MTX 280, no output

[ECM]

Working on an MTX Thunder 280 with no output.

Power supply is good with +/-25 volts.

Power to opamps is good with +/- 18 volts.

Outputs are not shorted, amp turns on and stays on just fine. No overheating and no signs of overheating.

Opamps appear to be fine. I bypassed the opamps by injecting an audio signal to pins 1 and 7, the opamp outputs (ne5532), with no change in amplifier output.

No change with the crossover engaged or off. EQ and Gain pots measure identical and turning them does nothing.

The only thing I have to go on now is this. When I play music thru the amp, I will hear output for a split second, then off again, over and over. Muting or protection circuit maybe?

Any ideas?

ECM

[Perry Babin]

The LM339 controls the output. When you have audio, do you have 0v on pin 13?

[ECM]

I'll check the LM339 Mr. Babin. The amp is at work, I'll be in on Monday.

[Perry Babin]

Since you don't have easy access to it, post the voltage on all of the pins of the 339.



Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:

[ECM]

Hooked the amp up this morning and it will not turn on!

I measured the voltage on the pins anyway:

1- 0.6
2- 5
3- 30.5 <--I have no idea why it's reading 30V. Checked it twice
4- .389
5- 3.2
6- .226
7- 3.2
8- 3.7
9- 5.03
10- 5.3
11- 7
12- 0
13- 0.6
14- 0.6

[Perry Babin]

What is the supply voltage on the B+ terminal? The supply voltage on this pin is typically the B+ input minus -.6v (drop across diode).

[ECM]

The input voltage is ~12 volts. I've had this issue before with my power supply and MTX. For some odd reason the amp affects the regulator in the power supply. I'm ordering a switching power supply off ebay today.

The amp in my other post, an identical MTX, turns on just fine.

[Perry Babin]

This comparator may have a shorted channel. According to the inputs, all of non-inverting inputs are higher than the inverting inputs so all outputs should be high. Is there an external component tied to pins 1, 13 or 14 that's pulling the voltage on that circuit down?

It may also have an open connection to the pull-up resistor(s). Pins 1, 13 and 14 probably go to a resistor and then to the base of a PNP transistor. There is likely another resistor between the base and emitter (which is tied to the 5v reg). These two resistors should be pulling those 3 pins up.

[ECM]

Pins 1,13, and 14 are tied together. There are no open lines or shorts between those pins and the resistors leading to the PNP MPSA56 transistor.

In circuit, the transistors all measure good. No shorts or opens and all are identical.

I'll get a few LM339's on order and see if that's the issue.

ECM

[ECM]

Installed new LM339, no dice.

Pin 3 still reading around 30V. I have a multimeter directly connected to the power supply and it's at 12V. Bad input power supply cap maybe?