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MTX 1501D (old design) power supply issues
MTX 1501D (old design) power supply issues
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Old 17th September 2019, 05:07 AM   #1
BumpinYota is offline BumpinYota  United States
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Default MTX 1501D (old design) power supply issues

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?
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Old 17th September 2019, 05:18 AM   #2
Perry Babin is offline Perry Babin  United States
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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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Last edited by Perry Babin; 17th September 2019 at 05:40 AM.
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Old 19th September 2019, 01:48 AM   #3
BumpinYota is offline BumpinYota  United States
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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....

Last edited by BumpinYota; 19th September 2019 at 02:11 AM.
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Old 19th September 2019, 02:53 AM   #4
Perry Babin is offline Perry Babin  United States
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Are the gate resistors back in the circuit?

The gate resistors and the driver transformer have to be in the circuit.
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Old 19th September 2019, 01:34 PM   #5
jkrokenberger is offline jkrokenberger  United States
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With the gate legs floating the fets may not remain 100% off and can cause conduction if power is applied. The entire drive circuit needs to be complete to test. You can remove the rectifiers to test power supply.
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Old 20th September 2019, 12:57 PM   #6
BumpinYota is offline BumpinYota  United States
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Quote:
Originally Posted by Perry Babin View Post
Are the gate resistors back in the circuit?

The gate resistors and the driver transformer have to be in the circuit.
Driver transformer is still in but the gate resistors were not. I will get the gate drive system running properly before testing next time. Thank you again for the help.
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Old 20th September 2019, 12:59 PM   #7
BumpinYota is offline BumpinYota  United States
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Quote:
Originally Posted by jkrokenberger View Post
With the gate legs floating the fets may not remain 100% off and can cause conduction if power is applied. The entire drive circuit needs to be complete to test. You can remove the rectifiers to test power supply.
Does the conduction that turns on the mosfets happen almost instantly?
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Old 20th September 2019, 01:33 PM   #8
Perry Babin is offline Perry Babin  United States
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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.
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Old 26th September 2019, 07:35 PM   #9
BumpinYota is offline BumpinYota  United States
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Quote:
Originally Posted by Perry Babin View Post
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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File Type: jpg 20190926_111008.jpg (779.4 KB, 24 views)
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Old 26th September 2019, 08:38 PM   #10
Perry Babin is offline Perry Babin  United States
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Just a note. The notch isn't necessarily wrong. The amp in the image below was working properly.
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File Type: jpg mtxdrivecircuit01.jpg (129.1 KB, 21 views)
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