Faulty MTX 500D

[Chuppster]

HUF75339P3 Fairchild Semiconductor | Mouser

Will these be a sufficient replacement for FET201-204?

 

[Perry Babin]

Those will work.

 

[Chuppster]

I replaced the resistors with 390 ohm and replaced FET201-204. Replacing the resistors was interesting - some of the leads were rather burnt and had seprated from the board, but I think everything is okay now, there seems to be a connection where there needs to be. No power, though. Now should I go for the components over the blackened area of the board, such as Q691-Q65?

 

[Chuppster]

I have replaced the power supply transistors as well as the blown out resistors. No LED, but the power supply transistors get plenty warm. Next, should I replace these?
http://www.bcae1.com/temp/mtx500dregulatordamage01.swf

Does anybody have the schematics for this amp?

 

[Perry Babin]

If you pull the rectifiers, does the power supply function without the FETs overheating?

 

[Chuppster]

I pulled the 2 rectifiers, still no LED. The power supply transistors don't' heat up, though.

 

[Perry Babin]

Is the area around the regulators discolored?

Have you checked the output transistors to confirm that they're not shorted?

 

[Chuppster]

I'm getting about 28 megaohms between pins 1-3 both ways on each of them. Defiantly not shorted. These are SSP45N20A, FET 101-104. The only discoloration on the board centers around those resistors I replaced.

 

[Perry Babin]

What about 1-2 and 2-3?

 

[Chuppster]

There are no shorts, regardless of polarity.
I seem to have mixed up my rectifiers. What are the model numbers for REC201 and REC202?

 

[Perry Babin]

Rec201 should be the positive rectifier. Rec202 should be the negative. Confirm that the center leg of the positive rectifier goes to the positive terminal of one of the rail caps and the center leg of the negative rectifier goes to the negative leg of one of the rail caps.

 

[Chuppster]

Okay, I put the FEP16CT in REC201, it appears to be positive.
http://www.mouser.com/ds/2/149/FEP16CT-188009.pdf

With them in, still no LED. Voltage between B+ and ground drops to around 8 volts when rem is applied.

 

[Perry Babin]

Do the rectifiers read as shorted in the board?

 

[Chuppster]

Only one way. Like it should, I presume.

 

[Perry Babin]

Do you read a short from the center leg to either of the other legs on either rectifier?

 

[Chuppster]

Okay, my bad, this multimeter is slower at reading than I thought. It reads a short between the legs 1-2, 2-1, 2-3, 3-2 on both.

 

[Chuppster]

Okay, my bad, this multimeter is slower at reading than I thought. It reads a short between the legs 1-2, 2-1, 2-3, 3-2 on both.

What I meant to say is that it reads resistance between the legs 1-2, 2-1, 2-3, 3-2 on both.

 

[Perry Babin]

What do you mean by 'resistance'? 0.1 ohms is resistance but also would indicate that there is a problem.

 

[Chuppster]

No resistance between legs 1-3, re stance between all other pins starts in the hundreds of ohms and climbs continuously.

 

[Perry Babin]

That's normal. It will save time if you take the time to explain precisely what you mean.

Remove the output transistors and confirm that there are no solder bridges between their pads. Does that allow the amp to power up?