MTX thunder 4200x no audio

[ian33]

hi all

This amp came in with broken rca input.I replace the rca input jacks but i do not get no audio.This amp has - + /14.3v on the rail is that normal?

All opamps have - +/9.1v

I am getting audio on most of the opamps using a small amplifer.
Any idea what to check next?

[Perry Babin]

Those voltages seem low. What do you read across the B+ and ground when you only have 15v of rail?

[ian33]

13.6v

[Perry Babin]

Post the DC voltage on all of the pins of the 494.

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:
Pin 15:
Pin 16:

[ian33]

494

Pin 1: 1.56
Pin 2: 3.06\3.08
Pin 3: .05
Pin 4: 3.31
Pin 5: 1.45
Pin 6: 3.25
Pin 7: .00
Pin 8: 13.06
Pin 9: .01
Pin 10: .01
Pin 11: 13.14
Pin 12: 13.11/13.14
Pin 13: 4.88
Pin 14: 4.88
Pin 15: 4.87
Pin 16: .04

[Perry Babin]

Follow the circuit back from pin 4. If it goes to a PNP transistor's collector, remove the transistor and power up the amp (all transistors clamped tightly to the heatsink and 10-15 amp fuse in the B+ line).

Does that allow it to power up and produce clean audio?

[ian33]

It is connected to MPSA56.I removed it amp plays good audio and has - +/22.32 rail voltage now.MPSA tested good off the board.where should i go from there?

[Perry Babin]

What is the base of the MPSA56 connected to? If it connects to one of the pins of the LM339, post the voltages on that IC.

[ian33]

The base is connected to pin 4 on the LM339 through a 10k resistor.It is also connected to the themistor through that same resistor.one end of the resistor is directly connected to the base of the same MPSA56 and the next end is directly connected to the emitter.

LM339

Pin 1: 4.69
Pin 2: 4.69
Pin 3: 12.39
Pin 4: 4.88
Pin 5: 7.46
Pin 6: 3.53
Pin 7: 4.88
Pin 8: .83
Pin 9: 3.36
Pin 10: 4.64
Pin 11: 3.36
Pin 12: .5mv
Pin 13: 1.1mv
Pin 14: 4.68


mpsa56 pads
c:.66
B: 4.74
E: 4.86

[Perry Babin]

Are you sure that the base isn't connected to another pin (1, 2 and 14?) of the 339 via a lower value resistor?

Are you sure that the A56 isn't leaky?