QUAD 520f dc offset

[fonon]

Hi,
One power transistor and some resistors in the output stage were burned out due to the short circuit. Damaged parts were replaced, but the amplifier not works properly. The output power in the repaired channel is lower and the output dc offset exceeds 100 mV. Any idea?
f.

[Tajzmaj]

My guessing....
1. You missed some destroyed part....(some diode)maybe
2.You have install some really strange output transistor or if you did not replace all 4 of them some other is ,,leaky,, or damaged one way or the other maybe it is actually open circuit....so replace all 4 of output transistors with something new and not too fast. MJ15003 or MJ15024 is good choice.
3. There is something oscillating.
4. Something is wrong with power supply and yours 0 is actually not there where it should be so check components there...
I will send you service manual for 606 amp which is
basically the same but it has 6 output transistors per channel man a little higher psu voltage. Everything else is more or less the same....So please email me because such file is too big to attach.
Very best regards, and report what was wrong....
Taj

[405man]

You can test the class A stage if you remove the output transistors and use a high value load, if you use a 1k resistor and a 47R resistor in series, you can connect headphones accross the 47R resistor and listen to the class A stage.

Stuart

[fonon]

Originally Posted by 405man

You can test the class A stage if you remove the output transistors and use a high value load, if you use a 1k resistor and a 47R resistor in series, you can connect headphones accross the 47R resistor and listen to the class A stage.

Hi,
Could You explain how this test helps me to resolve the problem described?
Regards,
f.

[fonon]

Thank You for answer.

Originally Posted by Tajzmaj

1. You missed some destroyed part....(some diode)maybe

The voltages on D4, D5 & D6 have more or less typical values.

Originally Posted by Tajzmaj

2.You have install some really strange output transistor or if you did not replace all 4 of them some other is ,,leaky,, or damaged one way or the other maybe it is actually open circuit....so replace all 4 of output transistors with something new and not too fast. MJ15003 or MJ15024 is good choice.

All power transistors were MJ15003. Only one transistor was replaced. The new transistor is G-series (Pb-free technology) while all others - not.

Originally Posted by Tajzmaj

3. There is something oscillating.

I don't find it.

Originally Posted by Tajzmaj

4. Something is wrong with power supply and yours 0 is actually not there where it should be so check components there...

There are not replaced parts in the power supply. It works properly.
f.

[Tajzmaj]

Because there you have floating ground time to time just so happens that one of the resistors or transistors at psu.....so check...but you will se at the service manual. It can be a lot of things wrong there...including dc servo tlc op amp....and so on ....so I really hope that you will find service manual useful
Very best regards, Taj

[fonon]

Taj, thank You for the manual. It's very useful, of course. QUAD has a little original circuit design. What is a general idea of working points adjustment?
f.

[Tajzmaj]

You cant adjust there anything. It is calculated so that everything is adjusted. But That is actually small class a power amp followed by unbiased class c output stage...hehe just to follow class A circuit. And there is of course dc servo to keeps output offset voltage at zero...or almost zero. Psu is set so that positive rail is actually a little higher than negative because that's the way to get symmetrical clipping....but of course it will work as well with equal psu rails. Just it will clip earlier at the positive side of the signal....
Regards, Taj