[Solved]Subwoofer Amplifier Voltage Problem?

[dundee]

Hi Guys!
I'm trying to repair a Active Subwoofer.

I found a shorted TL074 on the input Filter board:


After replacement of the TL074 the subwoofer worked very well for 30 minutes. Same issue --> TL074 shorted.

I measured +-20V on the voltage rails and thought everything is ok.

Later i found the schematic for the power supply and amplifier:


Specified were +-15V

I'm not really sure what's going on. I tested Q501, Q502, Q503 and all resistors around them.
Changed Caps C505, C507, C507, C508 and the Z-Diode. I noticed that R501 and R502 get really hot.

Would be nice, if someone has a tip for me.

[uncle_bob]

Hi

What is the voltage on the zener? It should be about 16V. If it's not then the new diode is blown. If it is, what is the voltage on the pass transistor's emitter (we already know it's 20V). Pass transistor Q501 has 16V on the base and 20V on the emitter ... pass transistor is fried. It may be fried as a short and takes out the zener each time you replace it. Good bet that the speaker got an over voltage event at some point (or just plain got run to hot). I'd replace both pass transistors.

Bob

[MosherIV]

I noticed that R501 and R502 get really hot.

Nothing to be worried about (unless they are glowing !).

It looks like they are being used as high side current shunts (current monitor) as part of some kind of transistor/zener diode driven linear regulator circuit.

[Anks]

Power supply is killing the opamp. Its being used in a situation were it is susceptible. Put some resistors inline with the supply. you could also measure the current over them with a scope. if its going into oscillation then its pulling current to produce sound way over the cut off point of the filter.

regardless more likely a bad cap.

[dundee]

I got it!

TIP31 and TIP32 were exchanged 

Must be a mistake of the manufacturer because i bought it cheap from a B-Stock.

Great support, thank you!