| Electronics > Repair |
| Marantz SR5200 surround amp odd buzzing sound on all outputs |
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| 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. |
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