MTX 6001 (red protect light)

[scampo77]

Can I use the 1N4742A as a replacement or should I find out a proper replacement? I dont want to lose more of these transistors.

And am I ok with the 33.75k ohms instead of 31k?

 

[Perry Babin]

The resistance is not a problem but if you have to order parts, you may as well order the right ones.

The 4742 shouldn't cause immediate problems but it would cause the regulator transistor to run hotter which could cause it to fail prematurely. Again, if you have to order parts, order the right one.

 

[scampo77]

Do you happen to know the part number? Or should I call MTX again?

 

[Perry Babin]

Which part number?

 

[scampo77]

The part number for the failed diode I tried to put the 1N4742A in place of. It has 4746AP marked on it and was wondering if this is part of, or an entire part number.

 

[Perry Babin]

1N4746A

It's an 18v, 1w Zener diode.

 

[scampo77]

Thank you very much for all the help and advice, I will get ordering and I will be back and a couple weeks.

 

[scampo77]

I installed my new parts today and I powered up via a big resistor and the speaker output slowly just climbed to 50v again. I shut it down right away and I did not seem to melt the transistors or diodes I just replaced.

I ran around the board checking for shorts and blown resistors again, and again nothing.

I am getting frustrated with this thing.

 

[Perry Babin]

Are the two regulators producing ±17v output?

 

[scampo77]

I hate to sound like an idiot but which ones are the regulators?

 

[Perry Babin]

Q400 and Q401. Measure the DC voltage on leg 3.

 

[scampo77]

Q401 is
B -6.2
C -7.5
E -5.5v

and

Q400
B 0v
C 8v
E 1.5v

At first power up the collector on both, runs away to +-50v then after about 4 seconds with power applied to the amplifier, both Q400 and Q401 snap back to the readings mentioned above

 

[Perry Babin]

Is the rail voltage on the output transistors dropping as well?

 

[scampo77]

The IRFp240 all have the same symptom on start up (run to +-50v then snap back to +-8v)

same thing on the speaker outputs except they both snap back to -7.5v

power supply is.... on the center leg of the 75339P i have +9v

 

[Perry Babin]

It sounds like the amp is either drawing excessive current or something is wrong with the 12v power supply.

 

[scampo77]

What would cause it to draw current if I don't have a speaker hooked up to it?

The power supply is fine, it powers up the rest of my broken stuff with no issues

 

[Perry Babin]

How much current is it drawing and what's the rated current of the 12v power supply?

 

[scampo77]

I Dont have any way to measure amp draw here with me today, but it is beating up on my PSU pretty bad. I drop from 14v to 9v and its a 5 amp supply. it is a safe bet that im using up everything the supply has to offer

the power supply of the amp gets hot very quickly, even though I have my big resistor inline on the way in to the amplifier.