QSC CX4T Help ....

[east electronics]

Even though the circuits is very simple i got puzzled ...

I am missing 15V rails from both ch

2K2 voltage drop resistors are fine also getting relatively hot
15 zeners are also fine
220uf capacitors are also fine
Removing op amps in case of serious shorts to relief the rails changes nothing

Even though schematic is available on the internet it seems that the actual amp has more parts on it than exist in the schematic .

Weird is that both ch have the same failure

here is a pic of the schematic

 

[sajti]

And what about the R613-D603-604-605-R612-D606-R625 chain? It can eats the current from the zeners too.

Sajti

 

[east electronics]

carefully measured all these parts out of the circuit with no results

 

[sajti]

+/-63V rails are OK?

Sajti

 

[east electronics]

yes very fine

 

[sajti]

yes very fine

OK, all logical steps done 🙂

You wrote the the 15V rails are missing. What does it means? Totally 0, or just less than 15V?

Sajti

 

[east electronics]

+rail is like 3.2V
- rail is like 1.6

Remember both ch are alike even though circuits are totally separated

 

[sajti]

+rail is like 3.2V
- rail is like 1.6

And this voltages are totally same, on both channels?

Sajti

 

[east electronics]

yes either exactly or pretty close

 

[sajti]

yes either exactly or pretty close

It's strange, as You wrote... Did You try to add some parallel resistors (say 10k or something like this) to the 2.2ks, to see if the voltages are changing?
The drivers Q603, 604 are OK?

Sajti

 

[east electronics]

I can do that if you like
Drivers have been checked and in general if both shorted this will mean no start up and some DC in the output ...
Output is at totally zero volts ...feedback chain also ...

 

[sajti]

I think, I got it!

Check the thermal protection! Q50, Q51, and the parts around!

Sajti

 

[east electronics]

Yes i actually missed this part of the schematic will check and let you know thanks

 

[east electronics]

This part of the schematic and existed on the second page missed that
Truly removing these transistors 15+15 rails come back to life

Thanks sajti very well done !!!

Now i will look what is the problems there

My very best !

 

[sajti]

You're welcome!

I'm interesting what was the root cause of the problem.

Sajti

 

[east electronics]

QSC have done lots of weird things starting from USA series ( eventhough weird actually work pretty well )

Please look at the circuit and share your opinion
we have noticed that thermistors that are stated in the schematic as 100R when cold in a room temperature measure 41 and 50 R each
So if these fail regardless the temp of the amplifier Q51 will shunt 15+15 rails to stop the amplifier ...

 

[sajti]

Be careful, as the protection circuit is related to the -63V rail. So the lower the ressitance of the PTCs are keep the amplifier working, as keep the base of the Q50 close to the ground. High resistance let the base to be negative, and trigger the protection, if I understand right.
So 40-50 ohm would be OK. Just chech the base voltage of Q50, it should not be lower then 0V.

Sajti

 

[east electronics]

4.7uf capacitor 160V was found leaky
Sajti really thanks for pointing to the right direction

It all started by looking at the schematic of the first amplifier that not includes the thermal protection circuits ....scrolling it a bit down made things clear for me ...

At your service Sajti
Well Done !!!
Kind regards
Sakis

 

[sajti]

I wish a long and trouble free run! 🙂

Sajti