Yamaha PX3 2x500W amp repair, one chanel in protect

Hi everyone,
I have a Yamaha PX3 amplifier with chanel A fault.
Fault message overcurrent.
Do you have experience with repair this type of amp?
I checked all the resistors, capacitors, diodes, FETs and none of them faulty, no short or break.
Swapped si8244 driver IC with the good chanel not helped.
Measured with scope si8244 pin1. PWM, and not get signal from YDA 177-V IC, and pin5. disable get signal, so disabled it.
At good chanel si8244 pin1. i can measure signal.
It has CQ-330H current sensors at output, i do not measure any anomaly, but do not know am i measure right.
The bad output checked with scope has spike pulse nearly 60V every 5 sec.
Could you advise what to check next?
Thank you!
 

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Hi bojlihal

How when did it fail? Playing at full power? Did it burn any speaker?

"The bad output checked with scope has spike pulse nearly 60V every 5 sec" sounds like it is in protection mode and every 5 sec it tries to start up.
Dispalay says Over Current.
I would still suspect the FETs, and would change them for new ones, just to be sure. And I would think the fets are for sure faulty and the there is a shoot through when starting up, and the over current protection goes into action.
The drivers si8244, you swapped them completely so the ones from the bad side now runs well in the "good" channel?
I would also have changed these, but if they are running it should be ok.
 
Measured with scope si8244 pin1. PWM, and not get signal from YDA 177-V IC...
No PWM should still mean a high or a low, which one is it?
The bad output checked with scope has spike pulse nearly 60V every 5 sec.
Could you advise what to check next?
Is there a load connected to the amplifier ? Also, if you have an oscilloscope, can you post some waveforms, maybe those of

  • PWM output
  • Post-filter /speaker output
  • Current sensor output ?

60V means that the high-side MOSFET has managed to turn on, probably due to the over-current protection being periodically withdrawn to check auto-recovery from overload. Alternatively, the MOSFET is continuously ON, saturating the output choke every 5 secs, also triggering the protection.
 
Thanks for the replys, i traveled to holiday for a few days, after i can continue the measures.
I do not know how it failed, i bought it with this fault. I think i will try to swap or replace the FETs too.
The good side PWM pin has sine wave on it, the bad nothing.
I will inspect the mentioned things!
 
Hi,
I measured some at si8244 pins, attached the pictures.
Current sensor output is the same as good chanel, around 2,5V
Disconnected HV +/- supply there is no fault.
Do you have some advise?
Anybody has service manual or schematic for this amp? It is hard to find the fault without these.

badch1.jpg badch16.jpg goodch1.jpg goodch16.jpg
 
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