Boston Acoustics GT-40

[mike49504]

Amp came in with 2 blown outputs.. I removed all outputs to test them ..

Outputs are out of the amp right now amp is still stuck in protection mode..

Heres what i get on the SG3525

Pin 1:0
Pin 2:5.18
Pin 3:0.02
Pin 4:0.26
Pin 5:2.06
Pin 6:1.42
Pin 7:2.04
Pin 8:4.92
Pin 9:6.10
Pin 10:0
Pin 11:5.86
Pin 12:0
Pin 13:13.88
Pin 14:5.77
Pin 15:13.64
Pin 16:5.18

Any ideas?

 

[Perry Babin]

The power supply appears to be working. Does it have rail voltage.

If so, post the DC voltage on all of the pins of the LM339.


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:

 

[mike49504]

Pin 1: 8.75
Pin 2: 6.95
Pin 3: 13.71
Pin 4: 2.68
Pin 5: 8.74
Pin 6: 5.62
Pin 7: 9.09
Pin 8: 6.96
Pin 9: 8.74
Pin 10: 2.34
Pin 11: 0.49
Pin 12: 0
Pin 13: 0.28
Pin 14: 2.34

 

[Perry Babin]

Can you take a 'good quality' photo of the bottom of the board under the 339 showing a couple of inches of the board around that area?

 

[mike49504]

Heres 2 pics of the area around the Lm339

 

[Perry Babin]

What pin(s) on the 339 read 0 ohms to the cathode of D18?

Is the power LED lit?

What is the DC voltage across DZ5?

 

[pergo]

those boards seem to be projected by Audiosystem(Steg)'s engineers.

Possibile?

 

[Perry Babin]

Steg and Twister both use circuits similar to this one.

 

[pergo]

if you have the schematic or block diagram, can you send me at pergo88 @ alice . it (without spaces..it's for anti-spam ) ?
Here in Italy i cannot find Twister's and Steg's schematics

 

[mike49504]

Pin 1 Pin 5 Pin 9 all show about 1.1 ohms to the cathode side or D18

Power led ans the led safe light are both lit..

voltage across DZ5 :4.89 volts

 

[Perry Babin]

Lift Q28. Does that allow the protect LED to go off?

If so, which pins on the 339 have significantly different voltage compared to the voltages you posted?

 

[mike49504]

If i lift Q28 the power led doesnt light anymore the Protect light is still lit

 

[Perry Babin]

Is the emitter of Q28 connected to the cathode of D14?

 

[mike49504]

OK i lifted Q29 instead of Q28.. With Q29 in circuit and Q28 removed amps protect light goes off..

Heres the voltages that are different on the 339

Pin 10: 0.2 was 2.34

Pin 13:12.29 was 0.28

Pin 14:0.3 was 2.34

 

[Perry Babin]

Have you done anything other than removing the output transistors?

Do you have any DC voltage across any of the channels?

Has any other work been done to any of the components around the LM339?

 

[mike49504]

No work has been done on this amp at all.. All i did was remove the outputs since 2 of them were defective..


With all outputs still out of the amp i get 0 volts across all channels

 

[Perry Babin]

OK. I'm trying to borrow one of these from a friend but it will be a couple of days before I can get it (if he has one).

 

[mike49504]

Ok thanks..

I did some testing..

If i put 1 IRF540 and 1 IRF9540 in 1 channel only the amp will produce audio..

If i try to put another set of outputs in the amp in any of the other 3 channels even with Q28 removed the amp will go straight into protection mode..

 

[Perry Babin]

Are you applying B+ and remote voltage at the same time?

Or are you applying B+ then remote a few seconds later?

 

[mike49504]

Im applying both B+ and Remote at the same time..

Sometimes if i do it that way the amp will protect other times it wont..


If i apply the B+ then the remote it will power up..


But if i put more the 2 outputs in it no matter what i do it will go straight into protection even with Q28 lifted