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Sansui 8 Issues And Schematic Question

R

Rob Louvasz

New Member
I tried posting this in the solid state forum with only one reply. I'd thought I would post it here. I'm working on a Sansui 8, the style with no protection relay and have run into a few issues. The left and right channels both have over 24v DC on the speaker outputs. I started with the right channel. So far I've pulled F004 and F004, TR's853, 855, 857, &859. When TR852 is still connected, there is 24v on it's base and collector. When they are disconnected, there is still 24v on the "O" and "P" positions coming from board F-1278. And there is still 24 volts on the HH output point. I downloaded the schematic from hifiengine and found some differences. My F1278 board does not have the R820 resistor just a jumper and none of the transistors match what the schematic except for TR801 and 802. Also the outputs were replaced with ON semi's. The unit was recapped but not by me. Any guidance would be appreciated.
 
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sanjib said:
Have you tried all three adjustments required in the amp boards? You have to do all three adjustments at the same time. Here is a nice thread on how to do it.
https://audiokarma.org/forums/index.php?threads/eight-driver-board-adjustments.707373/
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YES, I second and third that! Follow Jim's procedure in the link, and you may find you don't actually have a component issue. I say this from experience. I was getting 42V (!) at the speaker outputs on my Eight. Turns out the VRs were set far from where they needed to be. I would never have guess the adjustments could get you this far off from zero, but that was the case. I think the issue is instead of being able to only set bias and DC offset, unique to the Eight you also set the Differential Pair current and current balance, as detailed in Jim's thread. I'm not aware of another amp that was designed to have that adjusted outside the factory. A great feature, but one that can cause big problems if not set right. Try making the adjustments and see where you are at.
 
Thanks for all of the replies. After checking both F-1278 boards and replacing the old pots with multi-turns, I started the set up procedure as described in the service lit. I hooked up an 8ohm load and meter and tried to make the first adjustment. Even after turning down both VR's the voltage never changed. I'm going to get back into it today.
 
I read thru the link and followed the hook up procedures. I have four meters hooked up and an 8 ohm load on the right channel output, with the speaker selector on "A" . I also have the DC offset meter connected to the headphone plug. Now, I'm only trying to set the right channel. I have F003 pulled and the meter put in series. So, here's what I got: The bias is at 0.027 and doesn't change with adjusting VR803. The differential current pair only can be adjusted to 0.031 and won't go higher. The current balance is at 10.0 volts and won't change. The DC offset is at 17 volts at the head phone jack and won't change either by adjusting VR801. Also the load resistors get a bit warm, right around 160 degrees. Any Ideas??
 
Do not connect a load! If you have a DC offset it is going to sit there and draw current and then your bias measurement will be off. Also if you have a high enough DC offset on an amp and you connect a load you could blow the output transistors. Never connect a load until you have bias adjusted and 0V on the output.
 
I have an Eight that I'm procrastination with and it has a problem on one driver board (not the input pair guys! ;)), anyway...
Have you checked voltages per schematic? You can remove the driver cards and test, this will see if you have an issue downstream, it's best to test from the earlier stages.
On a previous amp recently, I had rail voltages at speaker terminals, it was found to be the transistors of the power supply board.
 
Here's the right channel update: I disconnected the load (thanks dr*audio) I was following the set up outlined in the SM, that's why it was still connected. Anyway it also helped that I switched the newer meter I was using to measure the bias to DC current draw instead of AC! I chalk that one up to old age.... I do have 6 meters hooked up at once to do the 4 adjustments but I also have two more for AC input current and voltage. So after all of this, everything was able to be adjusted! Bias came down, current, etc. I just picked up three more meters this morning along with some more test leads to be able to adjust both channels at the same time. I post some photos. Thanks to everyone for getting me this far!
 
Rob Louvasz said:
Here's the right channel update: I disconnected the load (thanks dr*audio) I was following the set up outlined in the SM, that's why it was still connected. Anyway it also helped that I switched the newer meter I was using to measure the bias to DC current draw instead of AC! I chalk that one up to old age.... I do have 6 meters hooked up at once to do the 4 adjustments but I also have two more for AC input current and voltage. So after all of this, everything was able to be adjusted! Bias came down, current, etc. I just picked up three more meters this morning along with some more test leads to be able to adjust both channels at the same time. I post some photos. Thanks to everyone for getting me this far!
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Great! Glad to hear you were able to make the adjustments. Similar to my experience. Couldn't believe I could get 41V at the outputs, but with the ability to make so many adjustments on the Eight, it can happen. :beerchug:
 
Well, here's the update. The right channel is pretty much OK. I've got 8 meters on the unit, 4 for each channel. But, the left channel still has issues. The bias won't move from 0.050. The differential current pair won't move, it's at .086. Current balance is at .027 and won't move and the DC offset is at 37 volts and won't change. I measured the DC offset at the headphone jack and the speaker fuse holder with the fuse removed and the reading didn't change. Any suggestions?
 
So, the right channel is behaving, the left channel not so much. I'm still getting over 30v in places where it should not be. I'm going to change out the two 2n4250's, apparently they can be a common issue.
 
OK. I changed out the 2n4250's and also TR806. I also found a heat stressed resistor R831. C802 was leaking. I also changed the other 2n4250's on the right channel board. Now I'm able to get the left channel to dial in. Right now I've got 10 meters hooked up, a variac and a dim bulb tester.IMG_1140.JPG
 
You seem to be so close to finding the issue, this is no easy feat with an Eight. I'm watching with with keen interest here and the things preventing me with continuing my fault finding (driver board) is time constraints and that I don't have 10 meters!! - Keep up the good work :)
 
Rob Louvasz said:
OK. I changed out the 2n4250's and also TR806. I also found a heat stressed resistor R831. C802 was leaking. I also changed the other 2n4250's on the right channel board. Now I'm able to get the left channel to dial in. Right now I've got 10 meters hooked up, a variac and a dim bulb tester.View attachment 2187347
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If I understand your posts, you are now able to properly set both channels following Jim's procedure? If so, sounds like you've solved the issue!
 
Yes, I should have been more clearer on that. I was able to get everything set using Jim's procedure. I'm going to let it play for a while and then check everything again. I think the main issue with the left channel was the incorrect resistors and the 2n4250's. I hope this thread helps other people in the future with their 8's.
 
Rob Louvasz said:
Yes, I should have been more clearer on that. I was able to get everything set using Jim's procedure. I'm going to let it play for a while and then check everything again. I think the main issue with the left channel was the incorrect resistors and the 2n4250's. I hope this thread helps other people in the future with their 8's.
Click to expand...
Great! I'm sure it will help others. Once I fixed the issues mine had when I bought it, it's worked flawlessly since. They are great little receivers, with just the right sound for me. :)
 
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