Samsung Home Theater HT-X40 - Shut Down on Protection Problem

A couple of days ago my Samsung home theater HT-X40 started periodically shutting down with "Protection" message. Resetting by the "Stop" button pressed for 5 seconds made it forget all settings but did not fix the shut down problem. From the cool start it took about 20 minutes until it switched off for the first time and 2...3 minutes after re-powering while it was still warm. That behavior clearly indicated a problem depended on heating.Disclaimer: Repairing electrical and electronic devices requires corresponding knowledge and skills and should be done by qualified professionals only. Electrical devices are sources of different hazards - electrical shock, skin burn, fire, etc. A DVD laser is a source of different hazards including eye, skin, fire hazards. This post is just a description of the problem in my particular case and not a recommendation for anybody on what to do and how to fix the problem whatsoever. The description is provided for informational purposes only, I bare neither responsibility nor liability and do not accept any claims for any injury and damage on your side.

A couple of days ago my Samsung home theater HT-X40 started periodically shutting down with "Protection" message. Resetting with "Stop" button pressed for 5 seconds made it forget all settings but did not fix the shut down problem. From the cool start it took about 20 minutes until it switched off for the first time and 2...3 minutes after re-powering while it was still warm. That behavior clearly indicated a problem depended on heating.

A promising thing was that all speakers were working fine every time when the theater was on. The fan on the rear side was working OK, air flow was relatively hot, but it was difficult to judge by that. Testing the speakers did not show any problem - each of them had normal resistance about 6 Ohm. Inspecting situation inside the theater I found the following problems on the speaker driver board (marked correspondingly in the following picture - click to see it in whole size):

1. the output driver heat-sink was very hot,
   
2. inductors corresponding to the central speaker were untouchable (burning hot),
3. 0.47 uF x 100V filtering capacitor in the central speaker channel had a visible crack on its body.
   

Test of the unsoldered capacitors clarified that 5 out of the 6 caps had good insulation (infinite resistance at 20 MOhm multimiter range) and only the one with visible crack showed 12 MOhm under the same measurement conditions. Each of these capacitors is connected between the two wires going to the corresponding speaker (in parallel to the load). It was probably happening so that under higher temperature insulation resistance significantly dropped down from the 12 MOhm to a value comparable to the main load, in its turn that increased the temperature more. The temperature kept increasing until the protection switched off the power.

For the first step I decided to exchange only that obviously broken capacitor and see if any other problem would surface after. The original capacitor was 0.47uF, 100V, film type:

I was lucky to find a cap of 0.47uF, 250V, same film type in an old modem. This is the board after replacement:

I intentionally distanced cases of the capacitors from the inductors on the board, because the inductors could be that heat source which helps to damage the caps. Since I had assembled everything back together the home theater has been working normally. In addition I increased the gap under the home theater by sticking 4 pads usually used on furniture legs to protect floor from scratches:

Temperature of air flow from the back panel fan has become significantly lower comparing to that time when the home theater was failing.

Technical Note
From many comments I can see that there is an interest in deeper technical details related to the problem. Here are some useful references:

1. Class D Amplifier describes principals of operation;
   
   
2. Class D Audio Amplifier Output Filter Optimization - the application note explains why output LC filter is required (in the beginning of "Output-Filter Design section). The reason is EMI (electro-magnetic interference).
   
   
3. 2 x 210 Watt STEREO DIGITAL AMPLIFIER POWER STAGE - datasheets for TAS5162, power amplifier from Texas Instruments with examples of typical schematics. It is probably not the exact power amplifier by part number from the Home Theater, but it's of the same class-D.
   

Long story short: Nowadays most of audio power amplifiers utilize Pulse Width Modulation (PWM), so called class D amplifiers, because of very good about 90% efficiency. PWM uses higher frequency Fpwm (100kHz and higher) comparing to the sound frequencies and the pulses have fast edges. That's why the output signal besides the sound frequencies also has Fpwm and bunch of its harmonics. The output low-pass filter (here built as passive LC) is needed to block Fpwm related frequencies inside the box - without the filter the wires going to speakers will irradiate these frequencies and may interfere with other electronic devices. Say, without such a filter you would still hear correct sound (probably with unnoticeable distortions), but some electronic equipment around can be affected (anything with "wireless", "radio", "sensitive", "low noise" characteristics is in the list).

I hope this technical note can help to understand better what's going on there and how to deal with the problem.

I'll keep updating the post. Read more...