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 10/22/2021: Since I first published this, I’ve gotten a lot of inquiries from people regarding repairing their L1s. For people local 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 five units and succeeded fixing all 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):

6 of 7 failures have been in the “Auxiliary Power Supply Board” (read below for more details). I recommend starting trouble shooting there if you L1 does not power up 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 – REPAIRED

Unit 2: D601 and D608 bad – REPAIRED

Unit 3: D601 bad – REPAIRED

Unit 4: D609 bad but also Transformer T601 open between pins 6 and 8. I was lucky in finding a broken wire that was accessible outside the windings. I reconnected the wire with some very carefully soldering. – REPAIRED

Unit 5: Had a working Aux Supply, but no sound from one half of the tower. The +/-27 power supply (there are three) for that channel power amp had smoked MOSFETs and other components. I harvested a +/-27V module from Unit 4 to get this one out the door, then repaired the supply by replacing two MOSFETs, two zeners, several resistors, and one rectifier.

Unit 6: A bit of a train wreck with many visibly charred components. I ultimately had to replace D606, D602, D614, Q601, R614, R620, R621, SCR601, TH603 and Z601. Whew! But now back from the dead. – REPAIRED

Unit 7: replaced D602 and D607 – REPAIRED

Tally so far on L1 repair is 7 for 7! 

Back to 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. To make matters worse they are completely uncooperative about supplying schematics (though I was able to find documentation 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.

Bose L1 Block Diagram

The below linked doc has schematics and instructions for disassembly. Even though it provides guidance, I still recommend taking LOTs of pictures. Especially of where connectors plug in.

Bose PS1-L1 rev1 Schematics

Dissembling the base is no small task. There are screws everywhere, and you have to pull almost everything 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.

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.

Warning #2! The output of the EMI board is about 330V and the big capacitors on that board can store that voltage for up to 30 minutes time after the base is unplugged. Check that the voltage has fallen to a safe level (<20V) before touching anything or removing any boards.

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 need 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 miscellaneous inside images below, mostly to remind me where all the connectors go: