No start up Punch 160a4

[brandes.cm]

This amp won't start up. I can apply remote voltage and no LED and draws very little current. The VREF pin of the 494 reads 4.84Vdc. Without rail voltage I have no +- supply either.

If the 339 senses a fault does that affect the 494?

[Perry Babin]

The 339 can shut down the amp.

Post the DC voltages on those two ICs.

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:



339
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:

[brandes.cm]

494
Pin 1: 0
Pin 2: 4.414
Pin 3: .055
Pin 4: 4.8
Pin 5: 1.46
Pin 6: 3.22
Pin 7: 0
Pin 8: 14.8
Pin 9: 0
Pin 10: 0
Pin 11: 14.8
Pin 12: 13.48
Pin 13: 4.85
Pin 14: 4.85
Pin 15: 4.85
Pin 16: 0



339
Pin 1: 0
Pin 2: .7
Pin 3: 14.7
Pin 4: .314
Pin 5: 2.397
Pin 6: 4.85
Pin 7: .457
Pin 8: 4.85
Pin 9: 4.85
Pin 10: 4.85
Pin 11: 9.16
Pin 12: 0
Pin 13: 0
Pin 14: 0

[brandes.cm]

Looking at the numbers it seems Vref needs to be higher. Could I just lift pin 14 and see if the 494 would start?

I lifted pin 14, it will try to start and blow the 5 amp fuse in B+ line.

cory

[Perry Babin]

Are you sure that pin 8 of the 339 is correct?

Does pin 7 of the 339 go to the positive terminal of a 10uf capacitor?

[Perry Babin]

You can't lift pin 14 unless you lift both 13 and 14 and tie them together. If pin 13 isn't driven high, it causes the outputs on pins 9 and 10 to go high and the amp essentially tries to drag the positive 12v supply output to ground.

[brandes.cm]

Pin 8 reads 4.85V, pin 7 ties to the + side of C5.

I lifted pin 14 of the 339, not the 494. I should have stated 339 in my post.

[Perry Babin]

Is pin 8 of the 339 shorted to pin 9?

Can you lift pin 8 and connect it to ground?

Did you check the outputs?

Did you try setting the bias pots to the fully counter-clockwise position?

[brandes.cm]

Not shorted.

Tied pin 8 to gnd, same outcome as lifting pin 14 of 339.

One channel of outputs were shorted those have been replaced. The other side measures ok.

[Perry Babin]

I think this is the over-voltage shutdown. 14.7 shouldn't be too much but you could try setting it lower to see what happens. Reconnect pin 8 to the board as it was.

If the amp tries to power up with lower voltage but draws excessive current, remove the power supply driver transistors marked 1A (or 1G).