Bose L1 Compact Speaker, TOP258 IC blowing

[Yambo]

Hey Folks my first post here,
I've recently got a Bose L1 compact sound system in for repair. The unit failed to power up one 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, but the thing was I couldn't get hold of any TOP258 ICs. So I placed an 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 have 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 a 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. C7 measured out of circuit was 2nF which I assume is ok. D2 tests out ok and I measured the resistance of it reversed bias and shows no value.

The full schematic can be found

Thanks in advance.
P.S: I have got another unit which came in for repairs and the capacitor and the resistor of the LPF (amp chip output)was burned up and bulged on that one. The SMPS IC was cracked on that one as well. I have checked those components on the unit which I'm currently working on and they test out ok.

[stj]

Re: Bose L1 Compact Speaker, TOP258 IC blowing

check r4, r12 and r13
if they are good then try replacing d2 and dz1
if those diodes in the snubber fail it can blow the internal fet.

[Yambo]

Re: Bose L1 Compact Speaker, TOP258 IC blowing

check r4, r12 and r13
if they are good then try replacing d2 and dz1
if those diodes in the snubber fail it can blow the internal fet.

All the resistors shows the correct values.
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?

[stj]

Re: Bose L1 Compact Speaker, TOP258 IC blowing

the tvs sounds good

[Yambo]

Re: Bose L1 Compact Speaker, TOP258 IC blowing

Thanks
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 the 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.

[stj]

Re: Bose L1 Compact Speaker, TOP258 IC blowing

X7R is 10% tolerance,
that may be why they stated it.