SX-850 In The House! Recap/Restoration Ahead!!

[markthefixer]

re Buzz : something to think about and to look for

I ended up buying this 3900 from Raccoon and fixed the hum issue yesterday. I want to post the fix in case someone else runs into this issue- i'm guessing this would apply to any of the receivers in this series. The solve- as mentioned by Raccoon, the previous owner had been in the unit prior and had done some repair work. There are 2 black ground wires going to pin 6 and 7 of the amplifier board. I was checking grounds, when I grounded pin 3 of the PS board the hum got cut in half- that's odd i thought. So i followed the schematic grounds and discovered the previous owner had reversed the grounds. Once I swapped them around the hum is now gone.

Cheers

Thank you for the follow up.

That makes sense.
The pin 6 ground is a current return path (pin 3 awr-210) and reference from the protection circuits back to the power supply, and they are purposefully isolated from the amplifier.
The pin 7 ground is a signal ground reference and slight (quiet!) very low current return (pin 21 awr-210) from the input stage of the channels.

on the power supply board there DELIBERATELY is no connecting foil between pin 3 and pin 21.
A power return ground can constantly vary a bit due to the current flowing interacting with the resistance of the ground path. If there's AC current on that - you hear hum.

Imagine the nightmare of hum between the grounded center tap of a transformer secondary and the main storage capacitors.

That is why grounds are not confused, or interchangeable.

now, looking at the 850, WHERE the ground goes is important:
the AWR-101 power supply has the SAME split ground situation,
20 & 21 are limited to the grounds of the filter capacitors, which should be separate wires back to the ground point, as they hit the center taps of the transformer and thus carry AC current on them, which the ac current will act in concert with the resistance to true ground and cause the line to hum.
while
1 & 12 are regulator references. also chassis grounded, but not sharing any wires to the star ground point.

 

[MarkyM]

Amazing, thanks for the detailed thread! I especially appreciate the post on cleaning/lubing the switches and pots, since I just bought some FaderLube to do my own 850 (I already have the D5).

BTW I don't have the buzz on the headphones that you describe.......

Enjoy!!

No problem! This receiver was a pleasure to work on.
Have fun with your 850!

 

[MarkyM]

re Buzz : something to think about and to look for
now, looking at the 850, WHERE the ground goes is important:
the AWR-101 power supply has the SAME split ground situation,
20 & 21 are limited to the grounds of the filter capacitors, which should be separate wires back to the ground point, as they hit the center taps of the transformer and thus carry AC current on them, which the ac current will act in concert with the resistance to true ground and cause the line to hum.
while
1 & 12 are regulator references. also chassis grounded, but not sharing any wires to the star ground point.

Update...the headphone jack hum/buzz issue on my 850 appears to be resolved!
I had some ground loop issues on my bench. I have a Topping D10S DAC and a MOTU M4 ADC/DAC that are both USB bus-powered. I was getting a buzz whenever the two of them were connected as well.
I added a USB power isolator on the M4's USB cable at the computer and lifted the ground on the 3-prong power cord of my scope.
The buzz is now gone with the speaker switches both off.
Thank you for all the help as always. Ground loops are nasty!

 

[skysailor64]

Update...the headphone jack hum/buzz issue on my 850 appears to be resolved!
I had some ground loop issues on my bench. I have a Topping D10S DAC and a MOTU M4 ADC/DAC that are both USB bus-powered. I was getting a buzz whenever the two of them were connected as well.
I added a USB power isolator on the M4's USB cable at the computer and lifted the ground on the 3-prong power cord of my scope.
The buzz is now gone with the speaker switches both off.
Thank you for all the help as always. Ground loops are nasty!

I have been following this and that's great news. Glad you got it resolved. I immediately went to check my SX950 since I have now moved it from the bench to living room after the recap and I wanted to see if the problem remained on my unit in this new electrical environment. To my dismay the problem remained. But I did discover that the level of the hum was affected by the tone on/off position with it being very much diminished in the tone off position. I also found that with the volume turned all the way down the problem remained no matter what source is selected. Only goes away when any one speaker set is selected with or without speakers attached. So the buzz/hum must be injected or created after the volume control. I will hunt around and check the ground paths that have been mentioned to see if I can resolve this as it seems it's not normal even though not intrusive when playing audio at listening levels. Comments welcomed.

 

[MarkyM]

Thanks, me too!
Mine was worse with the tone amp on as well.
Do you have any gear in that system with grounded power cords?

I ran some amplifier tests today on my 850 into my 8 Ohm dummy load. Clipping on both channels (per my Siglent scope) is symmetrical at about 25 vRMS (70V P-P) or just over 75 Watts/ch, both channels driven at 1kHz.
THD is below 0.04% at rated power (65W) and .02-.03 percent down to below 1 Watt. I use a MOTU M4 USB audio interface and Audiotester v3.0 software for the distortion and response measurements.
The volume control has an interesting taper. There's a spot between abut 10:45 and 11:15 where there is no change at all and then the level increases faster above that point. It is "slower" below that point.