NAD 3020i output voltage issues

[poundy]

been a while since ive asked for any help and i try not to these day as i want to be able to work it out for myself-but!
i have one here i just cant work out, both speaker outputs are -26v


when i got the amp both output transistors were blown on the right channel and fuses along with them as you woudl expect, so they were replaced.
I have since checked and tested all other transistors in the output Q401 through to Q418, both channels and all are ok.


the power sections on both rails have the correct outputs.


i have -26v on all 4 output transistor bases and emitters
i also have -26v on the feedback resistors R429 and R430, in fact pretty much -26v all over the place so i dont have a good starting place.


ive been trying to work out what what genericaly could affect both channels the same



any ideas?

 

[huggygood]

have you controled the power supply symetry ?

 

[poundy]

?? sorry i dont understand

 

[Mooly]

Maybe huggy means have you checked all the rails. Are all four supplies correct, in particular the two positive rails of +43 and the +32/28 volt rail.

 

[poundy]

all circled in green are bang on


i must say after changing the 2 blown transistors i want expecting this


its something common to both rails

 

[Mooly]

You need to check that those voltages are reaching the output stage at their destinations. Measure at these exact locations. It is possible a resistor is open.

Be sure to use the bulb while working on this.

And if they are present and correct then it gets more interesting

 

[poundy]

already checked at these points

Q407

E -26V
C -23V
B -26V


Q405

E -26V
C -24V
B -23V


R544 is shorted
Q415 collector is -26v


always have the lamp plugged in, and the light isnt on at all.

 

[Mooly]

Well there is your fault

If they are at -26v and yet you measure the correct positive values (as per your diagram in post #5) at the power supply then there has to be an open circuit.

They are the same point... yes Take a step back and recheck on the power supply and then on those points again.

 

[poundy]

yes right i see it.


bit of a mind fog, i have flu at the moment and its hard work!

 

[Mooly]

OK have a remeasure at the PSU to check it really was OK and then work from the power supply toward the output stage.

Hope you are feeling better soon.

 

[poundy]

so both R 437 and R 436 were open circuit


i noticed that both resistors were negative one side and positive the other, so it had to be around that location


all voltages now normal.


whats the odds agaisnt both going at the same time.ive put 1/2 w ones back instead of 1/4w

 

[Mooly]

excellent.

Unless you know the amps history its probably fair to say that anything might have happened to it. Maybe someone tried measuring voltages there and had a meter on a current range instead of voltage and zapped both in the same way. You'll probably never know.

 

[poundy]

very true.but ive started logging all the faults i find and resolve, that way next time it will give me a stear, and belive me if you had my brain you would need it

 

[Mooly]

Lol, it all comes with experience and practice. Making notes is good but eventually you will get beyond that and just work from gained knowledge. But it takes time.

 

[huggygood]

I've been doing troubleshooting for a little over twenty years and I'm learning it every day ...