Marantz SR5200 surround amp odd buzzing sound on all outputs

[poot36]

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.

[coromonadalix]

schematics : 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

[poot36]

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.  I fix one thing and something else either breaks or I find another issue.

[coromonadalix]

this maybe ?

https://www.avforums.com/threads/marrantz-sr5200-help.1519047/

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/

[poot36]

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.

[poot36]

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.

[coromonadalix]

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 ...

[poot36]

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.

[poot36]

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.

[poot36]

Well I tried the old trick of adding additional capacitance on all of the supply rails except for the B rails and no change.  I did try unplugging the front panel main connector and the buzzing did decrease but I think this is due to the VFD being off.  The amount of buzz then was similar to when I unplugged just the power to the VFD.  Not really sure where to go from here.

[coromonadalix]

this wont work,     you need to follow in the schematic all  the outputs of any preamp, adc dac's   going to the power amp input, 

stop permuting  boards   now you know  it is not easy, you're up for a long run,     as i see in many models like yours, it's a bug seen many times, as for a design defect,  could be, parts quality, could be

if it was mine,  go to the dump,  like many pioneer ones and others with dsp problems or similar,   your's i totally understand you want to repair it

start  as i wrote, dead short any/every  small / low level output signal to ground,  you should be able to find the culprit,  it could even generate that  buzz on the supply lines,  you would need to isolate all supply lines one after the other ...

patience and time ... 

[poot36]

Well I have managed to run the amp with only the B- and B+ rails along with the 6V rail that is needed to turn on the relay to enable to main power transformer.  Still had buzzing.
I then had this crazy idea to connect my headphones between the speaker ground and the center tap on the transformer for the B rail.  This also buzzed for some reason.
I checked the schematic and the board trace layout and there should be only a few inches of PCB trace between those points.
I checked the resistance with a multimeter and it was equal to the resistance of my probes which is around 0.4 of an ohm.
I also checked for voltage between these points and found none in both the DC and AC setting on my meter.
My scope did show some noise though so I am guessing there is an issue here.
Have you seen anything like this before or am I doing something wrong with my testing?

[coromonadalix]

really sound like open grounding, ground loop  not okay ?

unless maybe unless the amp section is self oscillating      is the amp section input signal grounded ?      connector cp201   put all of them to ground

[poot36]

I have tested ground to most major points but not all.  I will double check that.
Super odd that the ground for the amp has to pass through the DAC board to ground the volume board (found this out when I was pulling boards and the amp went into protect due to no amp ground).
The buzzing between the ground points sounds different from the buzzing from the amp section.
Not a lot of difference but some.
I will short all the inputs to the amp board to be sure but when I did the finger test it did not pick up the hum from my finger so I suspect the resistors that ground the amp are low enough for this to not work.

[poot36]

Well I shorted the inputs to the amp board on CNT503 on the volume board as it was easier to get to and there was still the buzzing sound (no change from when they were not shorted).  I tested all the ground points and they all tested good except for the grounds which traveled through the DSP board to the volume board and amp board (it may go to other boards but I did not check that) where the ground was about 1 ohm higher then the rest of the grounds.  I traced it down to the CS4228A chip that links the digital ground and the analog ground.  I also don't get the point of the cap marked C875 as it also links the digital and analog grounds but why it is needed when they are shorted in the chip is a mystery?  I also found a way to power up the amp board with out having any +6V rail powering the main board by jumpering some points on the power board that allowed the main relay to turn on power to the transformer.  I could not use the headphone output in this state (or the other time that I disabled the other voltage rails as the relay coil power is supplied from a 15V rail) so I connected directly to the output from the amp board on the main board and there was still the buzzing sound just way less.  Not really sure where to go from here.

[poot36]

Well I found 2 faults, one that I caused myself when I first started to repair the amp and another by something unexpected.
The first fault was a cracked trace around where I had removed the original main filter caps for the B rails.
This removed the high frequency filter cap C123 out of circuit.
This was not the fault that was causing the buzzing sound though.
I had previously mentioned that the amp board got its ground through the DAC board.
Well the around 1 or so of resistance between the digital ground and the analog ground on the CS4228A chip was 90% of the issue here.
I bypassed it with a jumper wire on the bottom of the PCB and now it is not making the low frequency buzzing sound.
I still have a high frequency but relatively quiet buzzing sound that mostly goes away when any digital input is selected.
I suspect it is related to my wire I had to run for the bad trace to the CS4228A chip is not the best routing I could have done as that wire is the LRCK wire so it will have relatively high frequency digital data on it.
It is also possible that my replacement DAC and ADC chips are somewhat fake but I doubt it as when I wipe the top of the chip with IPA nothing comes off it.
The VFD brightness now does not change the buzzing sound at all now as far as I can tell.
At least I am making progress.  Stupid complicated star grounding system.

[poot36]

Well I re-routed the wire for the broken trace and it made no difference to the sound.
I repopulated all the cards in the system and now have the VFD buzz sound present and adjustable with the brightness.
I just can't win here.
I wonder if that issue is related to me installing a thinner ground wire between digital ground and analog ground in an attempt to make this look cleaner?

[Fried Chicken]

Receivers are notoriously complicated devices.
Fixing any of the individual components onboard is already complicated enough, but receivers interoperate with all of them so it can be a nightmare... plus they weren't designed for repair like one might expect: chuck and buy new!  (such a bullshit reality).

Anyway, I can't offer specific help here, but instead can suggest: follow good repair practices.  Be smart in your approach, think about ideas before trying them, work carefully and diligently, don't create a bigger headache for yourself by throwing everything against the wall and seeing what sticks.