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Sansui Eight help - hum on one channel

bob777 said:
That might be the next thing to try. Not worried about current drive just try to get the voltages right around tr804 and tr851. Remove tr805,806,807, and tr808. That's bare minimum
Click to expand...
OK, thanks guys for homing in on this. Will double check TR801/2 are in correct. Then remove TR805-8, and check offset. Should get to this tonight.
 
Pulled TR805 and 806 and tried the driver board. DC offset was at about 38 volts. Pulled TR807 and 808. DC offset is now reading zero.

I'm assuming this indicates there is something wrong with either TR807 or 808? Should I do the same test with the other driver board?
 
Lets get one board working first.
Measure n and c again at tr851. That will tell the story.
Removing them does not tell us whats wrong. Just keeps the h output from being pulled to pos supply voltage.
 
bob777 said:
Lets get one board working first.
Measure n and c again at tr851. That will tell the story.
Removing them does not tell us whats wrong. Just keeps the h output from being pulled to pos supply voltage.
Click to expand...

OK, getting +48.4V on both N and C.
 
Problem still there, check r 5,6,7,8, r10, 13,14,and r15.
You verified tr801 and tr802, checked tr803 and Tr804.
There is still something wrong there.
 
Can all those resistors be checked in circuit? I assume we are looking for something open or very high resistance?
 
Yes except for 807, 808, and 810. Lift one end. First try swapping tr 803 and tr 804, just exchange them after testing them.
 
Just did a quick check. All resistors check OK in circuit. Will check TR803 and 4 next and then swap.
 
Tested TR803 and 804. Readings: hfe, Vf (mv)

TR803: 96, 610
TR804: 88, 610

Swapped TR803 and 804. Voltages: N=48.0V, C=48.0V. :(
 
Try adjusting vr801 and vr802 while monitoring the voltage at n or c. Remember these are 10 or 15 turn pots.
 
I can do that but these are not multi-turn pots. They are all single turn. The originals were single turn, and I replaced them with single turn Bournes pots as that is what was available.
 
bob777 said:
Did you verify they sent you the correct trim pots?
Click to expand...
Yes, the markings on the trim pots matched the ratings of the originals.

All trim pots are set to the starting locations as listed in the service manual before making adjustments. VR801 and 804 are set to midpoint, VR802 all the way counter clockwise (minimum position), VR803 all the way clockwise (minimum position).
 
Next try swapping tr801 and 802. Also inspect the solder side of this board. We are running out of ideas.
 
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