Memphis 16-ST1000D

[spooney]

This amp has had quite the power supply blowout. The driver transistors (Q15 and Q16) on the power supply driver card were burnt beyond recognition. Does anyone happen to know what these transistors are or maybe an upgrade/sub for them? Are there any other components on the driver card I should be concerned about? Power supply fets and gate resistors will be replaced with IRF3205’s and 22 ohm gates respectively.

 

[Perry Babin]

I use BD140s. The originals was 2SA1266s.

Install the transistors standing far enough off of the board so you can solder top and bottom.

 

[spooney]

Thank you for the super fast response Perry. That is great news. I just happen to already have those parts on hand.

 

[spooney]

Is there a way I can verify the power supply driver card is working without placing any fets in the power supply? Or will I need at least one fet per bank to get it to startup?

 

[Perry Babin]

You can look at the waveforms on the gate pads. Load each location with a 0.047uF or similar cap (gate to source) to confirm that the drive circuits are as they should be.

 

[spooney]

While awaiting a response I tried one fet and gate resistor in each bank and it did not start up. Im not sure if I have the caps on hand to try one at each fet location. Do you think it is advisable to try loading something like irfz44n’s into the amp to see if it will start up? I have those in loads but not enough 3205’s at the moment

 

[Perry Babin]

What do you mean that it did not start?

Did the 494 have life? Oscillation, drive pulses, 5v out...?

 

[spooney]

I did not see any waveforms on the gate pads of the power supply fets I put in each bank. There was voltage on various pins of the 494 but I am not familiar enough with it’s operation to know what voltage/signal I should be seeing in each pin other than VCC and GND. Shall I start with posting voltages on each of the pins of the tl494?

 

[Perry Babin]

On pin 5, you should have a sawtooth waveform. On pin 14, you should have 5v. On pins 9 and 10, you should have drive pulses.

 

[spooney]

I was not able to see any waveforms on pin 5 or drive pulses on pins 9 and 10.

Here are the DC Voltages of the TL494CN

1. 0 volts
2. 4.86 volts
3. 0.04 volts
4. 1.02 volts
5. 0 volts
6. 3.54 volts
7. 0 volts
8. 13.93 volts
9. 0 volts
10. 0 volts
11. 13.93 volts
12. 13.93 volts
13. 4.86 volts
14. 4.86 volts
15. 4.86 volts
16. 0 volts

 

[Perry Babin]

What's the resistance between pins 5 and 7?

 

[spooney]

16.5 mega ohms with the red lead on pin 7 and black lead on pin 5. With the leads reversed it reads as open or out of the range of my meter

 

[Perry Babin]

The IC may be defective. There is one other possibility. On some amps, there is a transistor that synchronizes the output and power supply oscillators. Check your board. If there is a transistor with one leg on pin 5 and another connected to pin 7, remove it. The base leg would be driven from the sync terminal of the audio PWM board.

 

[spooney]

Perry,
Is that transistor typically found on the main board or on one of the driver boards?

 

[Perry Babin]

Very near pin 5 of the 494 on the driver board.

 

[spooney]

I found the transistor. Not really sure how I missed it the first time. It was literally the first one I checked. With the transistor removed pin 5 now has a sawtooth waveform on it but still no output on pins 9 or 10. Is the transistor itself the issue or indicative of a larger issue?

 

[Perry Babin]

The transistor isn't likely to be the problem, itself.

Copy and paste and fill in the blanks (DC voltage) for 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:

 

[spooney]

Pin 1: 0
Pin 2: 5
Pin 3: 0.066
Pin 4: 5.61
Pin 5: 1.47
Pin 6: 3.7
Pin 7: 0
Pin 8: 13.9
Pin 9: 0
Pin 10: 0
Pin 11: 13.9
Pin 12: 13.9
Pin 13: 5
Pin 14: 5
Pin 15: 5
Pin 16: 0

 

[Perry Babin]

Find the transistor that's connected between pin 14 and pin 4 and pull it. This will defeat the protection circuit so you will have to take the standard precautions to prevent further damage.

 

[spooney]

Found the transistor and pulled it. No change other than now pin 4 is at 13.67 volts. Still have my sawtooth waveform on pin 5 and no output on pins 9 and 10. I should also note that prior to removing the transistor between pins 5 and 7 I also swapped out the TL494 for a new one.