Voxson HT-10 Home Theatre Repair
This 5.1 channel home theatre amplifier was brought in with an unusual problem. The system worked fine, but every time quiet voices were used on the video, the sound would drop out dramatically. I hadn’t heard of this before, so was keen to try and diagnose the fault. First things first, turning on the unit showed no signs of life.
This is not what the customer had complained about. After removing the back of the housing, cables that had been disconnected could be seen. I found out later that the customer had the back off to check it out for himself & had not refitted the cables. After they were reinserted into their sockets, the amp came to life.
The customer provided the remote control and this needed to be tested as well. This was easily done by using either a movie camera or mobile phone camera. All buttons showed the ultra-violet light flashing, proving it was operating.
This unit has a test cycle that can be accessed using a ‘test’ button on the remote control. Each channel is individually tested and sound can be heard through each speaker separately. After discharging the main filter capacitors, tests were carried out on the amplifier board, including capacitors, resistors and transistors. All of these tested within limits.
After powering up the amp, freezer spray was used on the power transistors in the hopes that this would change the fault conditions. However the problem remained the same.
After consulting with Jestine, it was decided to replace the power transistors. These were TDA7377 & TDA7265 amplifiers available out of Hong Kong for a reasonable price. Installation included an insulator behind the TDA7265, whereas the TDA7377 was bolted directly to the heat sink, using thermal paste on both parts.
After 3 hours of testing with the volume loud enough to annoy the neighbours, all sound frequencies were working well. A satisfied customer returned to pick up the home theatre for his next movie marathon!
If you are interested in a video of this repair, please follow the link below:
This article was prepared for you by Mark Rabone from Australia.
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Note: You can check out his previous repair article below:
https://jestineyong.com/electronic-kettle-not-turning-off-miraclemax/
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Parasuraman S
April 13, 2017 at 11:05 pm
Good! If only either Jestine Yong or you could clarify what defect in the transistor could trigger this fault or why a decision to change the output transistor was made, it would have been more useful! I am unable to relate!
Mark
April 14, 2017 at 10:38 am
Hey Parasuraman,
I'm sorry I can't provide any more diagnostic information. I had no schematic to guide me and after testing all the other components on the board, decided the power transistors may be faulty. As it turns out, this solved the problem.
Albert van Bemmelen
April 14, 2017 at 1:59 am
Apparently quite a lot of those TDA chips with lower or higher component numbers keep causing audio problems on these Jestine repair pages. I suppose Mark, that while mentioning freezing the Power Transistors, in fact freezing both the TDA7377 & TDA7265 amplifiers were meant. Good thing that although freezing them didn't help, replacing them worked out just fine.
Mark
April 14, 2017 at 10:40 am
Hey Albert,
Yes, both transistors were hit with the freeze spray and although freezing them made no difference, replacing them did the trick.
Henrique Jorge Guimarães Ulbrich
April 14, 2017 at 3:18 am
Hi, Mark, thanks for your contribution. I vaguely remember of this type of problem, long ago. To the best of my knowledge, there could be one unique cause: according to the data sheet from internet, the TDA7377 and the TDA7265 are both class AB amplifier ICs (they ar not single transistors). For an unknown reason (hidden inside the package), perhaps occurred an internal bias problem, shifting the working point out of the AB point, tending to class B (although not reaching it). This bring about crossover distortion, that is more pronounced at low levels (exactly according to the customer claim).
Mark
April 14, 2017 at 10:43 am
Hey Henrique,
Thanks for your input. I don't claim to understand all the workings of these transistors, however your description certainly seems to explain all the symptoms the amp was experiencing.
Henrique Jorge Guimarães Ulbrich
April 14, 2017 at 3:24 am
Complementing my comment: I supose the amplifier circuits have negative feedback. If they have, the crossover distortion could be lowered or even eliminated. Perhaps the negative feddback (if existing) was not so effective.
Isaac ganza
April 14, 2017 at 3:38 am
Did you test those transistors before mr yong suggestion to replace them. Isaac 4rm south-africa.
