Trnsfmr
Well-Known Member
The Eight was a revolutionary model for Sansui. For some reason, they decided to make the differential current, and differential balance, adjustable, on the driver boards.
And then of course, in Sansui's inimitable fashion, they put instructions for adjusting these parameters in the Service Manual, that an EE would find very hard to decipher. It reads like gobbledy gook.
Even though Justin and I figured it out, it's the myriad of ways they tell you to hook up the meter that is confusing. I think this was because there were no digital multi-meters at the time which would display + or - voltages, there was only a scale that started from zero and went up on an analog meter.
Making these adjustments was such a pain, probably their own techs couldn't do it, so they soon came out with the Eight Deluxe, which did away with the adjustments. This is a shame really, as these parameters have more to do with amplifier performance than anything else. When these adjustments are spot on, you get the lowest distortion and best slew rate possible from the design.
Well, now that we have digital multi-meters that will range over + and - voltages so easily, we developed a method for making the adjustments on the Eight very simple. We just rebuilt another one in our shop and went through the procedure again, so this time I took some pictures and thought I would share it here.
How to
First of all understand that there are four things to adjust, one of them being Bias, which everybody knows how to do. The remaining three are:
Differential pair current
The differential pair is fed through VR802 and R804 to a pair of 100 ohm resistor feeding the emitters of TR 801 and TR802. The idea is that no matter what happens signal wise, the total current is suppose to remain constant. That current is 1.5 ma. We can easily measure it by measuring the voltage drop across R804, a 330 ohm resistor. We want to measure 0.500v across R804. VR802 sets this current.
Current balance.
At a no signal condition, we want TR801 and TR802 to have exactly the same current going through them. The actual amount of current is not near as important as the balance. It's much easier to measure the balance instead of the actual current, so we measure the voltage between the two emitters, and set that voltage to 0.000. VR804 sets this balance
Speaker offset
This is no different than we do for any other amp. We like to measure at the headphone jack, one channel at a time to ground. The adjustment for this is VR801. VR801 will only be able to bring the offset in range once you get the other parameters close. Sansui specifies that a resistor (or speaker) be hooked to the output.
Each one of these adjustments affects the others. The way we deal with this is to have a meter for each parameter. We hook them all up at the same time, and just go back and forth with adjustments until they are spot on, with everything warmed up, of course. If you have only one meter, I would really recommend that you go out and buy a couple more. Harbor freight meters are fine, we buy lots of them, and they are cheap. The little one is really cheap, and there's no excuse not to have several.
I blow up meters all the time, much to Justin's amusement. We probably buy most of the meters that our local Harbor Freight sells. We're a good customer. HA!
First let's look at setup and then the adjustments.
In the first picture you can see where the meters are hooked into the driver board. We use the little mini-grabbers, they work great. The red and black leads from one meter are connected to R804. The yellow and white leads to another meter are connected to the emitters of TR801 and TR802.
In the second picture you can see 5 meters all hooked up at the same time. The first meter on the lower left is hooked to R804 (2v. range). The second meter, right next to it, is hooked to the emitters (200mv range). The third digital meter is on the shelf, hooked to Left headphone output (200mv range). The two big Tripplett analog meters are reading bias for the left and right channels. An analog meter is so much easier to read for some measurements.
We like to meter everything when we are adjusting an amp. If you can see everything, then there's much less chance of something happening, and blowing a pair of output transistors in a few seconds. The Trippletts are fused internally, so if bias goes over 1 amp, it blows. We also have 1 amp fuses in line to our bench speakers. After years of blowing things up, you get wary.
We put 20 turn adjustable pots in, because the old ones were so scratchy that they would blow fuses in the meters and speaker lines. It's a 47 year old amp, after all.
Step by step
So... if you changed transistors or other repairs to the driver board, set VR801, 802 and 804 to midrange. Otherwise just start from where they are. VR803 is bias and will be the first thing to adjust.
1. Turn amp on and immediately set VR803 for bias. Factory spec is 8-10ma.
2. Set VR802 for 0.500v. on the appropriate meter.
3. Set VR804 for 0.00 on the appropriate meter
4. Set VR801 for as low a reading as you can get.
5. Let it warm up and do 2 - 4 several times until it's stable. Bias will be pretty stable and mostly change because of warm up. The other three really affect each other and also change with warmup.
The factory instructions talk about test points 1-3. Forget about them. The above is all you need to do.
We change the differential pair as a matter of course on every driver board we work on. Usually KSA992's. If you have trouble getting measurements in range, change them.
