[Marantz SR5012] Center channel keeps going into ASO/Overcurrent protection.
First off I would like to thank this community as it has provided me with great information that has helped me greatly on my repair for this Marantz SR5012 AV receiver. I am creating this new thread as the other 2 that I have created here has had their original question/problem solved and I plan that this is the final thread on my repair. Please note that this is my first time repairing audio amplifiers so I may not be aware of common issues and things to know.
Lets start at what I have done so far. Random speaker popping on all channels is fixed by replacing the 2 large filter capacitors for the power amplifier's power supply. Blown fuses is fixed by insulating the output transistors from the heat sink with thermal pads.
Now to the current issue which I need to tell the whole story with this AV receiver. This Marantz SR5012 is bought used from Facebook Marketplace with the description describing that it has random speaker popping noise as a defect. This has been addressed with the already mentioned fix of replacing the filter caps. Once I had that fixed I tried playing some movies with it and on parts of the movie where there is loud sounds for the center speaker (and audible distortion) the AV receiver cuts off power and goes into ASO protection which according to the service manual is caused by “a short circuit occurred between the speaker terminals, or speakers with an impedance outside the rating where connected”. I confirmed that all speakers are connected correctly without any shorts and the receiver powers on completely with the speaker relays engaging. A few testing trials latter the receiver will now not power on and will go into ASO protection a second or two after turning on. I took out the amplifier board and found that Q428, the darlington 2SD2560 NPN output transistor for the center channel, has a short between collector to emitter. The corresponding emitter resistors, R511 and R512, have ohm readings in the kilo-ohm range which means they got damaged too from over current. So I had these components replaced thinking that that would be the cause of the problem. Had to replace that transistor twice as I accidentally damaged it since the heatsink had to be electrically isolated from all of the transistors. I also preformed the DC-bias adjustment and set all 7 channels to 8 millivolt as close as I could as specified in the service manual.
Put everything back together to find out that once again the problem of the ASO protection occurrence again with any loud sound for the center channel. I will not power on the AV receiver again to prevent any further damage to the center channel section of the amplifier as I fear that the original problem is still present.
Resources that I have looked into so far is this thread on this forum (https://www.badcaps.net/forum/showthread.php?t=73293) and this website (https://alpengeist-tvrepair.blogspot...g-rapidly.html), alpengeist's TV repair blog of a similar issue.
Taking a look at the service manual (link posted below) on page 31 which shows the schematic for the amplifier. ASO protection occurs if Q405 gets a signal to its base if any one of the over-current detection transistors “opens its gate”. For the center channel this is Q430. I am going to take out the amplifier board once again to probe all of the transistors on the center channel as specified in the above resources. Are there any other points to test before I go full parts cannon on this repair?
Marantz SR5012 Service Manual PDF: https://cdn.badcaps-static.com/pdfs/...c775c5d540.pdf
Lets start at what I have done so far. Random speaker popping on all channels is fixed by replacing the 2 large filter capacitors for the power amplifier's power supply. Blown fuses is fixed by insulating the output transistors from the heat sink with thermal pads.
Now to the current issue which I need to tell the whole story with this AV receiver. This Marantz SR5012 is bought used from Facebook Marketplace with the description describing that it has random speaker popping noise as a defect. This has been addressed with the already mentioned fix of replacing the filter caps. Once I had that fixed I tried playing some movies with it and on parts of the movie where there is loud sounds for the center speaker (and audible distortion) the AV receiver cuts off power and goes into ASO protection which according to the service manual is caused by “a short circuit occurred between the speaker terminals, or speakers with an impedance outside the rating where connected”. I confirmed that all speakers are connected correctly without any shorts and the receiver powers on completely with the speaker relays engaging. A few testing trials latter the receiver will now not power on and will go into ASO protection a second or two after turning on. I took out the amplifier board and found that Q428, the darlington 2SD2560 NPN output transistor for the center channel, has a short between collector to emitter. The corresponding emitter resistors, R511 and R512, have ohm readings in the kilo-ohm range which means they got damaged too from over current. So I had these components replaced thinking that that would be the cause of the problem. Had to replace that transistor twice as I accidentally damaged it since the heatsink had to be electrically isolated from all of the transistors. I also preformed the DC-bias adjustment and set all 7 channels to 8 millivolt as close as I could as specified in the service manual.
Put everything back together to find out that once again the problem of the ASO protection occurrence again with any loud sound for the center channel. I will not power on the AV receiver again to prevent any further damage to the center channel section of the amplifier as I fear that the original problem is still present.
Resources that I have looked into so far is this thread on this forum (https://www.badcaps.net/forum/showthread.php?t=73293) and this website (https://alpengeist-tvrepair.blogspot...g-rapidly.html), alpengeist's TV repair blog of a similar issue.
Taking a look at the service manual (link posted below) on page 31 which shows the schematic for the amplifier. ASO protection occurs if Q405 gets a signal to its base if any one of the over-current detection transistors “opens its gate”. For the center channel this is Q430. I am going to take out the amplifier board once again to probe all of the transistors on the center channel as specified in the above resources. Are there any other points to test before I go full parts cannon on this repair?
Marantz SR5012 Service Manual PDF: https://cdn.badcaps-static.com/pdfs/...c775c5d540.pdf
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