Mark
April 14, 2017 at 10:48 am
Hey Isaac,
They were difficult to test due to the nature of the fault, being intermittent and only effecting lower tones. However, if you could provide me with testing procedures for these transistors, I would be grateful.
As stated, they worked all the time, but only lower tones were effected.
Robert Calk
April 15, 2017 at 6:27 pm
Good job, Mark. It would be interesting to see what the Peak Atlas DCA 75 Pro says about those transistors.
Mark
April 16, 2017 at 3:12 pm
Hey Robert,
Interesting tester I haven't seen before
angelo
April 14, 2017 at 4:36 am
hmmmm...
it means the credit equally goes to Sir jestine
as well
congrats to the two of you..
Mark
April 15, 2017 at 9:19 am
I'm thinking most of the credit goes to Jestine. I have learnt so much from his book and being part of this blog among others that have a passion for electronics and that know far more than myself.
Yogesh Panchal
April 15, 2017 at 5:35 pm
Mark,
Good attempt for repair.
Paris Azis
April 16, 2017 at 1:31 am
Hey Mark
Interesting kind of failure! As I have seen the data sheets of both those ics, the TDA 7265 controls the mute and st/by functions by varying the voltage levels of pin 5. It seems that this voltage is therefore critical in terms of its stability for such a failure. The TDA 7377 includes a diagnostics terminal among its pins. This is pin No 10. It is an open collector stage and needs a pull up resistor to an external voltage reference source. If applied, the decoding of any type of the usual failures can easily identified before removing the ic. Just take a look on both data sheets for these ics for further details.
So, if you find the same trouble in the future you can test these voltage levels in order for you to know the reason of the failure. Perhaps this will not leave a question about the failure mechanism behind it.
Robert Calk
April 17, 2017 at 8:35 pm
Hi Paris,
My son has a USB battery pack that uses 2 18650 batteries. It also has a little flashlight that uses one 5mm white LED. The LED stopped working. So I took it apart and removed the LED. My Peak Atlas DCA 75 reads the LED as a 4.2V/or so zener diode with a Vf of about 1.5V. It's the first time I've seen a LED burn out and read as a zener diode. Have you ever seen that? I didn't write an article about it since it was such a simple repair. A new LED has it working just fine.
Paris Azis
April 21, 2017 at 4:22 pm
Hi Robert
Oh,I have seen many leds failing! Nothing is immortal in our world! The point is that if the circuit design is correct leds last about 50000 working hours. I am a little bit confused with your description. Does the Atlas Peak see the failed led both as a simple diode and as a zener as well? Or the led is fed from its source through a zener diode? Normally there should be a voltage dropping resistor between the source and the led in order to protect it from burn out. The calculation is straitfoward. It's a direct application of Ohm's law. You need to subtract the led's operating voltage from the total source voltage and then divide your result with the nominal working current of the diode. The new result will be the value of the voltage dropping resistor. If the calculation does not coincide with a standard resistor value, which is most likely, choose the next higher value available. Next, check the power this resistor consumes in order to choose a resistor of three times higher of that of your calculation finding. Again, power equals the square of the led's current, multiplied by the resistor's value. Use a resistor of three times this power (approximately again). Finally, if you take a decreased value of the led's working current in your first calculation, you will increase very much the longevity of your led. (Practically the calculation will give you a higher than the normally needed resistor).
I hope I have covered all your possible questions about the topic.
Paris Azis
April 21, 2017 at 4:41 pm
Robert,
If this circuit uses a 1,5V zener instead of a resistor as per the normal scenario I used above, keep in mind that the circuit behaviour will be different. The difference will be that the led, although healthy, will not light at all if the battery is discharged below a safety low level. This is because the zener diode will act as a switch not allowing current to pass through it. So the led will light only when the batteries are fully charged. With my scenario, the led will keep on lighting much below that safety voltage threshold, which is not good for the health of the battery. 18650 cells must not be disharged below 3 Volts and respectively not be charged above 4,2 Volts. This is their rule of longevity. Once again, the golden medium is needed!
