But, the AC Power cord along with a bunch of wires were routed into the speaker enclosure with some kind of adhesive glue to seal the gaps around the wires. I needed to remove the sub-woofer to see what was connected to the bunch of wires. Once done, there was this small PCB with just a fuse fixed inside. Also hidden away from view was the AC Power Transformer, which was fitted inside the same wall as the sub-woofer. It would be foolish to replace it with a new one without investigating why it blew. So, I proceeded to remove the transformer, which was held in place with two screws. Since the wires weren’t long enough, I had to remove the fuse holder PCB too and then desolder the primary connections of the transformer from the PCB. When I finally removed the transformer, I found the primary winding burnt and damaged. So, the transformer needed to be either replaced or re-wound. I chose the latter since the laminated core appeared to be of good quality. The label on the transformer indicated that it was a 230V-12V step down. In the meanwhile, I decided to check out the amplifier PCB. This was quite interesting and educative too. There was a very versatile audio amplifier, TDA7377, which is a quad audio power amplifier IC, and can be used in a variety of configurations. Then, there was LM1036 that contains treble, bass and balance circuits, and TL074 which is a quad FET low noise amplifier IC. A discrete diode bridge generated 12V DC, fed directly to the TDA7377, while other ICs were fed 9V obtained through an LM7809. I proceeded to download all the datasheets of these devices to enable me to check for shorts, voltages, signals on critical pins. #ALTEC LANSING ATP3 SPEAKERS ONLY DOWNLOAD#
Most importantly, I checked for shorts/low resistances between each of the four outputs of TDA7377 and GND. The resistance between 12V and GND was more than a k-ohm. Similar checks on other ICs also revealed an apparently healthy condition. I therefore proceeded to check the board by connecting a 12V battery. There was neither any sound nor power ‘ON’indication. I had to therefore trace at least parts of the circuit on the PCB to identify specific test points for identifying a fault.
It was during this process that I discovered that the ‘Power ON’LED was connected by tapping the transformer secondary winding instead of the rectified DC. A separate diode was used to generate a half-wave rectified DC for the LED.
I therefore connected the battery at the AC terminals of the diode bridge so that the battery could feed the LED too. The LED now lit up! As I moved the volume and tone controls, I could hear a faint crackle on the speakers, indicating that the board was working fine.
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