Hey Folks,
I've recently got a Bose L1 compact sound system in for repair. The unit failed to power up one fine day.
After opening the unit up the first thing I noticed was the TOP258 IC was cracked open. Before replacing the IC I had a look for any other components which might have failed. I checked for the obvious culprits and I couldn't find anything which has failed. I checked the diodes and resistors around the IC, checked the main filter caps and the electrolytics near the IC. Checked the secondary side for any shorts or failed caps and nothing. I checked the capacitors with the ESR meter, I removed some capacitors and checked them with my multimeter for the capacitance value as well.
So after much contemplation, I decided to change the IC only but the thing was I couldn't get hold of any TOP258 ICs. So I placed order for those. However I had TOP256 IC in my parts bin. It can handle low power than the 258 and to see if anything else failed I replaced it with it (until I got the 258). Powered it up and it was working. Obviously I didn't run it at full volume or had it on for long.
After week of intermittently testing again the unit stopped powering up. Opened it up the SMPS IC was cracked open. Same story as before nothing else seems to have failed. I now have got the TOP258 IC but am afraid the same thing would happen unless I could find the culprit.
I have checked the speakers thinking they might have partly shorted and might have been drawing too much current. They also test out ok. The amplifier IC is TDA8920B. On the datasheet the amp chip and the TOP258 seems to have all the protection in the world and seems to be bullet proof.
I was hoping someone could shed some light on how to proceed and what else to look for.
I have attached a pdf of the power supply schematic.
For a while I was pretty sure that the TVS diode (DZ1) might have failed as that would put high voltage on the drain pin, but it shows forward voltage of 0.6 and doesn't show anything in the reverse direction. I'm not sure how I can check it for proper 'TVS' operation.
The full schematic can be found BOSE L1 COMPACT SYSTEM SERVICE MANUAL Service Manual download, schematics, eeprom, repair info for electronics experts
Thanks in advance.
P.S: I have got another unit which came in for repairs and the capacitor and the resistor of the LPF was burned up and bulged on that one. The SMPS IC was cracked open on that one as well. I also checked those components on the unit I'm currently working on and they test out ok.
I've recently got a Bose L1 compact sound system in for repair. The unit failed to power up one fine day.
After opening the unit up the first thing I noticed was the TOP258 IC was cracked open. Before replacing the IC I had a look for any other components which might have failed. I checked for the obvious culprits and I couldn't find anything which has failed. I checked the diodes and resistors around the IC, checked the main filter caps and the electrolytics near the IC. Checked the secondary side for any shorts or failed caps and nothing. I checked the capacitors with the ESR meter, I removed some capacitors and checked them with my multimeter for the capacitance value as well.
So after much contemplation, I decided to change the IC only but the thing was I couldn't get hold of any TOP258 ICs. So I placed order for those. However I had TOP256 IC in my parts bin. It can handle low power than the 258 and to see if anything else failed I replaced it with it (until I got the 258). Powered it up and it was working. Obviously I didn't run it at full volume or had it on for long.
After week of intermittently testing again the unit stopped powering up. Opened it up the SMPS IC was cracked open. Same story as before nothing else seems to have failed. I now have got the TOP258 IC but am afraid the same thing would happen unless I could find the culprit.
I have checked the speakers thinking they might have partly shorted and might have been drawing too much current. They also test out ok. The amplifier IC is TDA8920B. On the datasheet the amp chip and the TOP258 seems to have all the protection in the world and seems to be bullet proof.
I was hoping someone could shed some light on how to proceed and what else to look for.
I have attached a pdf of the power supply schematic.
For a while I was pretty sure that the TVS diode (DZ1) might have failed as that would put high voltage on the drain pin, but it shows forward voltage of 0.6 and doesn't show anything in the reverse direction. I'm not sure how I can check it for proper 'TVS' operation.
The full schematic can be found BOSE L1 COMPACT SYSTEM SERVICE MANUAL Service Manual download, schematics, eeprom, repair info for electronics experts
Thanks in advance.
