This is a repair of Bose L1 tower PA system. These skinny tower speakers are somewhat of a marvel because they are designed to stand behind the musician (aimed RIGHT AT the microphone) and don’t generate feedback (in most situations). Ah, the miracles of DSP.
It turns out these PA’s have a frequent failure in their power supply. Read on for details.
UPDATE 7/7/2021: Since I first published this, I’ve gotten a lot of inquiries from people regarding repairing their L1s. For local people to northern CA, I’ve swapped working units for their bad ones plus cash. Then I try to fix the dead one I took in trade. All together now I’ve worked on four units and succeeded fixing three of them. I’ve also successfully repaired an aux supply board that someone extracted from their L1 and sent to me (Unit 3 below). For those trying repairs at home, here are my results (I’ll try to update as I see more units):
Firstly, ALL FOUR failures have been in the “Auxiliary Power Supply Board” (read below for more details). I recommend starting trouble shooting there but please note the dangers of troubleshooting circuits that are hot to the AC line (noted elsewhere in this post).
Unit 1: My first attempted repair which started this blog post – D608 open – FULLY REPAIRED
Unit 2: D601 and D608 bad – FULLY REPAIRED
Unit 3: D601 bad – FULLY REPAIRED
Unit 4: D609 bad but also Transformer T601 open between pins 6 and 8. This one’s not waking up unless I find a transformer somewhere. Also, it may have other problems that killed the transformer.
back the original blog post:
The tower speakers slide into the base where they are both supported and electrically connected to the amplifier. All the circuitry is in the base.
The bad news is if you have a problem with this first generation L1 (ours is about 10 years old), Bose is zero help. They will no longer repair them and only implore you to get a new system. Right. To make matters worse they are also completely uncooperative about supplying schematics (though I was able to find that on-line).
Our L1 suffered a power supply failure, which is apparently fairly common as these units age. The culprit is the “Aux power supply” shown in the block diagram at this link.
The below linked doc has schematics and instructions for disassembly. Even though it provides guidance, still take LOTs of pictures. Especially of where connectors plug in.
I pulled the base apart, which is no small task. There are screws everywhere, and you have pull almost everything in the base apart to get at anything useful. Below shows the top cover and control panel PCB after unplugging from the power chassis.
Pull the U-shaped top cover and you are in.
I pulled out the Aux Power Supply board (upper right corner) and the EMI filter board (lower right corner) in order to troubleshoot the power supply on the bench.
ONE HUGE WARNING! The EMI filter board and large parts of the Aux power supply are NOT isolated from the AC line! I powered this from an isolation transformer before hooking an oscilloscope. Make sure you fully understand what this means before proceeding with any testing. Unisolated circuits are VERY dangerous to work on.
Aux power supply board
I found some unrectified AC waveforms in places that should have been DC, and pinpointed the problem to a large rectifier diode that was open-circuit. After replacing this diode, all the Aux rails powered up, but many voltages were off (by 5V or more) from the spec’d values. I spent several days chasing this with no progress, until I guessed that the amplifier and the rest of the L1 circuitry might have to be connected to the Aux rails before they would read the correct voltages. This turned out to be correct. When I assembled and temporarily wired everything together, the Aux voltages read correctly. These rails are not tightly regulated and needed the load of the circuitry to pull them to their proper voltages.
Other misc inside images below