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.

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.

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

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

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. 


Aux power supply board

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


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


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



43 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


    • 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.


  2. I have three classic L1’s that have failed now. Its not just the power supply failing, sometimes its an amp. I was able to rewire one L1 so it’s the same as the ones with two amps instead of the three.. Trying to figure a new amp to power the “sticks” and use as side fill speakers. I see these sticks for sale all the time now. There must be a easy set up with a small amp?



    • I suppose it’s possible to gut the base and replace the electronics with your own, like maybe amps from Parts Express, but I think there would be a significant sound change. Though as a last resort, it might be worth a try.


  3. Hi, I have the same problem (will not power up) and was wondering if I could pay you to repair my unit. I love this system. I tried the newer model and simply do not like the sound. Much more harsh and feedback prone.


  4. Hi. You put some work into that. I wanted to point out though, you never did identify what part needed replacing! Anyone less knowledgeable about electronics but handy with a multimeter and soldering iron could probably do the same repair, if they knew where to look. If not actually marking the culprit on the picture, I’m sure some people would appreciate at least identifying the board stencil ID (D123? Z456? etc.) and maybe diode spec., if only for an idea of what to check first on their unit.


  5. I have 4 of these units 2 of the original (3 amp) and 2 of the later (2 amp) versions. In the past both the 2 amp versions have died and been repaired by Bose, currently I have 1 completely dead 2 amp version and a 3 amp version that appeared to enter a tripping state (all lights flashing) in now has no power light (green) but the phantom power leds (red) light up – any ideas ?


      • Hello I had my Bose L1 classic repaired the guy replaced all the amplifiers and my unit worked good for one show then at a practice the bottom tower popped loud and is now making a intermittent popping noise out of the bottom tower when I tore down the unit I checked the wires from the amp that go to the control board and the black and white wire that goes to the bottom tower has a spike on my meter is it possible that I have a faulty amplifier from the supplier. Any input will be greatly appreciated


  6. Hello
    I am so ticked with Bose that had written a scathing letter telling them to just ship their work overseas and have it done in sweatshops because that’s all it’s worth to me now! That obviously got me nowhere and I never got a response from “customer service”. Your site has been the first one I have seen that deals with this problem and actually has a resolve. I too am victim to the “it just won’t turn on any more and the fuse doesn’t work. Sounds like this could be a very lucrative business. I too am wondering if you will repair my L1 as I am not an electronics savvy individual. I hope to hear from you.


    • Hi. I’d be happy to try fixing peoples towers, but the problem is that I can’t guarantee success (I’m an EE, but not a pro service tech) and shipping would be prohibitive. I also wouldn’t know what to charge because it takes hours to carefully open up the unit. If you live within driving distance of 94022 or can stomach paying two way shipping, I’d be willing to try, and not charge unless I succeeded.


  7. I have a Bose L1 Classic
    A guy brought to me.
    I have checked the power amp direct Inputs on the PS1 they all work.
    The input channels seem to be receiving signal fine.
    But are not outputting to the power amps.
    Channnel inserts output.
    Have you run into this issue? The guy said he pluged in Speakers to the power amp outs when the tower was pluged in. Why I don’t know.
    It clearly says not to. To contact me


    • Sorry, no. The Aux Power Supply issue is the only thing I have dealt with. I have a link to the schematics. The only thing I can suggest is to plod through the blocks step by step to see where the signal stops.


  8. By looking at the pictures of the aux ps board, it looks like you swapped out MOSFET Q601 and thermistor TH603. Is that correct? If so, what did you use for the thermistor. The only 100C lug type I’ve found is a muRata and Mouser says it’s obsolete. Any help would be appreciated.


    • I may have pulled those components during my “shotgun” troubleshooting efforts, but I did not replace them. They are the originals. Are you sure they are bad? They should be pretty easy to test.

      Looking at the photo now, None of the diodes show rework, but I am sure that I replaced one. The Aux Power Supply photo might have been taken before I did that. I’m not sure now which diode I replaced, but it was one of large ones (sorry for not being more precise). The labels for three large diodes are visible (D605, D608, and D609). The labels for the other two (near R630 and R631) are not visible in the picture. I would check these 5 diodes first.


      • Yeah, I haven’t tested the mosfet and the thermistor. I just wanted to make sure I can get the parts. The diodes are no problem. I can get those for $.55/ea. Thanks.


  9. I just finished a repair on the aux supply. In my case, it was D602 and D607. Also, you’re absolutely correct that the supply will not output correct voltages unless it’s connected to the load(s).