P.S: I have got another unit which came in for repairs and the capacitor and the resistor of the LPF was burned up and bulged on that one. The SMPS IC was cracked open on that one as well. I also checked those components on the unit I'm currently working on and they test out ok.
Which LPF RC was burned up and bulged? If the IC is cracked that suggests some pretty serious overload condition. So the mosfet could be staying on too long which might suggest a feedback loop problem. Another possibility is HV spikes are damaging the IC so then you'd check all the snubber components. But to check all these parts is not easy because to test them properly, they'd need to be tested at HV. Even then, I've seen HV parts work fine until they get to a certain temperature and then they fail.
The LPF I mentioned was the high output of the TDA8920B chip.
I also do have a feeling that it could be the snubber elements on the HV side. I have checked all the resistors and the capacitors on that side and seems to be good. Only questionable part is the TVS diode as I can't check it on a component test either.
Could you also please advice on what kind of problem in the feedback loop could cause the issue.
Thanks
Correct way to repair of such power supply is to check (change) feedback parts, Optocoupler U2 and reference voltage U4 (TL432).
Also I recommend checking of all zener diodes.
Also I recommend checking of all zener diodes.
I'm waiting for some parts to arrive. I have checked all the zeners in diode mode and seems to check out.
I tried measuring the value of C42 but my meter does not read in the pico range. I'm not sure if there's any other way of checking that. Visually it looks ok no cracks or burned marks.
Update on the test for the TVS diode:
My test setup was isolation transformer -- variac -- rectifier --test circuit.
Important note - there were no smoothing capacitors connected across the rectifier output so the DC was not smoothed.
I had two 20k resistors in parallel connected to the TVS diode.
TVS diode specs according to the datasheet
Reverse Stand off Voltage (VR): 128.00V
Breakdown Voltage VBR @1mA: Min 143.00V , Max 158.00V.
Here are the results:
When I set the input voltage to 154V = I measure around 120V across the diode.
When I set the input voltage to 250V = I measure around 130V across the diode.
What would your thoughts be on the results?
I tried measuring the value of C42 but my meter does not read in the pico range. I'm not sure if there's any other way of checking that. Visually it looks ok no cracks or burned marks.
Update on the test for the TVS diode:
My test setup was isolation transformer -- variac -- rectifier --test circuit.
Important note - there were no smoothing capacitors connected across the rectifier output so the DC was not smoothed.
I had two 20k resistors in parallel connected to the TVS diode.
TVS diode specs according to the datasheet
Reverse Stand off Voltage (VR): 128.00V
Breakdown Voltage VBR @1mA: Min 143.00V , Max 158.00V.
Here are the results:
When I set the input voltage to 154V = I measure around 120V across the diode.
When I set the input voltage to 250V = I measure around 130V across the diode.
What would your thoughts be on the results?
Attachments
I have done so ESR meter measures displays 'good cap' with LOW ESR.
Removed from circuite and measured the capacitance value and it was around 477uF for one and 472uF for the other one.
So I'm thinking of changing all the resistors and the caps on the HV side. Since the resistors also looks a bit tarnished even though it measures ok. Also I'm ordering in some diodes.
Anyways I'm having some doubts about the capacitor. C42 is a 47pF 1kV Ceramic disc cap and the dielectric material stated on the service manual was X7R. I couldn't find the same from mouser.
I found same rating capacitors but different dielectric material. So my question is matching the dielectric material that important? and can I use MLCC? any merits to it?I'm asking for future reference too.
Anyways I'm having some doubts about the capacitor. C42 is a 47pF 1kV Ceramic disc cap and the dielectric material stated on the service manual was X7R. I couldn't find the same from mouser.
I found same rating capacitors but different dielectric material. So my question is matching the dielectric material that important? and can I use MLCC? any merits to it?I'm asking for future reference too.
Hmmm ... are you shure that the problem is the SMPS ? and not the load ... ?
It reminds me on a TV set which took the PS down occasionally and in the end it was the TDA.... amplifier chip which developed an internal dead short depending on temperature, mechanical stress a.s.o. Bond wire shorting to lead frame, something like that.
To fully protect a off-the-line SMPS against a dead short on the output is not trivial and you simply don't know whether the Bose design is fool prove.
