Hi guys,
I was working on this Onkyo SR508 with the left channel shorted. One of the power NPN transistor had shorted out and one half of the noble resistor was also open.
So I took the two power transistors and the noble resistor from a working channel that is not used. Amp was working perfectly when I tested it. I should ask is it ok to leave the unused channel without the two transistors and the resistor?
Ran it for sometime I heard a relay click not sure which one it was - no error message nothing and the amp kept on working. Turned off and on again no problem, was working. The relay clicks after couple of minutes of use. I ordered the power transistors needed and took the amp back to the owner’s place to be used for the time being.
I wanted to check whether the speakers had shorted as this was my suspicion for the original failure. The speakers were fine.
The owner was using it for sometime and he called me back saying the amp had died again.
I went back the following day to have a look. Left channel had shorted out again. This time the two power transistor that I had borrowed from the working channel had shorted out. The noble resistor seems to be intact this time. In hindsight I should have borrowed the drive transistors from the working channel, damn! However the drive transistors still measure ok even now. Another thing I would like to remark is that the power transistors base has shorted to collector and emitter (could this be a clue). The fuses on the B+ voltage has blown as well. I would have to bring it back to my place to further investigate. I was wondering if you all had any suggestion on what to look for and any common failure points I should check.
Further points/questions I have:
1. What causes the B+ voltage to change? In other words what should cause the SEC1H signal to go high turning on the relay which changes the B+ voltage. (What is SEC1H?-the SM does not explain this).
2. Could that be the relay I heard while I was testing it?
3. Could there be an issue in the temperature detection circuitry? I noticed that the big heat sink get quite hot even with no signal applied. (Not sure how hot it should get). I did follow measuring the idling current procedure mentioned in the service manual. It remains with in specs. Also the thermal paste has become crusty.
4. I am having a hard time finding the noble resistor, I need a 0.22 ohm 2W resistor - but apparently they are almost impossible to source according to some forums. I came across 5W ones could I use them instead? Do they have a ‘fusible’ purpose in the circuit?
5. I noticed on one of the channel (working channel) the noble resistor was touching one of the driver transistor and the resistors body has got some charring. Could this be an issue?
6. Also noticed some of the capacitors looked a bit dark not sure if its suppose to be like that.(not the electrolytic ones).
7. Why didn’t the amp go into protection mode instead of blowing the fuses?
Any further troubleshooting tips advice would be greatly appreciated.
Thanks in advance.
P.S I tried uploading some pictures and Pdf of the schematic for some reason it doesn't seem to go through. So here they are uploaded to a flickr album Onkyo SR508 | Flickr
I was working on this Onkyo SR508 with the left channel shorted. One of the power NPN transistor had shorted out and one half of the noble resistor was also open.
So I took the two power transistors and the noble resistor from a working channel that is not used. Amp was working perfectly when I tested it. I should ask is it ok to leave the unused channel without the two transistors and the resistor?
Ran it for sometime I heard a relay click not sure which one it was - no error message nothing and the amp kept on working. Turned off and on again no problem, was working. The relay clicks after couple of minutes of use. I ordered the power transistors needed and took the amp back to the owner’s place to be used for the time being.
I wanted to check whether the speakers had shorted as this was my suspicion for the original failure. The speakers were fine.
The owner was using it for sometime and he called me back saying the amp had died again.
I went back the following day to have a look. Left channel had shorted out again. This time the two power transistor that I had borrowed from the working channel had shorted out. The noble resistor seems to be intact this time. In hindsight I should have borrowed the drive transistors from the working channel, damn! However the drive transistors still measure ok even now. Another thing I would like to remark is that the power transistors base has shorted to collector and emitter (could this be a clue). The fuses on the B+ voltage has blown as well. I would have to bring it back to my place to further investigate. I was wondering if you all had any suggestion on what to look for and any common failure points I should check.
Further points/questions I have:
1. What causes the B+ voltage to change? In other words what should cause the SEC1H signal to go high turning on the relay which changes the B+ voltage. (What is SEC1H?-the SM does not explain this).
2. Could that be the relay I heard while I was testing it?
3. Could there be an issue in the temperature detection circuitry? I noticed that the big heat sink get quite hot even with no signal applied. (Not sure how hot it should get). I did follow measuring the idling current procedure mentioned in the service manual. It remains with in specs. Also the thermal paste has become crusty.
4. I am having a hard time finding the noble resistor, I need a 0.22 ohm 2W resistor - but apparently they are almost impossible to source according to some forums. I came across 5W ones could I use them instead? Do they have a ‘fusible’ purpose in the circuit?
5. I noticed on one of the channel (working channel) the noble resistor was touching one of the driver transistor and the resistors body has got some charring. Could this be an issue?
6. Also noticed some of the capacitors looked a bit dark not sure if its suppose to be like that.(not the electrolytic ones).
7. Why didn’t the amp go into protection mode instead of blowing the fuses?
Any further troubleshooting tips advice would be greatly appreciated.
Thanks in advance.
P.S I tried uploading some pictures and Pdf of the schematic for some reason it doesn't seem to go through. So here they are uploaded to a flickr album Onkyo SR508 | Flickr
