MTX Thunder 81001 component value/schematics

[Perry Babin]

Is that diode leaky/;shortd?

 

[Maxikill97]

It seems to read okay with the multimeter.
No strange readings.

 

[Perry Babin]

Reinstall the op-amp (a new one if you have one) to see if you get output from pin 1 with the diode out of the circuit.

 

[Maxikill97]

Installed a new op-amp. Drive disappears from U401 Pin2 and Pin3 pads.

 

[Perry Babin]

Do the op-amps have positive and negative supply voltages?

 

[Maxikill97]

Yes, + and - 15v

 

[Perry Babin]

I don't know what could be happening to the signal. The input impedance to that op-amp is in the mega-ohm range.

 

[Maxikill97]

Yes, that is true. A very high impedance between U401 Pin2 and Pin3 out of the circuit.
It's about 0.8Mohm

 

[Perry Babin]

Do you see a signal if you set the vertical amplifier on the scope to 0.5v/div?

 

[Maxikill97]

Maybe this has something to do with it.
U400 Pin1 and Pin7 output sine wave has 0v as DC voltage with a Utop of 450mV (P-P voltage is 900Mv)

U401 input Pin2 is a flat 1.15v DC voltage
U401 input Pin3 is a flat 0.53v DC voltage

 

[Perry Babin]

The input terminals 2 and 3 of U401 should be identical to the outputs of U400. If you touch the board around this area (not on a trace or pad), does the scope deflect any?

 

[Maxikill97]

Sorry, I don't know excactly what you mean.

Should I trace U401 Pin2 or Pin3 while touching the PCB? And look for bumps in the voltage?

 

[Perry Babin]

I'm looking for leakage/contamination on the board. If there is something like leaked electrolyte, the trace will deflect when touching nothing more than the board.

 

[Maxikill97]

I could not measure any deflections when touching the board on several places around that area.

I have also cleaned the board with isopropyl after soldering.

 

[Perry Babin]

Is R405 (7.5k) within tolerance?

 

[Maxikill97]

R405 is spot on. Reads 7500ohm

 

[Perry Babin]

The last time you installed U401, was it a new/unused IC? Or was it the one you removed?

 

[Maxikill97]

I installed a brand new NE5532

 

[Perry Babin]

Is one end of R405 connected directly to ground (0 ohms)?

 

[Maxikill97]

Yes, one side is connected to GND.

I found a critical thing.
The offset at U401 Pin2 and Pin3 explained in #110 is not supposed to be there I think.
I used an other device which can give a little stronger RCA signal. There was a low duty-cycle PWM signal at the optocouplers input.

The first photo is a sine wave (U400 Pin1 and Pin7), and the DC line is U401 Pin2.
IF the Utop from U400 Pin1 and Pin7 is rising above the DC offset at U401 Pin2, it starts to oscillate at the optocoupler (photo 2).
The strange wave is the signal at U401 Pin2 in that case.

So, signals above the DC offset from U401 starts to result in a pwm output. Directly explains why the dutcy cycle is so low.
Means that the DC offset at U401 Pin2 and Pin3 needs to be eliminated