Instead of replacing parts with no obvious signs of defect at random I would fit a pair of rather low rated fuses somewhere between SMPS and amp, near the connector maybe.
There are fuses towards the small signal stages but not towards the power amp, if I see that correctly. Not that a fuse will protect the PS but ...:
If the fuse blows, you know it was the amp.
If the SMPS blows and fuses stay intact you know it was the PS.
It reminds me on a TV set which took the PS down occasionally and in the end it was the TDA.... amplifier chip which developed an internal dead short depending on temperature, mechanical stress a.s.o. Bond wire shorting to lead frame, something like that.
To fully protect a off-the-line SMPS against a dead short on the output is not trivial and you simply don't know whether the Bose design is fool prove.
Instead of replacing parts with no obvious signs of defect at random I would fit a pair of rather low rated fuses somewhere between SMPS and amp, near the connector maybe.
There are fuses towards the small signal stages but not towards the power amp, if I see that correctly. Not that a fuse will protect the PS but ...:
If the fuse blows, you know it was the amp.
If the SMPS blows and fuses stay intact you know it was the PS.
Last edited:
Thanks for that suggestion. I also have been thinking about a fault on the secondary side could be taking out the SMPS IC. The TDA chip also was a primary suspect as I mentioned before the second unit I got had the output filter capacitor C8 bulged and the resistor R21 blown in half. Could this be an indication for a faulty IC or faulty cap/resistor.
Is there anyway to troubleshoot the IC?
There are fuses which goes to the preamp board but none on the supply rails for the TDA IC.
With the experience you had did the faulty IC take out the SMPS IC? and how did you find out that it was the TDA IC, is it with the suggestion you made with the added fuse which is pretty a neat idea. Thanks!
I wanted to ask how to add fuse when the rails are on the same board and not a seperate board?
Appreciate the help.
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The PS' s protection did its job and survived. The amp was on a sub assembly and I was able to remove it and hook it up to a lab PS. Mechanical pressure and/or heating with a soldering tip revealed the intermittant short.
Looking at the Bose PS schem I thought that the green square might be a cable and connector, which would make it easy to splice in a fuse, otherwise ....
Looking at the Bose PS schem I thought that the green square might be a cable and connector, which would make it easy to splice in a fuse, otherwise ....
Got stuck on a problem. The service manual says "*ALLRESISTORSINOHMS,5%1/10W(0603)."
I searched for those on mouser and the one which I found which matches the specs had a voltage rating of 75V. For instance the R1 (110k) which is the clamping circuit for the mosfet - wouldn't it exceed the rated voltage?
Its my understanding the typical voltage rating for 0603 size resistors are in the 75V range.
I'm slightly confused at the moment.
I searched for those on mouser and the one which I found which matches the specs had a voltage rating of 75V. For instance the R1 (110k) which is the clamping circuit for the mosfet - wouldn't it exceed the rated voltage?
Its my understanding the typical voltage rating for 0603 size resistors are in the 75V range.
I'm slightly confused at the moment.
You are correct wit your thoughts, at connections of this resistor it is possible to have about 150V which are clamped by DZ1, P6KE.
So correct answer is for the resistor 0603 is safe up to 75V, 0805 up to 150V, and 1206 up to 200V. Beware that for AC, you must check it against the peak voltage, not RMS value.
Maybe you should try some lover value connected in series (62kohms), on that way you will have about 150V rating.
So correct answer is for the resistor 0603 is safe up to 75V, 0805 up to 150V, and 1206 up to 200V. Beware that for AC, you must check it against the peak voltage, not RMS value.
Maybe you should try some lover value connected in series (62kohms), on that way you will have about 150V rating.
Hmmm ...
In my service manual all resistors are specified individually and no mentioning of them being all the same...
e.g. R1, 110K, RMG, 1/4W, 5%, 1206
or even
R21, 22 OHM, RMG, 1W, 5%, 2512
Power Amplifier Pcb Assembly - Bose L1 Compact Service Manual [Page 31] | ManualsLib
BOSE L1 COMPACT SERVICE MANUAL Pdf Download | ManualsLib
Attachments
"*ALLRESISTORSINOHMS,5%1/10W(0603)." - is written on the bottom of the schematic diagram.
