Saving the Sansui

Time is taking it's toll on all of our quadraphonic equipment.  It's a gradual loss that, overtime, you probably didn't notice was occurring.  Hoodcrow sent me some pretty technical papers on how you can tweak the QS decoders in the Sansui QRX-x001 receivers.  I'll try to break it down even further for you.  Just remember, I'm not an engineer...I don't even play one on TV!

First, take off the cover (I know...DUH).  When you are looking straight down into the beast, you will see most of the boards laying flat down.  There are 2 boards however that are vertical and held in by a clamp.  Congratulations, you've just found the QS decoder boards!

Sansui decoders require 25 volts to operate.  Chances are, your machine has drifted off of that.  You have to try to get 25 volts back into the boards.  This is the trickiest part of the alignment.  The pot that controls the voltage is well hidden on the board that the decoder boards plug into.  In fact, I had to remove these 2 boards to even see the control.  I can only speak from my experience with the 7001 for the location of this pot.  Located just in front of the large black caps (next to the power supply) is where the control is hidden.  In fact, you can't see it in the picture below, but it is sandwiched up in there!  You need to hook a digital multimeter up to pin 5 of the 2088 board and take the initial reading.  Mine was at 23 volts.  I pulled the boards and very slightly moved the pot clockwise.  These things are sensitive!  I turned it about 1/8 turn and the voltage shot up to 29!  Just tweak it a little.

Once you get the voltage taken care of, it's time for the fun part.  To align the decoder you will need a signal source that generates a 1khz tone.  I created this on my computer in Sound Forge.  You need a Left only tone, a Right only tone, one tone that is mono (equal in both channels) and 1 tone that is mono (equal in both channels) but 180 degrees out of phase.  I used my trusty Heathkit audio scope to check my work, but you can use the VU meters on the 8001/9001 units or some outboard VU's hooked up to the speaker leads...or wing it by ear!

First, play the LEFT 1khz tone into the Sansui in QS mode and adjust VR4 until the output on the RIGHT front and back channels are low, but equal.  The Right Front and Back channels are always out of phase with each other.  When they are equal in volume, that is known as "strict imbalance".  

Now, play the RIGHT 1khz tone.  Adjust VR3 so that the LEFT front and back channels are low and equal.

Now play the mono tone (equal in both channels) into the Sansui.  Adjust VR2 so that the REAR channels are low and equal in level.

And finally, play the 1khz tone that is 180 degrees out of phase into the receiver.  Adjust VR1 until the FRONT channels are low and equal in power.

That's it!  You will be amazed at the difference.

"Quad" Bob Herndon says...

The main thing I would CAUTION all of you on is the 25VDC adjustment!  As Tab notes it doesn't take much of a "tweak" on the adjustment pot to change the voltage considerably. If you don't have a very accurate Digital Volt Meter....don't do it! And if you do, be VERY careful!  If you ramp that voltage up over 34- 35vdc or below about 18vdc you could smoke your decoder boards and destroy those irreplaceable QS chips. That power supply is critical for the chips to operate/decode properly.  I use a DVM that reads to "thousandths" and calibrate the supply to 25.001 VDC "under load" or with the decoder boards present.....also note that the voltage WILL vary depending on whether you have the boards installed or not!  If you can't adjust the pot with the boards in place, BE EXTREMELY CAREFUL about checking voltage both with the boards in place and out and don't exceed the ranges I mentioned above!
  The other pots, if misadjusted will screw up your decoder....but won't damage anything even if you run them through their full range from stop to stop. 

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