Repairing a Sony 5.1 surround system

[Felto1]

Hi,
Im trying to repair a Sony model no. (STR-K750P) 5.1 surround sound system. The problem is that one of the output channels,
where you hook up the 2 wires that goto the speakers, is making a crackling sound and does not produce the sound needed.
I do not have a schematics of the board so that I can follow this one output channel and diagnosis it. So what I did is followed
the traces that goes to one that's at fault but I cant seem to find anything. I've tested a few transistor and resistors that are connected
but everything seems to check out ok. Does anyone know what could be causing this crackling and no sound output? Or does anyone
know where I can find or download a schematics to this type of system so that I can further diagnosis it. Thanks in advance.

 

[tunedwolf]

I have had a quick peek at the schematic for your amp and I can tell you that you are in for a really wild ride with this...

You should check the voltages around IC701 and IC702 and compare them to each other, with the same signal being applied to the amp. You will also need to check the voltages around IC501. If any look suspicious, you will need to check the limiter and booster circuits and their associated feedback paths. If they are all good, you will need to delve further back into the preamp, processor, selector and associated circuits. You have not been specific about which channel is faulty, or on which settings the fault presents itself etc so it's rather hard to help you.

A word of caution, the amps are direct coupled, so one slip of your meter lead shorting something will destroy components instantly and in a cascading manner.

If little of what I have mentioned here makes sense to you, my best advice to you, if you really want to save the amp, is to let a good engineer deal with it for you

 

[Felto1]

I have had a quick peek at the schematic for your amp and I can tell you that you are in for a really wild ride with this...

You should check the voltages around IC701 and IC702 and compare them to each other, with the same signal being applied to the amp. You will also need to check the voltages around IC501. If any look suspicious, you will need to check the limiter and booster circuits and their associated feedback paths. If they are all good, you will need to delve further back into the preamp, processor, selector and associated circuits. You have not been specific about which channel is faulty, or on which settings the fault presents itself etc so it's rather hard to help you.

A word of caution, the amps are direct coupled, so one slip of your meter lead shorting something will destroy components instantly and in a cascading manner.

If little of what I have mentioned here makes sense to you, my best advice to you, if you really want to save the amp, is to let a good engineer deal with it for you

I don't think that amp is totally gone. Its just that one channel that's not cooperating. Its the channel that drives the center speaker hookup. All other speakers are working fine and no other issues.

Its making a crackling sound when the center speaker is on and no other sound is coming out of it. I will check the voltages that you specified at those junctions and will get back to you.

Can you send me a schematic for this radio so that I can view it? or point me to a website to view it there. Thanks for the response.

 

[Felto1]

I don't think that amp is totally gone. Its just that one channel that's not cooperating. Its the channel that drives the center speaker hookup. All other speakers are working fine and no other issues.

Its making a crackling sound when the center speaker is on and no other sound is coming out of it. I will check the voltages that you specified at those junctions and will get back to you.

Can you send me a schematic for this radio so that I can view it? or point me to a website to view it there. Thanks for the response.

Ok I also measured the Voutputs on pins 2 and 3 on the power amps (IC701, IC702) and I do not get a reading voltage with my voltage meter. Should I be getting some voltage at these junctions when the power is on?

I also checked Vcc pinout and no readings there either. Is this the right way of checking these pinouts with my volt meter?

 

[tunedwolf]

Now that I know you are having trouble with the Center output, it is driven by IC501 (STK350-230). You should find approx 53V on Pin 9, -53V on Pin 7 relative to Gnd on Pin 8. Your signal enters the device on Pin 1 and your drive is output on Pins 5 & 6 and should be close to zero volts dc, max of 1.1V. The limiter consists of Q501, 502 and associated passives and your booster circuit consists of Q503, 504 and surrounding circuitry. There is a current detection circuit built around Q505, 6 and the take off for the power protection circuit is built around Q540. It's possible that the audio signal makes it all the way to the protection relay (RY501, driven by Q550) and there's actually nothing wrong with the amp. So check for audio prior to the relay in case the contacts are dirty or in case the relay is actually faulty.

After checking, the schematic is out there in the wild, google is your friend

 

[Felto1]

Now that I know you are having trouble with the Center output, it is driven by IC501 (STK350-230). You should find approx 53V on Pin 9, -53V on Pin 7 relative to Gnd on Pin 8. Your signal enters the device on Pin 1 and your drive is output on Pins 5 & 6 and should be close to zero volts dc, max of 1.1V. The limiter consists of Q501, 502 and associated passives and your booster circuit consists of Q503, 504 and surrounding circuitry. There is a current detection circuit built around Q505, 6 and the take off for the power protection circuit is built around Q540. It's possible that the audio signal makes it all the way to the protection relay (RY501, driven by Q550) and there's actually nothing wrong with the amp. So check for audio prior to the relay in case the contacts are dirty or in case the relay is actually faulty.

After checking, the schematic is out there in the wild, google is your friend

Thanks for the helpful information. I checked the relay that feeds to the center output and indeed it is firing. Im thinking maybe it might be that IC501 (STK350-230) chip that is faulty. How can I check that specific chip to see if it is
working? I checked all of the resistors and transistors in the limiter, Booster and surrounding circuitry for any shorts. Everything checks out ok. I have found a schematics online so that I can view this a lot better. Any other information
will be greatly appreciated. Thanks again.

 

[tunedwolf]

Feed the amp with a known signal and check for clean audio going into pin 1, check all of the supplies to make sure that voltages are correct, check for clean audio prior to the protection relay. If the voltages look approximately correct and you have clean audio coming in and you are certain that the fault doesn't lie in the limiter or booster circuits, then substitute the amp chip

I know you said the protection relay was firing ok, but that doesn't mean that the relay contacts themselves are making it, so check for clean audio before the relay contacts, say at the junction of the emitter resistor pair in the booster circuit (R516) before doing anything else, if it's there and not all crackly, then replace the relay