Im having problems pinpointing the the real problem with this thing.When I got it all the 50volt 220uf input caps had exploded.The stuff is everywhere inside it.
The 2 A56 drivers and the 10 ohm resistor that drives half of the small transformer are toast.So are the 5 HUF75339 that are on that side of the PS.The other half of the PS was fine though.
I removed all 10 75339's,replaced all 4 A56 drivers,the 2 10ohm resistors and the 10 50volt input caps.I applied power to test the wave on the gate resistors before installing 10 new mosfets and the same 2 A56 with the 10ohm resistor went out again.So this time I also replaced the TL494 along with the 4 A56 and 10ohm resistors and the same thing happened.But this time I noticed A small puff of smoke come from D209 which is connected to another A56(Q214) next to the other ones.I have a feeling there is something more going on here then just replacing the bad parts.
Anyone seen this before?Could the small transformer be shorted?Ive never seen one go bad before.What is the purpose of Q214 and D209?
This amp had lots of black rubbery glue securing the 10 caps to the board and some of it is on the small transformer and surrounding SMD's.I had to remove 95% of it to replace the caps.Could there be some kind of issue with this glue do damage to certain parts like the with the old Yamaha M-80 house amps?
The rest of the amp seems fine but wont know till I get the PS going.The 15v regulators aren't shorted.Output mosfets seem fine.Im very familiar with the 5532 and Opto problems with these but havent seen a PS problem like this with exploded Caps.Im hoping someone can point me in the right direction so I dont waste time with trail & error approach since I dont have a schematic.
The 2 A56 drivers and the 10 ohm resistor that drives half of the small transformer are toast.So are the 5 HUF75339 that are on that side of the PS.The other half of the PS was fine though.
I removed all 10 75339's,replaced all 4 A56 drivers,the 2 10ohm resistors and the 10 50volt input caps.I applied power to test the wave on the gate resistors before installing 10 new mosfets and the same 2 A56 with the 10ohm resistor went out again.So this time I also replaced the TL494 along with the 4 A56 and 10ohm resistors and the same thing happened.But this time I noticed A small puff of smoke come from D209 which is connected to another A56(Q214) next to the other ones.I have a feeling there is something more going on here then just replacing the bad parts.
Anyone seen this before?Could the small transformer be shorted?Ive never seen one go bad before.What is the purpose of Q214 and D209?
This amp had lots of black rubbery glue securing the 10 caps to the board and some of it is on the small transformer and surrounding SMD's.I had to remove 95% of it to replace the caps.Could there be some kind of issue with this glue do damage to certain parts like the with the old Yamaha M-80 house amps?
The rest of the amp seems fine but wont know till I get the PS going.The 15v regulators aren't shorted.Output mosfets seem fine.Im very familiar with the 5532 and Opto problems with these but havent seen a PS problem like this with exploded Caps.Im hoping someone can point me in the right direction so I dont waste time with trail & error approach since I dont have a schematic.
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The transformers do sometimes short.
Q214 is the transistor used for switched power for the 494 and other circuits.
I can't see any purpose for D209 in normal operation.
Q214 is the transistor used for switched power for the 494 and other circuits.
I can't see any purpose for D209 in normal operation.
Ok.I removed the small transformer and applied a high frequency square wave to it thinking I would be able to see a difference between the good and bad winding.Nothing obvious stood out so I removed one from a scrap amp and got similar results only the voltage was lower then the original transformer from the amp that was killing the drivers.
I didnt want to gamble with the second one so I took a third amp that I had removed the heat sink and all parts attached to it from and found it to have a good wave form on the gate resistors.
I swapped the small transformer,the 4 a56,10ohm resistors,TL494,Q214,D209 and D101.
Everything seems fine.I have a good wave form from all 10 gate resistors and D209 didnt fail this time.
I dont have any of the 75339 mosfets so I am going to use IRF3205 in there place.Since this amp has 10 ohm gate resistors with a single 39 ohm resistor driving each bank(5) of them.Is there any changes that should be made?Or are the mosfets similar enough to the originals?
I didnt want to gamble with the second one so I took a third amp that I had removed the heat sink and all parts attached to it from and found it to have a good wave form on the gate resistors.
I swapped the small transformer,the 4 a56,10ohm resistors,TL494,Q214,D209 and D101.
Everything seems fine.I have a good wave form from all 10 gate resistors and D209 didnt fail this time.
I dont have any of the 75339 mosfets so I am going to use IRF3205 in there place.Since this amp has 10 ohm gate resistors with a single 39 ohm resistor driving each bank(5) of them.Is there any changes that should be made?Or are the mosfets similar enough to the originals?
You can use resistors in place of the two primary windings of the driver transformers to see if the drive circuit is OK with the transformer removed.
I don't remember if this amp has a regulated power supply. If it does, reduce the 12v supply voltage to force the duty cycle to go to ~50% (max). If the amp draws more current at low voltage than at higher voltage, the value of the gate resistor needs to be reduced.
If it's unregulated, power the amp up for a few seconds at idle to confirm that it's not drawing excessive current. If it is not, unclamp the power supply FETs (but NOTHING else), lift them from the heatsink and power it up for a few seconds at a time to see if the FETs get hot quickly. If you can leave it powered up for 30 seconds or more and the FETs remain cool, the resistor value is likely OK (this is for an unregulated supply only).
I don't remember if this amp has a regulated power supply. If it does, reduce the 12v supply voltage to force the duty cycle to go to ~50% (max). If the amp draws more current at low voltage than at higher voltage, the value of the gate resistor needs to be reduced.
If it's unregulated, power the amp up for a few seconds at idle to confirm that it's not drawing excessive current. If it is not, unclamp the power supply FETs (but NOTHING else), lift them from the heatsink and power it up for a few seconds at a time to see if the FETs get hot quickly. If you can leave it powered up for 30 seconds or more and the FETs remain cool, the resistor value is likely OK (this is for an unregulated supply only).
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