MTX 1501D (old design) power supply issues

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This is a bit odd.

Amp died from vibration damage from several output fets legs breaking off. Power supply fets and 2 gate resistors all let out the magic smoke. Removed all damaged components, cleaned up the board. All fet pads read independent of each other using diode setting. Fitted new mosfets in (same type as original and did not fit gate resistors). Applied B+ and Ground to terminals for just a moment then removed - thank goodness too, as the power supply fets got VERY hot for that half second.

Remote was not given 12v at any point. Will the power supply fets turn on full time without a drive signal given to them? I'm at a loss given that there is always full time B+ on the center leg and the 3rd leg of the fet drives the transformer, whose center tap is ground. So the only way for the Fets to get crazy hot like that is if they all blew somehow instantly or were all turned on full time somehow?
 
The regulators will fail in these amps within seconds if not clamped to the heatsink and will typically do a lot of damage.

When the power supplies fail, the PNP driver transistors and the 10 ohm resistors on them sometimes fail but they won't get the drive signal past the driver transformer.

You may have open gate resistors..
 
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(All this testing was done with the amplifier off.) I also removed both gate resistors as they were very very toasty. With the gate resistors removed there should be no drive to the power supply mosfets, but with the amp off it shouldnt matter right?

However those power supply mosfets heat up QUICK if ANY voltage is applied to the main B+ and ground terminals no matter how brief its application. (Power regulators shouldnt be on if the amp is off correct?)

AFAIK arent the power supply mosfets electrically isolated (via the power transformer) from the output side of the amp? So blown output mosfets wouldnt cause excessive current draw with the amp off correct? It is behaving as though the fets themselves are shorting internally acriss the drain and source but they test out perfectly fine on my transistor tester....
 
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The gate of the FET is essentially a perfect small capacitor with essentially no leakage. Virtually anything can charge the gate to where it will conduct. This can be as simple as touching the gate. It may even be possible to have the gate charge from the capacitive coupling to the drain. Once charged, it remains charged until the voltage is drained. This is typically done by the drive circuit.
 
The gate of the FET is essentially a perfect small capacitor with essentially no leakage. Virtually anything can charge the gate to where it will conduct. This can be as simple as touching the gate. It may even be possible to have the gate charge from the capacitive coupling to the drain. Once charged, it remains charged until the voltage is drained. This is typically done by the drive circuit.

Thank you for the help Perry!! I got the amp fired up today!!

For anyone else that might see this, attached is a picture of the drive wave going to the gate pad of the powersupply side. That notch on the upswing went away once i fitted the 75339P3 power supply mosfets!
 

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Well ive discovered something that also makes perfect sense but i hadnt anticipated.

But if you place ground of your oscilliscope to the amp's main ground, then place the probe on the aluminum heatsink with time divide set to 5us and voltage divide to 1v, you can see the ouptut switching wave (57kHz) being induced in the heat sink!!

You would not believe how long i spent trying to find the short to the heat sink...until i momentarily fired up the board on my rubber mat on the desk with the heat sink in my lap a good 6" away! LoL
 
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Heatsinks aren't always grounded. On some of these amps, there was a small capacitor (0.1uF?) between a screw and ground. You can add one if your amp doesn't have one. Paralleling a 1k resistor with the cap is a bit better because it drains off any DC as well.

This is only to shunt capacitively coupled noise to ground. If you have a bad insulator that's shorting to the heatsink, that's an entirely different issue.
 
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