heat problem: changed MPT50N05E to IFRZ44PBE

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coryvirtue

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Joined 2014
I am trying to resurrect an old Soundstream MC500, 4 ch. car amp. I bought it burnt out about 20 years ago... It needed new power supply FETs. The original ones were MTP50N05E (not made anymore) and were crossed with IRFZ44PBE FETs. They are installed. When turned on, there is proper output voltage across the rectifiers (good), but, in about 20 sec. the toroidals get extremely hot and if the power is left on (connected to heat sink), the amp's heat sink gets crazy hot.

Everything looks good and working. No shorts or burnt out FETs

The power supply has it's own board and it is not connected to any other circuit. It is run with a TL494CN chip.

Do I require different gate resistors for the IFRZ44 FETs? Original resistors are 47 ohm. Maybe something is up with the TL494 chip? Any thoughts?
 
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coryvirtue

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Joined 2014
Immediately off the TL494 chip, there is a 150 ohm resistor on both pins 9, E1 and 10, E2. There is a connection to one side of the gate leg resistor of each FET. E1 goes to 4 FETs and E2 goes to the other FETs. The resistors immediately coming off each gate leg of the IRFZ44 is 47 ohms. So there are 8 of these in total because I have 8 IRFZ44 FETs. (4 channel amp, two power supplies)

So, if I check the resistance from the chip pin leg, E1 or E2, to the pin legs of the IRFZ44, it will read 197 ohms because the reading will go through both resistors.

I might be calling the pull down and gate resistors wrong, or backwards... sorry.
 
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Perry Babin

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Joined 2003
bcae1.com
The attached is only an example. In this amp, R208 and R209 are the pulldown resistors. The 47 ohm resistors are the gate resistors.
 

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coryvirtue

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Joined 2014
I changed the resistors to 100 ohms. Substantially better!. Now, how warm do toroidals supposed to get in normal operation? When I turn the power on for about 2 min, it is warm to the touch, not hot. Should I decrease the resistor value more, say to 50 ohms? I have 57 ohms on hand...

SemperFi, the FETs blew years ago from the other owner. I did replace all output transistors/FETs on the output and power supply boards. All good. But, on this amp, the power supply board is completely separate from the other boards, so I knew it is related to the power supply board because the output board I disconnected from the power supply board
.
 
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Perry Babin

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Joined 2003
bcae1.com
You can't go too low because the IC may not be able to handle it.

Wade Stewart designed this amp. I can't find his email address right now. If anyone knows how to contact him, please post the information here.

Wade would be able to give definitive answers and solutions.

Post a photo of the board.

Is it the main power transformer that's getting hot or secondary filter inductors?
 
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coryvirtue

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Joined 2014
Main power transformers. The problem is probably using the 'recommended equivalent FETs'. They obviously are slightly different... I believe the originals were 50N05E but now I am using the IRFZ44PBE because the 50N05E are not available.
 

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