ok i have this amp for repair it had a burnt power supply,and the relay doesn't engage.i changed the drive transistors, gate resistors.but the power supply fet's sitll over heat and short. what else would cause this?
With this driver board, I've heard that replacing the 2 TL072s on the driver board will restore audio. I've never run into this fault so I can't confirm that this information is accurate.
Have you run all of the pots and switches through their entire range to see if the amp will produce audio?
When amps using this board are going into protect for no apparent reason (all FETs OK), the two MMBTA42s in the center of the board are generally the cause.
Have you run all of the pots and switches through their entire range to see if the amp will produce audio?
When amps using this board are going into protect for no apparent reason (all FETs OK), the two MMBTA42s in the center of the board are generally the cause.
Can you power it up long enough to get a DC voltage reading on the gate legs of the power supply FETs?
If so, what is the DC voltage on them?
If so, what is the DC voltage on them?
Did you try pulling the rectifiers to see if it would power up and not stress the power supply FETs with the rectifiers out of the circuit?
Does the gate waveform go all of the way to ground?
If so, is the falling side of the waveform perfectly vertical all of the way down to about 2v?
If so, is the falling side of the waveform perfectly vertical all of the way down to about 2v?
For reference, post a photo of the waveform on pin 9 of the TL494 (Ka7500 or whatever driver IC they're using).
You should calibrate your probe. There should be a calibration test point on the front of the scope. Touch the probe to it and adjust the trimmer capacitor on the probe until the top and bottom lines of the calibration waveform are flat.
Are all of the gate waveforms (on each of the power supply FETs) the same?
The switch point seems a bit high (on the falling edge of the gate waveform) and that could be why the FETs are overheating. The ripple on the bottom of the gate waveform is making it a bit difficult to understand precisely what's happening. Can you clip the scope probe ground clip a bit closer to where the third leg of the FETs connects to the ground plane?
Are all of the gate waveforms (on each of the power supply FETs) the same?
The switch point seems a bit high (on the falling edge of the gate waveform) and that could be why the FETs are overheating. The ripple on the bottom of the gate waveform is making it a bit difficult to understand precisely what's happening. Can you clip the scope probe ground clip a bit closer to where the third leg of the FETs connects to the ground plane?
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