Stuck on an MA Audio power supply problem

[Scanlines]

I'm stuck on this Power Supply repair.
The power supply flamed out on this MA Audio SY7011.

I replaced the power fet's (IRFP064N's)
checked the fet dirvers (totem pole)
checked the gate resistors

Amp went into protect and shut down when I powered it up.
I pulled the output rectifiers out of the circuit to isolate the PS
AND now PS fires up, but draws higher current than it should and
I noticed a "bump" in the bottom of the drive wave.

With PS FET's out of the circuit, I get clean square wave drive.
As soon as I install FET's, the drive gets dirty.

I have pulled the main secondary windings and small secondary off of the board to isolate the toroid. Still the same situation.
There seems to be 1K pull down resistors in the circuit which test OK as well




Here's some pictures of the wave bumps.

Here's a rough schematic of the transformer config.

Any suggestions ?

[Perry Babin]

Please try uploading the images again.

[Scanlines]

Sorry, the links worked on the preview.
TRy again.


gatesofmos.jpg picture by UCN_photo - Photobucket


gatesofmos.jpg picture by UCN_photo - Photobucket


gatesofmos.jpg picture by UCN_photo - Photobucket


gatesofmos.jpg picture by UCN_photo - Photobucket

[Perry Babin]

Instead of aligning the trace with the middle reference line, (no input to the scope), set it on the second to last major division from the bottom (the top of the first major division on the bottom of the display). Set the scope to DC coupling and 2v/div. Set the timebase to whatever you had it set to before. Post the waveform on the gate of one of the 064s (assuming that all are the same).

[Scanlines]

4VperDIV.jpg picture by UCN_photo - Photobucket



4VperDIV.jpg picture by UCN_photo - Photobucket


Perry;

The first one is 2v/per
and the 2nd one is about 4v/per so you can see the whole wave.

I hope the pic's are not too dark to see the grid or if you need a closer look, I'll get another shot if you need it.

[Perry Babin]

I don't see any problem with this drive signal if the FETs were in the circuit.

Do you think you could have a shorted transformer winding or a cracked core?

Do all of the power supply FETs heat up at about the same rate?

[Scanlines]

Both sets of FET's get equally warm.

I'm really used to a very clean, sharp square wave at the gates of these guys and not the little "ringing" and "rounding".

I am going to try to do a ring test on the pri and sec.
I don't think the core is cracked, but I don't have a total view on it yet.
Doesn't act physically intermittent at all.

Here are some waves on the FET gates with the secondaries reconnected.
Not sure, maybe your trained eye may see something.

Thanks for bearing with me.

wave2.jpg picture by UCN_photo - Photobucket

wave2.jpg picture by UCN_photo - Photobucket

[Dr Zeus]

Is this a MaxxSonics amplifier? Do you see R60 and R60A anywhere on the board? If it is, I've fixed a few of these which when the PS failed and also caused the drivers to fail. A1275 and C3228?

[Scanlines]

I don't see any indication of Maxxsonic. There is an R60, but no R60A.

I have pulled these drivers and tested them and even swapped them because I have had these types of non-linear problems in the past caused by drivers.

These drivers are A1023's and C1027's.

I tried a ring test on the transformer but my set up is for flybacks with more resistance.

[Perry Babin]

If this is again at 4v/div, the bump in the gate waveform could be a problem. Its peak is just above the threshold of the FETs and could be switching them on again for a fraction of a second. This wasn't in the original photos.