Robert Calk
April 22, 2017 at 11:16 am
I already know that. What I'm saying is that the LED quit lighting up and when I tested it with my DCA 75 Pro it said that the LED was a 4V zener diode. I've never seen anything like that before and was wondering if you had. Usually when a LED quits working my DCA 75 Pro says, "no component detected", which is what I expected. But for some reason this LED burned out and somehow reads as a zener diode - the first time I have ever seen that happen before.
Paris Azis
April 23, 2017 at 12:39 am
Hi Robert
It is known that leds behave like zener diodes. They have excellent stabilizing characteristics for that purpose and I have seen them used as zeners many times.
Now, in your case, I understand that your tester tells you "no component detected" if the crystal junction of the diode is open. This is of course natural to happen. But if the instrument tells you "this is a 4V zener diode", this is strange indeed because the led should work normally in this case. Perhaps without full brightness but it should work.
Anyway I don't own that type of tester and cannot give you any logical explanation about its result. I either use my digital multimeter in the diode function or my analog multimeter in resistance function, which has a special range for led diode testing. The difference between the two instruments I use, is that in general digital multimeters cannot test diodes with forward drop voltages greater than two volts, or even less than that depending on the manufacturer, whereas my analog multimeter has a special range for leds, this is the Rx10 range, which lights up any led with Vf up to 3V providing maximum current of 15mA. So I have the information of both the forward voltage drop and the current draw of the led. This is therefore a complete dynamic test of led diodes, which is the most reliable one.
Well,as far as I know about leds, the Vf of the low brightness types is between 1,5 and 1,7V. The high brightness types extend to 3,2V, the nominal current for both being usually 20mA. But I don't know about leds with Vf above 3,2V. At least I have never came across such one.
It would be therefore interesting if you tried to connect that failing led to your bench psu, limiting its current to say 10mA and the voltage around 5V and tell me what happens...
It would also be interesting for me to know what your tester tells you when you connect to it a low brightness led...
Robert Calk
April 24, 2017 at 8:14 am
I'm pretty sure that the LED was a ultrabright since it was used as a flashlight. I replaced it with a ultrabright and it works fine. These ultrabrights have a Vf of about 2.8V. I can't find the old LED right now. I don't know if I misplaced it or threw it away when I was half asleep sometime.
Paris Azis
April 24, 2017 at 2:55 pm
O.K Robert, understood. Whenever it will be comfortable to you, just remember and do me the favor of testing a low brightness led with your tester and let me know what message you get. I am only curious if it sees a typical diode or anything else.
Mark
April 16, 2017 at 3:26 pm
Hey Paris,
Thanks for your comments. I have a lot to learn about multi-pin power transistors, IC's and many other components! I am grateful for all the patient help I receive from others on this blog. I learn something new every day. You have helped me to better understand the data sheet and that will help me to have more confidence on my next repair.
Actually I have a small MP3 player that only works on standby. So I'm sure that will be testing my skills when I get to it.
Thanks again for your explanation.
Paris Azis
April 21, 2017 at 7:27 pm
Hi Mark
Just keep in mind that there are no power transistors with more than three pins. So don't get confused about that. But, as it almost always happens, there are some minor exceptions to that. These include the RF power transistors which have two emitter terminals (but only one emitter inside their case) and power MOSFETs (usually bricks) with four terminals, the fourth being the so called sense terinal (for measuring the drain current through them). Thus any component having more than three pins is an I.C (although there are exceptions again, such as I.Cs having three pins i.e the TL 431 or even I.Cs having two pins only, like the TAA 55O). This is the marvelous world of electronics!
Humberto
April 19, 2017 at 11:53 am
Thanks for sharing your experiencies with us by means of Jetine Yong`s blog. Good for you.
nangoli
April 22, 2017 at 5:52 am
hi engineer thanx 4 best articls evea