Bose L1 Tower PA Repair

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.

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1st generation Bose L1 tower PA system

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.

Bose L1 Block Diagram

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.

Bose PS1-L1 rev1 Schematics

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.

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Top cover and control panel after disconnecting from power chassis
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Internal power chassis removed. Power stuff is in here. Note the four fans.

Pull the U-shaped top cover and you are in.

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Power supply and amplifier circuits finally visible. The “Aux power supply” culprit is in the upper right corner.

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. 

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Aux power supply board

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Aux power supply back. Note that everything on the left side of this board is HOT TO THE AC LINE when input power is connected.

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

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Control panel view of some connectors that must be removed before the power chassis can be separated from the main body.

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Back of control panel close up
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Back of control panel close up
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Back of control panel
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Internal base plate for the tower support and speaker connector

 

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2 thoughts on “Bose L1 Tower PA Repair

  1. We had an issue with our house PS1 sound system, coincidentally also 10yrs old. 3 years of faultless performance followed by an amp which failed to power up immediately when switched on….sometimes it would be 5 mins or up to an hour or two before the green led would light up .
    Hopefully this will be resolved with your help and a fabulously knowlegible friend. I’ll keep you updated. Bose were unsympathetic to the point that I felt like a cheapskate nuisance even enquiring about repair & all but told me to throw my PS1 in the trash and buy a new rig!! Watch this space for the next news

    Like

    • Hi, Sorry for the delayed reply. I’m new to WordPress and don’t think I got notified of your comment. The symptoms are similar to mine, however we didn’t have any period where it worked for minutes before failing. It just didn’t turn-on one day. The Aux supply is still a likely culprit, but the problem is that this is thing is a bear to open-up and has a lot of messy “spaghetti” wiring and connectors to keep track of. I’m sure your friend knows that the first thing to check are the power supply voltages. It’s almost certain that some are off. Then the chase is to figure out if the cause is a dead capacitor or something else. good luck.

      Like

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