alpine pdx 4

[DREW028]

i pulled the rectifiers out and they tested good.I desolderd the gate and source on all output FETs. 3 of the 8 tested good. Should i replace all of them?

 

[DREW028]

Is it ok to install the new power FET's and power the amp without the output FET's connected?

 

[Perry Babin]

It's generally OK but I've never tried it with this particular amp. As always, be observant. If something appears stressed when power is applied, shut it down.

 

[DREW028]

The amp turned on but was pulling for 13v to 9 in a pulsing manner. The blue l.e.d 8002 ( same side as the red fault l.e.d) was also going on and off with the pulse. I turned it off and left it. When I returned tocheck the FETs, with just b+ and ground, q9902 and q9904 started to smoke. After I removed just the two FETs the amp was able to turn on as it did before when none were in (2 blue l.e.ds light up, one goes out and the red turns on)

This is my first attempted at a repair and I'm a little discouraged.

 

[Perry Babin]

This wasn't a good choice for a first amp.

Did you check the drivers and gate resistors for the power supply FETs?

 

[DREW028]

I checked the 2 gate resistors r9808 r9807. They read 10.7k but are suppose to be 22ohm.

 

[Perry Babin]

You'll have to replace those. They are 22 ohms, not 220 ohms.

Pull the 4 IRF3205s and power up the amp. Measure the DC voltage on pins 5 and 7 of IC9802. Post the voltages.

 

[DREW028]

Pin 5: .007
Pin 7: 9.06

 

[Perry Babin]

Post the voltage on all of the pins of that IC.

 

[DREW028]

Pin 1: .016
Pin 2: 0
Pin 3: .016
Pin 4: 0
Pin 5: 0
Pin 6: .118
Pin 7: 9.06
Pin 8: .119

 

[Perry Babin]

I think you have the pin numbering wrong. Download the datasheet to get the pin configuration.

Post the voltage on the 494 also. Look up the datasheet if you don't know how the pins are numbered.

 

[DREW028]

Thanks, I will check tomorrow. I was going off pin configuration in the repair manual you sent me.

 

[DREW028]

You were correct.

Ic9802

Pin 1: .002
Pin 2: .019
Pin 3: .004
Pin 4: .019
Pin 5: .123
Pin 6: 9.06
Pin 7: .123
Pin 8: .002

Ic9701

Pin 1: .004
Pin 2: 4.438
Pin 3: .102
Pin 4: 3.647
Pin 5: 1.688
Pin 6: 3.459
Pin 7: .004
Pin 8: 9.06
Pin 9: .019
Pin 10: .019
Pin 11: 9.06
Pin 12: 9.06
Pin 13: 5.051
Pin 14: 5.051
Pin 15: 5.051
Pin 16: .004

 

[DREW028]

I'm looking at a function generator kit http://item.mobileweb.ebay.com/viewitem?itemId=300663054044&index=4&nav=SEARCH&nid=51509321741

It's puts out up to 1mhz. Is this ok or can I do better for a few dollars more?

 

[Perry Babin]

Why do you want a function generator?

What is the DC voltage on the 3 terminal of Q9701.

 

[DREW028]

Pin 3 of q9701 is 5.008.
A tech at work suggested I get one if I was serious about doing repairs.

 

[Perry Babin]

I have one that I use about once a year. Most people use sine wave tracks on CD or some other source. Unless you have a limitless budget for equipment, spend the money on something else.

Is Q9701 shorted?

 

[DREW028]

Im at work but pin 1 had a lower voltage reading than the rest. Pin 2 was 5.xx. i will check when i get home.

I dont mind buying spending the money, its the space im more concerned about.

 

[Perry Babin]

Don't worry about checking that transistor. I think the amp may be in protect mode. Have you checked the output transistors to see if any were shorted?

 

[DREW028]

Yes, there was 1 that measured shorted between the gate and drain and 4 that measured below .3v between the drain and source on diode test. The legs (gate and source on outputs FETs) are still desoldered from the board as i mentioned before. When its first powered on both blue l.e.ds light up then 1 turns off and a red l.e.d turns on.

Pin 1 of ic 9701 has a trace from the secondary of the transformer. Could this be why is reading the fault since the power supply fets arent in?