MTX TA 81001

[S Merlington]

This amp powers up but doesnt produce audio..

Reading some other threads i got some info on the amp..


Heres what i have on the OP-AMP closest to the OPTO_COUPLERS

Pin 1:-13.91
Pin 2:0.23
Pin 3:0.08
Pin 4:-15.50
Pin 5:-13.62
Pin 6:0
Pin 7:14.23
Pin 8:15.05

I have Positive 81 volts of rail and negative 82.2 volts of rail..

I also have + - 15 volts of regulated voltages on the op-amps..

I also have audio on the 2 large SMD Caps near the RCA Jacks C306 and C307

The other thing i noticed is theres no voltage on the gate legs of the power supply fets ..

Any suggestions ?

 

[Perry Babin]

This power supply drive the FETs differently. You can't use voltage to troubleshoot it. If you have the tutorial, read the 'MTX Power Supplies vs Other Amplifiers'.

The op-amp appears to be defective but the input is very near the supply voltage so the output may be inverted from that and may not actually be defective.

 

[S Merlington]

So try replacing the op-amp to start with?

 

[Perry Babin]

Yes. It's difficult to tell for certain (in this instance) if it's defective from the voltages on the terminals.

 

[S Merlington]

Ok replaced the op-amp with a brand new one ..

Heres what i have now..

Pin 1:-13.64
Pin 2:0.14
Pin 3:0
Pin 4:-15.44
Pin 5:-12.49
Pin 6:-11.44
Pin 7:-12.58
Pin 8:14.99

Still no output..

Is Q405 (C1URA) a muting transistor) if so can i pull it to see if the amp has output or leave it alone ?

 

[Perry Babin]

Post the DC voltage on the other op-amp in that area.

 

[S Merlington]

NE5532

Pin 1:0
Pin 2:0
Pin 3:0
Pin 4:-15.48
Pin 5:0
Pin 6:0
Pin 7:0
Pin 8:15.43

 

[Perry Babin]

Double-check your numbers. This op-amp feeds the other op-amp so the input to the second op-amp shouldn't be significantly different than the voltage on the output pins of this op-amp.

I missed the question about the muting transistor. Don't remove it.

 

[S Merlington]

trippled checked the voltages and i come up with the same thing as posted..

 

[Perry Babin]

What's the DC voltage measured directly across R402?

What's the DC voltage measured directly across R404?

 

[S Merlington]

R402 0 volts

R404 0 volts

Both measured directly across them

 

[Perry Babin]

Post the exact readings from your meter. 0 isn't the same as 0.000. If it's 0.000. Post that.

Looking back over the numbers, I noticed that the original op-amp had 0v on pin 6 but now has significant voltage. Do you now have significant DC voltage across the speaker terminals?

 

[S Merlington]

00.00

measured directly across the speaker terminals i get 00.05

 

[Perry Babin]

Is it possible that you damaged one of the pads for the op-amp you replaced?

 

[S Merlington]

all op-amp pads are in perfect condition..

 

[Perry Babin]

With the black probe on the amp's ground terminal. Do you read ~0v on one side of R414 and ~-11v on the other end of that resistor?

 

[S Merlington]

I have to replace a couple of things in that area that i found to be defective For 1 i found R417 to be out of tolerance.. Its a 100 ohm resistor and was measuring 217 ohms off the board..

I will update you once i replce thoose couple of things

 

[S Merlington]

With black meter probe on amps ground terminal i get


1 side of R414:0.70

Other side -11.56

 

[Perry Babin]

That would seem to indicate that the op-amp is defective or there is a solder bridge.

 

[S Merlington]

a solder bridge between the pins of the op-amp or the resistor and the op-amp?