MTX 1501D

[m4ick]

This had a blown power supply and all of the outputs were in tact.

I replaced the 39R2 resistors and all 8 75339ps. It powers on an oscillates but FET260 (IRFB42N20D) gets hot EXTREMELY fast. Within seconds you cant touch it. In the 81000D I just fixed that part did not get that hot that fast.

I'm not familiar with this section of the amp, if I had to guess it appears to be a voltage regulator. Any idea why it would be heating up so quickly?

 

[m4ick]

Also FET260 is not shorted and I have all 3 LEDs lit.

 

[Perry Babin]

Compare the resistance across pins 5 and 8 for each of the opto-couplers. Is one significantly lower than the other?

 

[m4ick]

No, they both read ~990 ohms and slowly rising while charging the caps.

I did notice though that one of the caps marked 220 10A has a VERY small dent in it. It's the one right next to the mounting screw. In this amp it says "e2 220 10V". I don't know if it would affect FET260 but I thought you should know.

 

[Perry Babin]

On the opto-couplers wit hthe black probe on pin 5 and the red probe on pin 8, what is the DC voltage for each one?

 

[m4ick]

+27v and +28.2 after muting is released. The +27 seems to drain much more quickly after powering off.

 

[Perry Babin]

27v is too much. The regulated voltage should be very near 20v. The gates of the FETs may break down with anything over 20v. This would drive about 27v to them.

 

[m4ick]

I have no idea where the voltage regulators are in this amp. Where should I be looking for this over-voltage?

 

[Perry Babin]

What's the voltage across D260?

 

[m4ick]

26.7v

 

[Perry Babin]

That's a 20v Zener. You can confirm by looking up the markings. It needs to be replaced. When you order them,order extras.

Post the DC voltage on all 8 pins of the UC3845.

Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

 

[m4ick]

Do you know the package type of that diode?

Do you want the voltages on UC3845 before or after the diode is replaced?

 

[m4ick]

I was just looking around in this amp and Q240 is missing, it is present in your photos.

EDIT: Nevermind, it's been modded to be 1 ohm stable. I noticed it is missing the other 1501d they gave me. No wonder its blown.

 

[m4ick]

Ok, so my meter had a low battery and I was getting high voltage readings.

I read 19.2vDC across the zener.

Do you want voltages on the UC3845 referenced from battery ground?

EDIT: I'm trying to figure out why FET260 is getting hot.

 

[m4ick]

UC3845 referenced from battery ground:

Pin 1: 2.1v
Pin 2: 2.5v
Pin 3: 0v
Pin 4: 2.4v
Pin 5: 0v
Pin 6: 2.4v
Pin 7: 10.8v
Pin 8: 0v

 

[Perry Babin]

FET260 will get hot if it's not clamped down.

With all transistors clamped to the heatsink, does the amp produce clean audio?

 

[m4ick]

It powers up and produces audio. I can feel FET260 getting warm but I left it idling for about an hour and it seems stable. I'll test it on some subs soon.

Thanks.