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 https://elektrotanya.com/bose_l1_compact_system_service_manual.pdf/download.html

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.

 

Hi Yami,
Interesting. Are the soldered connections on T1 ok?
Please keep us posted.
Good luck..........

 

Hi Yami,
Interesting. Are the soldered connections on T1 ok?
Please keep us posted.
Good luck..........

Thanks @twohats, yes the solder connections seems to be ok. Also did check for open windings as well also seems to be intact.

 

Check that it is set for the correct input voltage.
Check all the components on the TOP258 D connection, especially the value of the capacitors.

 

Check that it is set for the correct input voltage.
Check all the components on the TOP258 D connection, especially the value of the capacitors.

Thanks @AlbertHall, yes the input voltage is set correctly. I have gone through all the capacitors with the ESR meter checks out ok. I guess I need to check the TVS diode in a test circuit can't think of anything else at the moment

 

I have gone through all the capacitors with the ESR meter checks out ok.

Just to confirm, the ESR meter also measures the value of capacitance?
I have come across circuits where those capacitors were low vaue and this increases the voltage spike and can then damage the MOSFET or transistor driving the transformer.

 

Just to confirm, the ESR meter also measures the value of capacitance?
I have come across circuits where those capacitors were low vaue and this increases the voltage spike and can then damage the MOSFET or transistor driving the transformer.

The meter I use displays the ESR value and says "cap is good if < ...". I took out some capacitors out of circuit and test them with my DMM to get the capacitor value. I didn't take all of them out though.
Also I didn't check the non-electrolytics with the ESR meter. I checked C7 which in the clamping part of the circuit with my DMM.

 

You really should check the values of C7 and C43. That will probably require at least one end to be disconnected from the board.

 

You really should check the values of C7 and C43. That will probably require at least one end to be disconnected from the board.

C7 I checked completely removed from circuit and measures as 2nF. Suppose to be 2.2nF but shouldn't it be fine?
I shall check C43 as well.
Just to reconfirm you don't mean using the ESR meter right (for C7 & C43)?

 

Just to reconfirm you don't mean using the ESR meter right (for C7 & C43)?

Confirm the capacitance is correct.

 

Update:
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?

 

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.