BLAUPUNKT VA4100 schematic needed - diyAudio
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Old 18th October 2010, 10:25 PM   #1
ultra is offline ultra  Greece
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Default BLAUPUNKT VA4100 schematic needed

For the last two days i have been trying to repair this 4 channel amp.
When i opened the cover i found out that someone has tried to repair the amp before me.
Many SMD components were re-soldered in a bad way.....
Anyway, the amp wouldn't power up so i used my lab power supply to power the secondary side (the audio channels).
All 4 channels did not draw excessive current but one of them had 30VDC on the output.
After repairing, i looked the preamp section that tested just fine.
So far, all seemed OK, except for the PWM.
Whenever i tried to power it up, the amp got started but only for a second-no excessive current drawn...just about 1.5A.
It uses the TL494 chip and i noticed that pin4 which is designated for dead time control (and maybe for protection sometimes), after one second measures 5V.So thats the cause that the amp shuts down.
I think that if i disconnect pin4 from the circuit and ground it, it will work.
If some member could produce a schematic, i could repair the amp without canceling the protection scheme.
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Old 18th October 2010, 10:35 PM   #2
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Do you have any DC on any channel as the amp tries to power up?

Remember to check the bridging terminals, the non-bridging terminals are likely ground.

Did you have any open or out-of-tolerance emitter resistors?
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Old 19th October 2010, 01:35 AM   #3
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All 4 channels are working properly. DC offset is below 50mV. Output voltage without a load is 13.6VAC before clipping (due to supply voltage limit). I cannot make power measurment as i am using a Hameg bench power supply with current limit at 0.5A per rail.
Idle current for all channels is around 50mA per rail. The amps have a fixed idle current sensed by a transistor between the Sanken power transistors.
As a ground, i use the ground plane between secondary snap capacitors.
With my oscilloscope (that also displays frequency), i can momentarily measure 38.6KHz pulses to the gates of the IRF3205s.
Output voltage of the PWM power supply is +/-31VDC but as the PWM shuts down this voltage decreases slowly.
Now, i noticed something really strange:
When i power the amp from my 13.5V/68A SMPS, it shuts down really fast.
But when i powered it from the Hameg power supply (current limited at 0.5A),
the amp stays on longer (4-5 seconds) before shutting down. So when 13.5V are current limited to 0.5A, the amp stays on LONGER.
That is how i measured voltage present at pin4 of 494-it starts around 0,3V and grows up to 4.95. Then the PWM shuts down.
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Old 19th October 2010, 01:44 AM   #4
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I did not have any out-of-tolerance emitter resistors. Two 0.22R/2W resistors are used in parallel per power transistor.
The reason one channel had DC on its output was a bad soldering to the driver transistor. I did not replace any parts in the audio section.
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Old 19th October 2010, 12:04 PM   #5
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Can you post a photo of the inside of the amplifier?
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Old 20th October 2010, 03:58 PM   #6
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I hope this pic helps...
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Old 20th October 2010, 05:24 PM   #7
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There is no attachment or link.
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Old 20th October 2010, 07:42 PM   #8
ultra is offline ultra  Greece
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Sorry, i took a high resolution photo that i couldn't upload....here it is...
Attached Images
File Type: jpg IMGP0323.JPG (493.4 KB, 52 views)
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Old 20th October 2010, 08:53 PM   #9
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What is pin 4 connected to on the board?
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Old 21st October 2010, 11:31 PM   #10
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Pin 4 of TL494 is connected through a 47uF/16v and a 47K to Gnd.
It is also connected through a 4R7 to the collector of a transistor and then to another one, with several resistors around. This whole circuit ends up in a thermistor mounted on the heatsink. Although the component measures OK (12K cold), it seems that some parts are faulty and give a false alarm to the 494. This makes sense because voltage of pin 4 TL494 increased slowly up to 4.95V, forcing the Ic to have infinite dead time. And maybe thats the reason why the amp stayed on for 4-5 seconds.
What i did, was to simply remove the 4R7 and i cancel the entire thermal protection circuitry. Then i installed a Normally Open thermal switch cutting off at 75 Celsius. Now the amp works OK even with some modification.
Thank you very much Perry for your help.
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