Audison SRx2 muting problem

[Mote]

I was given Audison SRx2 amplifier to be repaired.

I think something failed in the muting circuit. PS is working properly, amp does not draw excessive current. Input signal is clear and present on all opamps in the preamps section, volume and crossover pots, switches are working properly. No audio on speaker terminals.

I can not find audio signal on any transistors in the audio section of the amplifier, it is very difficult to tell what is what because of all of the SMD components, impossible to see where the traces are going. I found a SMD transistor near thermistor which at first gave me a reading that it is shorted, when this transistor is removed, then amp produces some audio but not anywhere near the power it is supposed to put out. I have not replaced the transistor yet because I have to order suitable replacement.

As I wait for the new transistor to arrive, anyone has the schematic for this amp? Just in case replacing it does not solve the problem.

 

[Mote]

Seems that it is not the problem, I tried a 2SC1815 as a temporary replacement in place of that transistor but that does not allow the amp to produce audio.

Anyone has had similar problems with Audison amps?

I will try to contact Audison as well.

 

[Perry Babin]

What is the DC voltage measured across the base (red probe) and emitter (black probe) of the two transistors in the photo?

 

[Mote]

Transistors are with BBV marking, found from LRx 1.400 service manual, that these are the muting transistors. I get 0,63V across base and emitter (two legs which are on the same side) on both of them.

 

[Perry Babin]

Lift those transistors to see if the amp will produce clean audio. If it does, you'll have to follow the circuit back to see what's driving those transistors.

 

[Mote]

Without those I do not get clean audio, only some crackling when gain is turned way up.

 

[Perry Babin]

Follow the circuit back from the collectors of these transistors. It likely goes through a capacitor, then to a 1k resistor then to an op-amp. Do you have clean audio on the output of that op-amp?

 

[Mote]

I checked that yesterday, I had clean signal on the pads of collectors for those transistors.

 

[Perry Babin]

How close is the schematic diagram that you have?

 

[Perry Babin]

Do you have both positive and negative regulated voltage on the SMD resistors marked 123 located near the emitter resistors?

 

[Mote]

There is 1,5V and 0,7V on 123 smd resistors near the emitter resistors.

 

[Perry Babin]

To make sure that you're checking the right resistors, confirm that one end connects to two 68 ohm resistors. If so, the other end of the 123 marked resistors should have ±15v. If there is no ±15v, you need to determine why the regulators aren't producing output.

 

[Mote]

Yes, I was checking the right resistors.

When I remove the transistor, which I talked about in the first post, then I get -9 V and +12 V on the resistors.
Interesting that transistor in or out, opamps still get their -+12V.

 

[Perry Babin]

Does the amp produce audio with that transistor out of the circuit?

 

[Mote]

Yes, but output power is quite low.

 

[Perry Babin]

Is it clean but low or do you mean that it clips at very low power?

 

[Mote]

It is clean but low.

 

[Perry Babin]

That sounds like a muting or gain problem.

Does the gain pot work?

Do you have the gain full up and the crossovers set to flat/off?

Is your signal source grounded to the amp ground?

 

[Mote]

Gain pot works, I can see the amplitude of input signal changing.

If the gain is full up, then there is some clipping of the signal on the opamps, so I have gain set at about 90%.

Crossover works, I have it set to full.

My signal source is not grounded to amps ground but I took a wire from amps ground to PCs chassis (signal source), did not improve the volume.

 

[Perry Babin]

What is the amplitude of the voltage when it clips on the op-amps?

Set the scope to 5v/div and confirm that it deflects to 2.5 divisions when touched to 12v. Post a photo of the clipped signal. Also measure the voltage on the power supply pins of the op-amp when it's clipping.