  10. My L1 has not been used for 2 years (will not power up). Tried to find someone in northwest NJ that could repair (besides Bose) but no one had a schematic. Does anyone know who can do the repair in the NJ area?


  11. GeorgeU. I do not know yet of a place in NJ for you, but there is a place in GA, and a place in CA that I took my two units to. The guy’s repair time is about 2 days, but shipping might be prohibitive for you. Also, he is not cheap. About $400 for the repair, but you get a warranty. He dd tell me that the Classic L1 was better built than later models. If you have the Classic with no power up, chances are 1 of the amps blew and that blew the Aux Power Supply. The main Power Supply is probably okay, but 1 or more amps went out and took out the Aux unit. The Classic is an easier diagnosis and fix than the Model 1. You might be able to find a shop in your area for the repair if you have the Classic L1. The Model 1 combines the amps and the power supplies, making dianosis harder and time consuming.


    • I would like the information on the repair guy in California. I have 4 L1’s, 3 of which are not powering up. As everyone has stated, Bose only wants to sell the new model 2. I’m in California and maybe I can get mine repaired.


  12. Guess I should have used spell check. Sorry for the misspellings. O.C. Electronic Repair is the place I go to in Southern California. Modular Electronics in Roswell Georgia is the other place.


  13. Lens42, Thanks for all the information. Very much. Mistakenly leaving my unit on overnight was the last thing I did with it. Now nothing. Power is a bit dirty around here. I’ve taken the unit apart (Note to others: there’s a couple sneaky screws and a ground screw that will impede taking the power supply cabinet out – you’ll find them.). Can I send you the PCB(s) for testing? Glad to pay, success or otherwise. Drop me a line and I’ll fill you in.

    Thanks again – really nice work here.
    Andy Quinn


    • My base on an L1 Model 1 blew up. I could not repair it but I saved the column radiator, I mounted each section on poles and fed them with a Crown 1000 amp in stereo. Incredible sound.


  14. If it’s any help to anyone (and while I have no connection this company other than as a very satisfied customer) I would highly recommend I was told by Bose that my L1 Model 1 unit (which completely died) was unrepairable. I couriered it from Scotland to the company in Buckinghamshire, where a gentleman called Clive fixed it up so that it now works perfectly.

    Liked by 1 person

  15. Hi! i also have a faulty PSU. D607 was short circuited, i replaced it and now i get some voltage on the secondary side, but very low. like 1V on most of the rails. I checked all other diodes (I trippled checked D202) and they all test fine. also de MOSFET is OK.
    If i test the aux board only connected to the High voltage DC rail comming from EMI Board, and nothing else connected to it, i should get voltages on the output connector going to preamp section right? they should be higher than expected and not as low, right? i tried to connect all boards, and the only thing i see is the phamtom power led comming up but intermitently. power led doesn’t turn on. I’m also assuming that i don’t need to have the speaker connected, right?
    thanks guy for any available help. (i also replaced all electrolytic caps)


    • *i meant i tripple checked D602, it is fine. I’m probably just going to replace the PWM IC and check the feedback network for any faults..


    • Yes, JohnD. When I first activated the board after replacing the diode, the output voltage were all pretty odd. I can’t recall if they were higher or lower than expected, but 1V on the rails does not seem right. My test was with no loads on the Aux supply. Only the EMI board and Aux power board were powered during the preliminary test. The voltages then all fell into line when amplifier loads were connected.

      Thinking about it, it seems logical that many outputs would be low if no loads are connected. IIRC, the circuit drives a transformer with multiple output taps. One output is fed back, so if there is no load on that output, the circuit will only need to switch infrequently to satisfy the fed-back output. That infrequent switching will not generate much voltage on the other “slave” outputs, so they may read very low. The fed-back output however (I think is was +15V) should read above or near the correct voltage. IIRC, I think that one read high and pulsed above 15V, because it had no load to discharge its capacitors.

      You should be able to verify operation of the PWM chip without pulling it (but maybe swapping is easier). Chips like this have a reference and feedback input. If the feedback input is below the reference input and the chip is not switching, then the chip is at fault. But if the chip it switching and nothing is happening at the transformer primary, then something else is wrong (maybe the power MOSFET). If the regulated output is low, but the feedback input to the PWM is “lying” and fooling it into thinking the output is in regulation (causing it not to switch), then something in the feedback path is bad. A big warning re checking the PWM is that parts of the circuit are hot to the AC power line, so the EMI board needs to be powered from an isolation transformer before you can safely probe these circuits. From the sound of your comments. it appears you are likely familiar with un-isolated AC risks. If not, get some technical guidance before proceeding.


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