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sansui 881 bias adjustment issue

S

special_20

Active Member
so I've recapped my Sansui 881. I did one board at a time and always checked her on a dim bulb tester and did a basic function test between each board. I am now setting the final DC offset and Bias adjustments before buttoning her up and putting her back into services.

I have the DC offset successfully zeroed: <1 mv on both channels measured at the speaker output terminals. I've turned both bias trim pots to their respective low points. I've pulled the fuses (F07 and F08) as described in the manual and am measuring in DC/mA across the fuse terminals. I read ~ 0.16 mA across F07 but VR03 seems to do nothing (i.e. I turn the trim pot and the mA reading on my DMM doesn't change)?

I did notice that the manual illustration has fuse F10 pulled and being measured across. Is there an error in the manual and I'm reading across the wrong points? Am I doing something wrong (e.g. reading in the wrong setting)? Note: I have not replaced the stock variable resistors
 
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Seems to me there (may) be an error in that manual (gasp). I would suggest tracing the fuse back to the output stage to be sure you are measuring across the correct fuse. Remember getting weird readings and determined I was actually measuring the lighting circuit once :( thus the comment.
-Lee
 
The best way to set bias is to leave the fuses in an measure the voltage drop across one of the white ceramic emitter resistors. The value of the emitter resistors is 0.33 Ohms, and the manual calls for 30mA current. So by Ohm's Law we multiply 0.03 x 0.33 = 9.9mV across any one emitter resistor. Adjust for that voltage and you're done. Use a mini clip lead on your meter, don't use a needle probe or you'll slip and short something out.
The reason you got no reading on current may be due to a blown fuse in your meter, or you may have had it on volts rather than amps.
 
Thanks dr*audio! Measured the dc volts across r23 and r24 and was able to adjust bias to spec (9.9mv). Got her burning in now :music:
 
So I watched a youtube video on this that said to measure off of F9 and F10. With my meter set to 200mA DCA, I couldn't get a reading. It's a janky old meter, but it worked to set the DC offset out of the speaker terminals. So I came here, tried R23 and R24, and I still can't get a reading when set to DC2v. I (now see this was stupid, but) slightly moved the trim pots for the DC Bias when I was reading 45.5 volts across F9 and F10, but that (i now realize, obviously) didn't change the reading. So now I'm in a situation where I don't actually know where my bias is set and can't seem to get a reading on it. Anybody have any ideas? Am I just enough of a newb to have used the wrong setting? Do I need a different meter? I know this is a zombie thread but any help is appreciated.
 
...and now its running really hot. This is what I get for trying to learn, lol. I'll try grabbing a new DMM in the morning.
 
To use the current method you have to remove the fuse and connect your DMM (set to mA) leads to the fuse clips and adjust for the proper amount of current.

Craig
 
llwhtt said:
To use the current method you have to remove the fuse and connect your DMM (set to mA) leads to the fuse clips and adjust for the proper amount of current.

Craig
Click to expand...
Thanks! Is it just one fuse or both? And I’ve got the amp on for about 5 minutes with the tone controls zeroed and the volume all the way down, right?
I’ve tried the dmm on all the DCA settings and got nothing. And is it just the one fuse that comes out, or both? I’m about to run and pick up a new meter just to see if it’s not that, but I couldn’t get a voltage reading across R24 either. I’m so close, I think. I’ll report back with a new meter. Wonder if the batteries are just old?
 
I think you need a new meter. Also get meter leads with mini clips on the ends so you can clip them on the resistor and not have to hold them, you could slip and short something out.
Do not run it for 5 minutes if it's running hot! Turn on, make the adjustment, then turn it off and connect to the other channel then turn it back on and make the adjustment. Monitor for 20 minutes, re-adjusting as needed, carefully disconnecting the probes and going to the other channel. This is why it's good to have 2 meters. When it stops changing, you're done.
 
Usually DMMs have fuses in them to protect them when the current is too much. F07 for the left channel and F08 for the right channel, 30mA for each.
 
Well this is a hassle. New meter, and I’ve tried F7-F10 and it goes into protection when I put the leads across the terminals and reads an overload. But when I pop the fuses back I I can read the offset and get it zeroed no problem. Bad fuse in the meter? I did get a pop out of one of the fuse terminals at one point. User incompetence? I’ve rebuilt engines and never felt this incompetent. Yikes am I bad at this game. But it works, so just use it? Can anybody tell me what I might be doing wrong?
 
I have been working on cleaning up an 881 over the past few weeks and finally got it all back together last night and set offset and bias. I used the method described above by Dr*audio (thank you for that!). A couple of things you might want to double check, are all of the fuses still OK? I would test all of them 1-10. I accidentally blew a fuse while testing r23 and found that the amp would still come out of protection.

Also can you share the type of meter you have? If it is an auto-ranging variety that has the ability to manually set the voltage range you might want to try that. I'd like to take a look at the manual if there is one on-line. The same setting you use for offset should be used for bias if following the emitter resistor method.
 
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