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Rebuilding another Pioneer SX-850 Hum issue

Rob357

Rob357

New Member
Hello folks! My SX-850 is on the bench for some new shiny parts. I had enough for the power supply board and ordered kits for the flat amp/tone board and the amp drivers and adjusters.
After the power supply was done, I powered it up and success! Speaker relay clicked in. Great, I thought. Not so fast, now I have a pronounced hum on both channels. I will be checking voltages and the offset/idle settings this afternoon.
AWR-101C
I have the version with the 47uf/10v instead of the 220uf/10v.
Q5 had been replaced by something that tested as a darlington. Not sure why anyone would do that.
I put in the KSC2690AYS per the list.
Any and all thoughts will be greatly appreciated.

Rob
 
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OK voltages look good except the 5.4v is about 6.2v
pin 13 is -13.2 on my list of voltages it shows -51.5v but my schematic it shows that -13.5 is right.
When I move the ground wires around, the hum changes. wires on 1 and 12, like the hum is being induced by something.
The DC offset and Idle are very close to spec.
Hmmm....
 
Here's another clue for you all (The Walrus is Paul)
Or two clues depending on if they could be related.
1- I have been using my AKG headphones to do the checking. While wearing them I started moving the controls to see if that made any difference.
2- I pushed in the speaker A button. Hum gone. Same with B. I hooked up a speaker, it sounds fine. What the....
Am I now grounding the power supply through the amp somehow?
 
headphone ground (pin 10) and speaker lamp ground (Pin 8) are (or should be) separate wire runs.

Headphones and speaker switches are on the same printed circuit board.

kicking in the speaker ALSO turns on the speaker light, which sort of gets a ground reference back to that section of the power supply board. it might be floating.

specifically, look at the grounding of pin 1 of the AWR-101 board.

look at the schematic and see how the grounds are drawn with heavy lines and
how different sections of the ground are separated and tied to different pins on the power supply board.
 
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