deeper into the unit.

 

[sirstinky]

If the speakers have separate leads for tweeter and woofer input then there are separate amps in the main unit. So it is possible to have a bad bass amp and still get treble.
The woofers are OK since you get sound when you connect them to the tweeter amp terminals.
Could be a bad ground connection or a blown fuse if it has any. Search for a service manual if you want to go deeper into the unit.

Hey thanks! I didn't know it had 2 amplifiers. How do I test for a bad ground? They must be really small because when I was inside, I only saw a transformer (probably 4x4" square) and some heat sinks with the usual PCB's with lots of diodes, caps, resistors, a relay, and some quad flatpack IC's. The PCB arrangement is unique with a separate board for the RF module and a main board with a separate PCB that has the display and all the front panel controls. There's a single fuse that comes off the mains input, but that's it for fuses. I will try to find a service manual, but that might be difficult for a unit that's going on 13 years old.

Thanks for the info. Any other suggestions are very welcomed.