EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: poot36 on January 06, 2025, 12:24:20 am
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I have this strange issue with what I think is 60 or 120 HZ buzzing in all audio outputs on this amp. I have confirmed that it still happens when the amp board signal input is disconnected from the rest of the unit. I have looked at the output from the headphones and the speakers with a scope and for some reason the output with the noise is inverted between both outputs. The output on the headphone has short almost square wave looking negative pulses that do correctly trigger off the line setting of the scope. The speaker output has long positive going pulses but would be a mirror image of the headphone output if it were inverted. The strange thing is the the buzzing sound does decrease if I dim or turn off the VFD on the front of the unit. I have checked all the voltage rails and they look fine to me but I could be wrong. The B+ rail has around 300mv of ripple and the B- has about 250mv of ripple on it. The ripple is higher on the B+ rail due to some I suspect protection related circuitry being powered off it. I know that the B+ rail does drop fairly quickly when the power is removed and the amp board is disconnected from the B power supply. The negative rail does not do this. All other rails are provided by linear regulators and are very smooth. Other then this issue the amp plays clean and does not sound distorted.
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schematics : https://www.dclabs.lt/dainiaus/wp-content/uploads/2017/09/sr5200_k2g.pdf (https://www.dclabs.lt/dainiaus/wp-content/uploads/2017/09/sr5200_k2g.pdf)
i would isolate all the possible sections, short their inputs to ground and follow the buzz on the supply lines, maybe some broken ground link(s)
as the schematic shows you have some star groundings on the psu section, follow them entirely,
do check if this ground is the same for the power amplifier section, and if the continuity is near zero ohms
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Thanks for the reply. I have checked all the grounds and they are provided from the 3 pin connector from the power transformer for the B voltage rails. I have removed all I can from the unit and it still makes the buzzing sound. I even removed and then grounded the volume board (this is where the amp board gets it's ground from) and I still had the buzzing sound. I did try grounding out the outputs on the volume board to the amp board and it made no difference. The solder connections on the bottom of the board look good. The star ground is connected to the volume board in a bit of a round about way on the PCB but it is connected. I do not have the original B filter caps in the unit as they were starting to bulge but the replacement caps were working fine in another amp that I scrapped due to the HDMI switcher chip being bad. I have had no end of issues with this amp trying to repair it: https://www.eevblog.com/forum/repair/marantz-sr5200-amp-damaged-center-channel/msg5600771/#msg5600771. (https://www.eevblog.com/forum/repair/marantz-sr5200-amp-damaged-center-channel/msg5600771/#msg5600771.) I fix one thing and something else either breaks or I find another issue.
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this maybe ?
https://www.avforums.com/threads/marrantz-sr5200-help.1519047/ (https://www.avforums.com/threads/marrantz-sr5200-help.1519047/)
https://www.devroom.io/projects/repair-marantz-sr5200/ (https://www.devroom.io/projects/repair-marantz-sr5200/)
try in direct as they say and see if it sound okay, if not, you have 2 ic's to change ??
in the past many receivers / sound system did had the dsp processor going bad, and in some cases the board was not made available for stupid licences issues ?? solution was to transplant from another or dump them :(
this page has some services bulletins who could be worth to check
https://www.diyaudio.com/community/threads/marantz-sr7200-buzzing-noise.371020/ (https://www.diyaudio.com/community/threads/marantz-sr7200-buzzing-noise.371020/)
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I have been able to fix the issues that I had in the previously linked thread. It was the DACs and ADCs that were causing that issue. I am now dealing with this new issue. The buzzing still happens even when the volume board is removed from the system (the amp board connects to this board and not the main backplane board). None of the service bulletins seem to relate to the issue I am having. I think I am going to try an external power supply to the amp board and see if that changes anything.
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I have tested the amp board on a separate power supply and it still buzzes. I have also powered the amp board with out the unit turned on and there is no buzz. I have a recording of the buzzing sound attached and also a sample of the sound when I press the dimmer button on the front panel.
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well it seems you still have another failed part to find
have you tried the direct mode .... means no dsp of any kind on the signal path
thoses series have their quircks up to the 7000 series .... many problems with parts it seems ...
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I can double check that. But I suspect that will still have buzz as I have ran a test where i completely disconnected the amp board inputs from the unit while maintaining a ground connection and it still buzzed.
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Well I found out that the S-direct can not be enabled when the headphones are connected so I had to turn it on when they were disconnected and then plug in the headphones. I could not hear a change in the buzzing. I also tested with speakers and the buzzing was the same with the S-direct on or off. I even removed the DSP board and added a ground jumper to the volume board so that the amp board was grounded and the buzzing got worse. I had my ground point on the edge of the PCB close to the front of the amp. I then moved it to the RCA connectors on the back of the PCB and the buzzing reduced but was still the same as if the DSP board was installed. I will admit that I do not have any screws in the back of any of the boards (where the RCA jacks are) currently but as most of the screw holes are not metal so I doubt that would make a great difference. I am still thinking it is related to the front board but that is very hard to test as the power button is also on that board and the other thing is I have not touched that part of the amp so I would doubt that would be causing the issue.