old first amp

[rrae]

I believe it's first austria amp with model fa 4410. I can't get sound. There is no voltage on middle legg for output transistors. Power transistors are IRFZ44N's, one is shorted.
I believe these diodes in picture, are zeners 1n4744? Am I right? One is shorted, and I want to replace.
I think amp is in protection mode, but protection light is not on, only power light. I can post voltages from KIA494P.
Sorry for no macro, if need I can take good macro from any place of PCB.

[Perry Babin]

If they're connected between the base and emitter of the PNP driver transistor, they're likely 1N4148s or the equivalent.

I'm assuming that you've already replaced the shorted FET.

Do you have both positive AND negative rail voltage on the center legs of the output transistors?

Do you have both positive and negative regulated voltage on the power supply pins of the op-amps?

Have you tried operating all pots and switches through their entire range with speakers connected to see if it ever produced any sound (any type of sound, audio, noise...)?

[rrae]

I didn't test for audio. I want to replace these failed components. 1n4148 look smaller than that diode on board.

[Perry Babin]

The diodes may (should) have numbers on them. Desolder them and see if you can read the numbers.

The 4744 is a Zener diode and would not be connected as I described.

Do the solder connections on the board appear to be original?

[prairiemystic]

In your pic, the upper little diodes look like clamps for the gate drive on the SMPS. They rarely fail. Check Q512 and neighbor are probably shorted and giving you a misleading reading.

1N914/1N4148 are the same size but a bit low current, I would guess a fast-recovery diode like a 1N4935 would be tougher.

Note that zener diodes show zener symbol on pcb (see ZD502 silver far right, 1/2W)

[rrae]

Well, replaced diode, and power transistors. I got rail voltage in outputs.

[Perry Babin]

What did you use as a replacement for the diode?

[rrae]

1n4148 is used for replacement. There wasnt any numbers or marks on old diode which i desoldered from board.

[rrae]

Well, I replaced KIA494P with TL494CN, and it goes in protection mode, with KIA it didnt went in protection.

[Perry Babin]

There's no reason for the amp to go into protect with the TL494. They're supposed to be identical in function.

If you can't find the fault, post the DC voltage on all 16 pins of 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: