ZX750.1 up in flames.

[dartssport]

Im back in the heat sink, i rechecked the outputs, I have 6 IRF9640 and 4 IRF640, none seem to be shorted.Any suggestions what to go next. with it cycling so quickly will it be hard to get any real readings?

 

[Perry Babin]

Did you replace the driver transistors for the power supply FETs? They are Q05 and Q11 near the 494. Check them for open junctions as well as for leakage and being shorted.

There is at least 1 screw near the TL494 that has a silver solder pad around it. The screw with its lock-washer must be installed and tight. If that's in place and the amp won't power up, try twisting/pushing/pulling on the power transformer. Does that, at any point, allow the amp to power up?

If not, I'd recommend removing the main rectifiers to see if that will allow it to power up. After removing them, make sure that there are no solder bridges between the pads on the top or bottom of the board.

 

[dartssport]

I replaced Q05 and Q11 nothing seems out of order there. I twisted and pulled on the power supply nothing, the main rectifiers are FMU34S and FMU34R?

 

[Perry Babin]

Yes. Those are the rectifiers.

Before you pull them (after removing the amp from the sink), power the amp up for a few seconds and then remove power. Which of the heatsink mounted components begin to heat up?

 

[dartssport]

The fets start to heat up first after 3 cycles

 

[Perry Babin]

Both the power supply and the output transistors are FETs. Which are you referring to?

 

[dartssport]

power supply

 

[Perry Babin]

If the output transistors are remaining completely cool, remove the rectifiers and see if the power supply will power up without the FETs overheating.

 

[dartssport]

The rectifiers are out and the amp powers up with no issues.

 

[Perry Babin]

Check the rectifiers to see if they're shorted.

 

[dartssport]

Nope rectifiers are ok

 

[Perry Babin]

How did you confirm that the power supply was operating?

 

[dartssport]

I think Im a jackass, when I said after I removed the rectifiers it powered up with no issues, I mean it powered up and nothing got hot. sorry Perry

 

[Perry Babin]

Do you have approximately 5v on the gate leg of all of the power supply FETs?

 

[dartssport]

Yep still getting around 4-4.5 volts at the gate legs

 

[Perry Babin]

That would mean that the supply is operating.

Do you have a scope?

Does your 12v power supply have a variable output (voltage)?

 

[dartssport]

No dont have a scope, or adjustable power supply.

 

[Perry Babin]

Do you have access to either?

 

[dartssport]

Its a possibility on the adjustable power supply, I was thinking of ordering a PC scope with that work (Doesnt do much good right now)

 

[Perry Babin]

I'd recommend that you buy an older model analog scope instead of a digital scope.

What is the variable power supply that you may be able to borrow?