• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Bias Help Sansui 661 (771)

robz2000

robz2000

New Member
Hey everyone.
My issue, and I have had this happen twice now, is a sansui 661 (or 771) the bias sits at 29 to 31 and doesn't move past that up or down. If I turn the trim pot, it'll go up or down 1 mv. Same behavior both channels. The correct number is 19mv and I cannot get close to that.
The odd thing, it doesn't run hot. So I doubt this reading is even accurate and that seems to rule out a bad pot. I've tried both mV/mA DC and AC on an auto sensing multimeter. i have successfully done bias adjustments on countless other units.
I am just baffled on this. And like I said this is the second time I've had this situation where I just can't seem to set bias on a Sansui. Any assistance would be appreciated
 
Last edited:
Register to hide this ad
How are you measuring the bias?. The manual says 19 mA, not mV, and the method is to put an milliameter in place of the fuse. This can cause problems with the meter, and if that is how you are doing it, start in the Amp mode to make sure current is ok, then switch to mA if it is within range. Easy to blow fuse in the meter, but only affects current reading.
The simplest way to avoid some of these issues, is to put a 1 ohm resistor across the fuse holder, instead of your meter, and measure voltage drop. 19 mV is 19 mA across a 1 ohm resistor. Some people actually solder two end caps from a blown fuse to a resistor, and clip it into place. YMMV, and I don't like use the meter across the fuse holder with the fuse removed method.
Best (IMHO of course) is just to measure voltage across one of the .33 ohm emitter resistors. The problem is that the fuse method includes all the amp circuit, not just the current through the output stage. The current draw of the rest of the amp is about 8mA for a 661 or about 9 for a 771. Which suggests actual current through output stage should be about 10 mA, or 3.3 mV across the emitter resistor.
All offset and bias adjustments are DC, so don't use AC settings.
 
Appreciate the reply. i have used both mA and MV, both DC and AC. just to be 100% sure.
i am curious about using the alternate method, across the .33 ohm resistors. i do not see any of the .33, but the ones in front of the output transistors, are .33Ohms. Are those the ones you are referring to?
 
Yes, .33 Ohms is what you are looking for. A safe setting is 6.6mV across the .33 Ohm resistor, which give you 20mA.
 
robz2000 said:
Appreciate the reply. i have used both mA and MV, both DC and AC. just to be 100% sure.
i am curious about using the alternate method, across the .33 ohm resistors. i do not see any of the .33, but the ones in front of the output transistors, are .33Ohms. Are those the ones you are referring to?
Click to expand...
The .33 are on the same board as the fuses and the output transistors (1500 I think)
 
thanks -- will try this this eve and see what i get. If i don't like what i see, i am going to replace all the trim pots and resistors on the driver board as my next step. Any recommendation on trim pot replacements? there is a good sansui transistor list on this site i can use for those.
 
For trim pots I use whatever sealed single turn pots have the correct value and close enough lead spacing that it will fit.
 
steve / Dr Audio - big thanks. i was able to adjust to the 6.6mV easily as planned on those .33 resistors in front of the output transistors.
really impressed with your solution here, and i'd assume always check bias this way in the future.

what was your math to come up with that 6.6mV value? and most importantly, is that something i can use on any amp/receiver going forward? or is it not quite that simple.

really appreciate your help. this site is such a treasure of good folks.
thanks again.
 
robz2000 said:
what was your math to come up with that 6.6mV value? and most importantly, is that something i can use on any amp/receiver going forward? or is it not quite that simple.
Click to expand...
It was based on 20 mA across the .33 resistors so the math is .020 * .33 = .0066 so the result is 6.6 mA.
 
It's Ohm's Law: E = I X R,
Where E is voltage
I is current
R is resistance.
And yes, you can adjust any amp using this method, with one exception below. 20mA - 40mA is a good, safe range. Some amps or receivers will not stabilize at 40mA, you have to use 20mA.
The exception to this method is if an amp has a single power supply, instead of positive and negative supplies. This kind of amp has the output at 1/2 the supply voltage and an output capacitor to block this DC voltage from getting to the speakers.
In this kind of amp there will be 2 adjustments; one for the bias and one to adjust the output for 1/2 the supply voltage.
You measure the supply voltage and then measure the voltage on the input side of the output capacitor, and adjust for 1/2 the supply voltage.
Then you adjust the bias, measuring across one of the white ceramic emitter resistors as above.
 
You must log in or register to reply here.
Back
Top Bottom