That's it. A long explanation, but the drawing pretty much explains it. I had to draw it out, because I have never found a good schematic for the Eight that I could print out or capture. Hope this helps.
And then of course, in Sansui's inimitable fashion, they put instructions for adjusting these parameters in the Service Manual, that an EE would find very hard to decipher. It reads like gobbledy gook.
Even though Justin and I figured it out, it's the myriad of ways they tell you to hook up the meter that is confusing. I think this was because there were no digital multi-meters at the time which would display + or - voltages, there was only a scale that started from zero and went up on an analog meter.
Making these adjustments was such a pain, probably their own techs couldn't do it, so they soon came out with the Eight Deluxe, which did away with the adjustments. This is a shame really, as these parameters have more to do with amplifier performance than anything else. When these adjustments are spot on, you get the lowest distortion and best slew rate possible from the design.
Well, now that we have digital multi-meters that will range over + and - voltages so easily, we developed a method for making the adjustments on the Eight very simple. We just rebuilt another one in our shop and went through the procedure again, so this time I took some pictures and thought I would share it here.
How to
First of all understand that there are four things to adjust, one of them being Bias, which everybody knows how to do. The remaining three are:
Differential pair current
The differential pair is fed through VR802 and R804 to a pair of 100 ohm resistor feeding the emitters of TR 801 and TR802. The idea is that no matter what happens signal wise, the total current is suppose to remain constant. That current is 1.5 ma. We can easily measure it by measuring the voltage drop across R804, a 330 ohm resistor. We want to measure 0.500v across R804. VR802 sets this current.
Current balance.
At a no signal condition, we want TR801 and TR802 to have exactly the same current going through them. The actual amount of current is not near as important as the balance. It's much easier to measure the balance instead of the actual current, so we measure the voltage between the two emitters, and set that voltage to 0.000. VR804 sets this balance
Speaker offset
This is no different than we do for any other amp. We like to measure at the headphone jack, one channel at a time to ground. The adjustment for this is VR801. VR801 will only be able to bring the offset in range once you get the other parameters close. Sansui specifies that a resistor (or speaker) be hooked to the output.
Each one of these adjustments affects the others. The way we deal with this is to have a meter for each parameter. We hook them all up at the same time, and just go back and forth with adjustments until they are spot on, with everything warmed up, of course. If you have only one meter, I would really recommend that you go out and buy a couple more. Harbor freight meters are fine, we buy lots of them, and they are cheap. The little one is really cheap, and there's no excuse not to have several.
I blow up meters all the time, much to Justin's amusement. We probably buy most of the meters that our local Harbor Freight sells. We're a good customer. HA!
First let's look at setup and then the adjustments.
In the first picture you can see where the meters are hooked into the driver board. We use the little mini-grabbers, they work great. The red and black leads from one meter are connected to R804. The yellow and white leads to another meter are connected to the emitters of TR801 and TR802.
In the second picture you can see 5 meters all hooked up at the same time. The first meter on the lower left is hooked to R804 (2v. range). The second meter, right next to it, is hooked to the emitters (200mv range). The third digital meter is on the shelf, hooked to Left headphone output (200mv range). The two big Tripplett analog meters are reading bias for the left and right channels. An analog meter is so much easier to read for some measurements.
We like to meter everything when we are adjusting an amp. If you can see everything, then there's much less chance of something happening, and blowing a pair of output transistors in a few seconds. The Trippletts are fused internally, so if bias goes over 1 amp, it blows. We also have 1 amp fuses in line to our bench speakers. After years of blowing things up, you get wary.
We put 20 turn adjustable pots in, because the old ones were so scratchy that they would blow fuses in the meters and speaker lines. It's a 47 year old amp, after all.
Step by step
So... if you changed transistors or other repairs to the driver board, set VR801, 802 and 804 to midrange. Otherwise just start from where they are. VR803 is bias and will be the first thing to adjust.
1. Turn amp on and immediately set VR803 for bias. Factory spec is 8-10ma.
2. Set VR802 for 0.500v. on the appropriate meter.
3. Set VR804 for 0.00 on the appropriate meter
4. Set VR801 for as low a reading as you can get.
5. Let it warm up and do 2 - 4 several times until it's stable. Bias will be pretty stable and mostly change because of warm up. The other three really affect each other and also change with warmup.
The factory instructions talk about test points 1-3. Forget about them. The above is all you need to do.
We change the differential pair as a matter of course on every driver board we work on. Usually KSA992's. If you have trouble getting measurements in range, change them.
That's it. A long explanation, but the drawing pretty much explains it. I had to draw it out, because I have never found a good schematic for the Eight that I could print out or capture. Hope this helps.
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