You're right the service manual with the parts list is different. Will double check the size again.
You're right the service manual with the parts list is different. Will double check the size again.
Further development:
Turns out the resistors I needed were actually 1206 as mentioned in the parts list. I'm still waiting for the parts including the TOP258.
Anyway I have a got a theory of why the units failed from a recent experience. I had a similar kind of system (JBL EON one) come in for repair with similar kind of problem. The power supply had failed. However this time the customer did notice some subtle distortion coming from the high frequency driver when it failed. Repaired the power supply powered it back on and there was no sound from the HF tower. Measured the resistance of the HF tower no obvious fault, there are total of 6 drivers - 2 in series and the pair is paralleled with each other so total of 3 rings.
So I opened up the tower disconnected all the drivers and checked each driver with a tone generator and one of the driver was quieter than the rest and it was distorting. So I bypassed that driver which also bypasses the one connected to it in series (all the drivers shows same dc resistance). The unit started to work. (the customer is going to replace the complete tower).
Thinking that the same thing might have happened to the bose L1 units I opened one of the tower. The drivers were connected in the same manner total of 6 drivers but the wires were wrapped to the terminals. I am a bit unsure on unwrapping the wires since I don't have a wrapping tool. So I used the tone generator with all the 6 drivers connected. I thought I heard some distortion.
Could this cause the failure of the TOP IC? As I read the TDA8920B (amplifier chip) sort of shut down for 100ms if it detects impedance is below the threshold as a protection. So
Could I unwrap the wire from the terminals to check the drivers individually and re-wrap them and solder them?
I also have a some question about how I am testing the drivers - I'm using a small headphone amplifier to drive it, could the distortion I hear be due to the amplifier not being able to handle the load?
Any suggestions and thoughts are greatly appreciated
Turns out the resistors I needed were actually 1206 as mentioned in the parts list. I'm still waiting for the parts including the TOP258.
Anyway I have a got a theory of why the units failed from a recent experience. I had a similar kind of system (JBL EON one) come in for repair with similar kind of problem. The power supply had failed. However this time the customer did notice some subtle distortion coming from the high frequency driver when it failed. Repaired the power supply powered it back on and there was no sound from the HF tower. Measured the resistance of the HF tower no obvious fault, there are total of 6 drivers - 2 in series and the pair is paralleled with each other so total of 3 rings.
So I opened up the tower disconnected all the drivers and checked each driver with a tone generator and one of the driver was quieter than the rest and it was distorting. So I bypassed that driver which also bypasses the one connected to it in series (all the drivers shows same dc resistance). The unit started to work. (the customer is going to replace the complete tower).
Thinking that the same thing might have happened to the bose L1 units I opened one of the tower. The drivers were connected in the same manner total of 6 drivers but the wires were wrapped to the terminals. I am a bit unsure on unwrapping the wires since I don't have a wrapping tool. So I used the tone generator with all the 6 drivers connected. I thought I heard some distortion.
Could this cause the failure of the TOP IC? As I read the TDA8920B (amplifier chip) sort of shut down for 100ms if it detects impedance is below the threshold as a protection. So
Could I unwrap the wire from the terminals to check the drivers individually and re-wrap them and solder them?
I also have a some question about how I am testing the drivers - I'm using a small headphone amplifier to drive it, could the distortion I hear be due to the amplifier not being able to handle the load?
Any suggestions and thoughts are greatly appreciated
Does anyone know of a source for the POTs on the Bose L1? I'm looking for VR200, which has a description of VR, 20KAX2, RD902-40-15F, ALPHA and a vendor part number of 4751-0750+0.
In searching Mouser I don't find anything. Searching the web I found a few "20KAX2" type POTs, but none of them look remotely close to what is on the PCB.
Thanks, gabo
In searching Mouser I don't find anything. Searching the web I found a few "20KAX2" type POTs, but none of them look remotely close to what is on the PCB.
Thanks, gabo