CT Sounds AT900-1D

[imraneesa]

Changed power mosfets and output mosfets.
Outputs are 6 IRF640N and 6 IRF9640.
Amp pulls plenty current. IRF9640 gets very hot. With current limiting resistor amp pulls 3amp and giving very clean output.

 

[Perry Babin]

With these driver boards, the first thing I do is replace the Q7 and 11. They run hot and sometimes check OK with a meter but won't perform 100% at high frequencies.

How clean is the waveform for the 9640s?

 

[imraneesa]

I have changed the two transistors. Attaching 9640 gate signal.

 

[Perry Babin]

That glitch on the falling side shouldn't be there. What does the output of the LM211 look like?

 

[imraneesa]

LM211 output

 

[Perry Babin]

Go to 2v/div and 5us and repost

 

[imraneesa]

Ok

 

[Perry Babin]

Are all of the 470 ohm resistors near Q7 and 11 within tolerance?

Did you use exact replacements for Q7 and 11?

 

[imraneesa]

Yes all are good with in tolerance. I replaced exact transistors. 1D and 2D. MMBTA92 AND MMBTA42 pair.

 

[imraneesa]

Power supply it had 1010 mosfets with 22ohm gate resistors. I changed them to 3205 with 47ohm. I hope that is not a problem..

 

[Perry Babin]

I don't think the PS is a problem.

The signal should be cleaner. Is the signal from the TL072 swinging cleanly about ±3.5-4v (at least)?

Do you get a cleaner signal if you ground the scope to the ground terminal of the audio driver board?

 

[imraneesa]

All the small power voltages dipping so much. May be that is the reason. But if I remove the driver board all voltages normal.
I don't know which is the ground pin of driver board. Is that which connects to secondary ground?

 

[imraneesa]

TL072 pin 1 signal.

 

[Perry Babin]

How difficult would it be to remove R24 and install a 10k resistor across pins 1 and 2 of the TL072?

 

[imraneesa]

Not so difficult. I did it and the amp comes on but no oscillations. It is idling at 0.6amps only.

 

[Perry Babin]

Drive something of about 50Hz into it and look at the output of the TL072 and the LM211. The output of the TL072 should be a fairly clean 50Hz sine wave. The output of the LM211 should be clean and square on top and bottom. With an LCD display on the scope, I'm not sure if it's a real problem or simply dithering.

 

[imraneesa]

Both signals are very clean. Attaching pictures.

 

[Perry Babin]

Signal on the emitter of Q14?

 

[imraneesa]

Ok

 

[Perry Babin]

Is the 180 ohm resistor within tolerance?

Are the positive and negative supplies on the large driver transistors on the driver board clean (180 ohm in the circuit, 10k resistor off)?