Ground Zero 24 volt amp repair

[PQatPIT]

Hello! Whoaa, my first post here.

I have a problem repairing Ground Zero 24 volt amp. First I found power supply fets (IRF3415) shorted, their driver transistors (2SA1023, now replaced with 2SA1124) shorted and TL494 controller pins 10 and 11 shorted and between pin 8 and 10 was 15 ohms. There was also a couple of shorted output transistors Snaken 2SA1693 and 2SC4466 at two channels, other two channels were probably never used.

Parts are replaced, and I also replaced gate resistors of supply fets, and added 1k pull down resistors from gate to ground. Attached is scope picture of voltage at the gates of supply fets. When powering it up at first there is about 300 mA current draw slowly rising, at the moment of picture there is almost a 5 A draw and rising. If remote is not pulled away the fets get too hot and will fail. All this when amp section voltage rails are cut off so no load for power supply.

Any ideas? More pictures will be sent if needed.

PQ

[Perry Babin]

The gates should be pulled closer to ground on the falling edge of the waveform.

Can you post photos of the waveforms on pins 9/10, the base of the driver transistor, the emitter of the driver transistor and the gate (if it is not the same as the one you posted). Ground your scope probe to the ground terminal of the amp when capturing the waveforms.

I think the problem is likely going to be the driver transistors.

[PQatPIT]

Thank you for answering!

Pic 11 shows outputs of TL494, probe 1 is pin 10, probe 2 is pin 9.

Pic 12 shows emitters of driver transistors, probe 1 is on transistor which is connected to pin 10 of TL494.

Earlier picture was the gates of fets, and probe ground was at GND terminal of amplifier.

There is also four 1N4148 signal diodes between emitters and bases of driver transistors. I find it odd that there is two diodes per transistor, one this direction, other is opposite direction. Should they really be 1N4148's?

[Perry Babin]

There's only one photo. I'm assuming it's '12'. Post the other photo.

One diode passes current from the 494 to the FET gate resistors to turn the FETs on. The other helps protect the PNP pulldown transistor when the FETS fail. I think they're generally 1N4148s.

[PQatPIT]

Thank you!

Picture on post #3 was "11", here is "12".

This forum software doesn't allow uploading multiple images unless hosted elsewhere, or am I missing something?

Didn't yet try lower value gate resistors, will do if necessary.

PQ

[Perry Babin]

This problem (drive signal not going to ground) goes back to the 494. You could try using lower value pulldown resistors on pins 9 and 10 (minimum of ~300 ohms) but I really think the drivers are the problem. If you look up the datasheet for the drivers, you can see that they have a very low current rating. If the current for the drive circuit is near the current rating for the drivers, the DC gain will be VERY low and can cause this problem. The original transistors are rated for much higher current. (50ma vs 800ma).

If you have 2N6491s on hand, try installing them in place of your drivers. The pin configuration is reversed so you'll have to install them facing the opposite direction in the board. If you have MPSA56s, you can use them for testing if you reverse legs 2 and 3.

[PQatPIT]

Thank You for help!

I'll try lowe value gate resistors and MPSA56's and let you know what happens.

Again, Thank You!


PQ

[Perry Babin]

The lower value pulldown resistors would connect between pins 9/10 and ground. These are not the gate resistors. The gate resistors are probably OK.

[PQatPIT]

Hello!

Attached pic shows what happens at gates of power supply FET's. Now there is 470 ohm pull down resistors to supply gnd between MPSA56 and gate resistors. There is no pull down resistors at pins 9 or 10 at PWM controller TL494, before MPSA56's, this is just like original.

This is when amp draws 1,1 amperes and it keeps rising, no DC voltage on channel outputs and seems that +- 15 volts at op amps is ok. No output transistors get hot or warm, power supply FET's heats only a bit.

Is this ok? Thank You in advance!

PQ

[PQatPIT]

Well, after five minutes of listening some background music pwer supply FET's get quite warm and MPSA56's get too hot to keep finger on.

Something is still not right, lacking pull-down resistors between TL494 pin 9, 10 and MPSA56's maybe?

PQ