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Behringer Sub TDA8954 repair
Behringer Sub TDA8954 repair
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Old 23rd November 2015, 03:44 PM   #1
Alexander Baev is offline Alexander Baev
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Default Behringer Sub TDA8954 repair

Hi.
Can you please help me to repair Behringer Subwoofer (B1200D Pro). I didn't find the schematic. I used TDA8954 datasheet as a reference. I attached the schematic from the datasheet.

I replaced the chip but the subwoofer stayed silent. After checking I found a broken leg of Llc1 (restored it) and strange value of Rvssa (150 ohm instead of 10, changed it).
Now all voltages seems ok (+-37V), but OUT1 is -20V, OUT2 is -12V.
Each measurement was done in seconds (less then 5 sec I suppose), because TDA8954 worms up greatly, and I had to remove it from the heatsink to be able to measure.
Can I check something more?
Is the chip dead again?
Thanks.
Behringer Sub TDA8954 repair-2-210-audio-power-amplifier-class-png
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Old 23rd November 2015, 04:35 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The first thing to do is place a heatsink on the ic. With no load,VDD and VSS OK? Does it oscillate pin 10 and pin 14? If not it is probably damaged.
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Old 23rd November 2015, 05:32 PM   #3
Alexander Baev is offline Alexander Baev
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Quote:
Originally Posted by JonSnell Electronic View Post
The first thing to do is place a heatsink on the ic. With no load,VDD and VSS OK? Does it oscillate pin 10 and pin 14? If not it is probably damaged.
With no load VDD and VSS are ok (+-37V). There is no oscillation on pins 10 and 14 (just constant negative voltage: -20V and -12V...
Ok, it seems it is damaged. What should I check before replacement? How can I find the reason of the damage?
PS: The voltage from Rvssa is applied to VSSA pin of the chip. Before the replacement of Rvssa there was -3V. Can this be the reason of the chip damage?
Thanks.

Last edited by Alexander Baev; 23rd November 2015 at 05:39 PM.
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Old 23rd November 2015, 08:31 PM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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VSSA is the driver supply internally. I should think the chip just died, either of natural causes or overload. Make sure the Boot Strap capacitors are not leaky!
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Old 26th November 2015, 07:13 PM   #5
Alexander Baev is offline Alexander Baev
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Quote:
Originally Posted by JonSnell Electronic View Post
VSSA is the driver supply internally. I should think the chip just died, either of natural causes or overload. Make sure the Boot Strap capacitors are not leaky!
Is there some technique for testing without expensive equipment?
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Old 26th November 2015, 08:35 PM   #6
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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It is class D and requires at least a reasonable oscilloscope otherwise it is all guess work.
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Old 27th November 2015, 08:36 AM   #7
Alexander Baev is offline Alexander Baev
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Quote:
Originally Posted by JonSnell Electronic View Post
It is class D and requires at least a reasonable oscilloscope otherwise it is all guess work.
I have an oscilloscope with 10MHz bandwidth. Is it enough? Where I can get the information on the techniques needed for testing D class amps? What about the leakage?
If my questions are too obvious for D class amps builders please give me the reference where I can learn the theory.
Thanks.
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Old 27th November 2015, 12:32 PM   #8
fireworks is offline fireworks  Switzerland
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A 10MHz oscilloscope is enough for repairing most class D amps. You might also need an ESR meter which is about $15 from EBay to check the capacitors. With some effort you could use the oscilloscope as an ESR meter.
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Old 27th November 2015, 02:06 PM   #9
Alexander Baev is offline Alexander Baev
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Originally Posted by fireworks View Post
A 10MHz oscilloscope is enough for repairing most class D amps. You might also need an ESR meter which is about $15 from EBay to check the capacitors. With some effort you could use the oscilloscope as an ESR meter.
But what should I check before turning on an amp after changing the amp chip (or for example making the amp from scratch). I ask because I changed TDA8954, turned the amp on, and the chip has died, and I even don't know why. Two possible reasons are mentioned in my first post...
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Old 27th November 2015, 03:25 PM   #10
DIYBras is offline DIYBras  Brazil
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Maybe the -VSSA pin is the most negative pin in the internal IC substrate and because of 150R "initial" value, the voltage here goes less negative than output and this *maybe* have destroyed the chip due some internal parasitic thyristor or bipolar action.
For example, chips like TDA7293 explode if the output -V pin is more negative than pre -V pin.
So *maybe* with all correct values and legs in place, the amplifiers works again.
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