Polk Audio PA880

[S Merlington]

This amp has FQP50N06's with 100 gate resistors..

Would the Z44,46's work as replacements?

And if so do i need to change the gate resistors?

 

[Perry Babin]

Either should work with the 100 ohm resistors but you need to force the supply to full duty cycle (assuming that it has a regulated supply) and confirm that the FETs remain cool at idle.

 

[S Merlington]

Replaced power supply fets..

Amp still doesnt power up ..

Heres what i get on the 494

Pin 1:0.00
Pin 2:1.90
Pin 3:0.06
Pin 4:0.00
Pin 5:0.48
Pin 6:0.60
Pin 7:0.00
Pin 8:2.69
Pin 9:0.00
Pin 10:1.27
Pin 11:2.68
Pin 12:2.68
Pin 13:1.88
Pin 14:1.88
Pin 15:1.88
Pin 16:4.42

 

[seankane]

Check to see where your remote voltage stops betwwen the terminal and the IC.

 

[S Merlington]

What pin or pins of the 494 shouls have 12 volts ?

 

[seankane]

Pin #12 is Vcc it should have battery voltage.

Pin#13 is the output it should have battery voltage.

Look up the data sheet and see which pins are what.

 

[Perry Babin]

Pins 8, 11 and 12 should have B+. Pin 13 is the mode pin and is generally tied to pin 14.

 

[S Merlington]

Pins 8,11,12 should have 12 volts is that B+ or remote voltage so i can back track the circuit?

 

[Perry Babin]

Backtracking would start at those pins and go backwards in the circuit.

 

[S Merlington]

I tracked it back to a 4458 IC

Heres the voltages i get on it..

Pin 1:1.43
Pin 2:1.91
Pin 3:0.81
Pin 4:0.00
Pin 5:6.82
Pin 6:12.55
Pin 7:1.42
Pin 8:13.72

Does this appear to be ok??

If so ill back track furhter

 

[Perry Babin]

The 4558 isn't likely to be used to supply power to the power supply IC. It may drive a transistor that drives voltage to the PS IC but it won't drive it directly. The voltage is likely supplied via the collector of a PNP transistor.

 

[S Merlington]

I found a B647 that deliveras the 12 volts to pin 8,11,12..

Is there a good sub for the B647 ?

 

[Perry Babin]

BD140

 

[S Merlington]

Replaced that same problem only 2 volts on pin 8..

There are only 2 other transistors in that area A1015's

If i find 1 to be defective is there a good replacement for thoose?


Also there is a diode connected to pin 8 (ZD1) 1 side has 2 volts other side is 0.00 volts and 2 volts measured directly across it..

I that normal and i need to back track more?

 

[Perry Babin]

Post the DC voltage on all 3 legs of the transistor.

 

[S Merlington]

Leg 1:13.90
Leg 2:13.60
Leg 3:13.10

 

[Perry Babin]

Either that transistor doesn't go to pin 8 or the voltage you read on pin 8 was wrong.

 

[S Merlington]

There is an A1015 transistor the voltage on that is

Leg 1:13.80
Leg 2:0.8
Leg 3:13.90

 

[Perry Babin]

Do you see a 0 ohm connection to either of these transistors from pin 8, 11 or 12?

 

[S Merlington]

The middle leg of the BD140 connects to a 100 ohm resistor then goes to pins 8,11,12..


If i place one probe on pin 8 and the other probe on the middle leg of the BD140 i get 100 ohms on pin 8 ..

So im assuming that the BD140 is connected to pin 8


I get 13.89 volts on 1 side of the 100 ohm resistor but only 2.74 volts on the other side..


Could the 494 possibly be bad?