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Old 19th June 2006, 04:13 PM   #1
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Question New member in trouble, needs your help!

Hi everyone, I am a new member here. I am happy to be here, as I found this site a terrific source of the most valuable information in electronics and it has a brilliant community
I have an Audiobahn amp, huge, 2k watts, which I am trying to fix. Problem: power supply is not working (it is huge 2-transformator, 16 MOSFETS). When I first got it DBL494 was totally dead and all MOSFET's in power supply were fried.

I replaced all MOSFETS - it isn't powering on.

Replaced DBL494, test run, and what is happening:
- DBL494 is getting REALLY HOT very fast. It will die a painfull death in a matter of seconds.
- MOSFETS are getting really hot too (will be dead very fast too); they are seems to be not switching anything. It's like DBL isn't generating anything, and keeps MOSFETS constantly open
- current draw > 25A
- If i short thermal protection - MOSFETS are fine; but DBL are getting HOT even quicker.
- All huge diodes are fine together with all capacitors - tried unsoldering; same >25A draw

Please, help me out!!! What will be next to look at? Unfortunately, I don't have oscilloscope now. Thank you
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Old 19th June 2006, 06:41 PM   #2
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The 494 chip will be driving a pair (or more) transistors, if theses transistors (or even one) is shorted or open the mosfets will conduct hard (which it sound like with the 25A current draw with no output diodes) and the 494 may be driving a short.

If the mosfet blew, they will usualy take out the gate resistors and the driver transistors and posibly the PWM(dbl494), as always if you had access to a scope you could remove the mosfets and see what they are getting for a drive waveform. (should be close to square)
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Old 19th June 2006, 08:12 PM   #3
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Thanks! Mosfets are driven by 2 transistors, which I tested to be fine with multimeter. I need to doublecheck that, 494 definitely driving a short Removing them tonight...
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Old 20th June 2006, 12:45 AM   #4
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Did you remove them from circuit? to test them properly you have to remove them from circuit or else you may be testing the 494 if on of ther junctions is open in the transistor.

BTW the last amp I fixed that had 2 transformers had 4 pairs of driving transistors (1 transformer has 2 pairs).

Look at figure 2 here to see how they use the bipolars to drive the fets.

In my own experence I've noticed that the transistor that pulls the gate to ground usualy goes open causing the fet not to turn off and large amounts of current to be drawn.

check the things I suggested and get back to us.
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Old 20th June 2006, 03:27 PM   #5
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Thanks you, I really appreciate it! This schematic really helps. I'll try pulling them off tonight for a check. The idea, that effect can be caused by burnt to open state transistor haven't came to my head
Ironically, I have another old, but still not forgotten amp (Rockford Fosgate), which is doing same thing, but with output MOSFETS, which are never closing - frying speakers It is not helping, that mjority of transistors there are in SOT23 case
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