Picked up a QRX-5500A from a nice seller on e-bay. I really enjoy Sansui quad and have the first and last variations: QR-6500 and 4500 (first Sansui Quad receivers) and QRX-8001 (last line of Sansui quad receivers). So, what I was missing was what happened in the middle as Sansui quad technology evolved. I've been keeping my eye out for something in the QRX-6500/5500 or 5500A line. The 5500A (and 7500A) were the last modification before the last iteration of quad processing that was used in the QRX-x001 line.
Anyrate, this one arrived safely, and looked promising with all knobs there, lights working, good case, etc. (but needing an external cleaning.)
Everything appeared to work on first power up, first on DBT, then on mains power. Had the normal scratchy pots, but nothing unexpected. External was dirty but a good cleaning of the face/knobs would solve that. Reconditioning the wood case was also in order. Took off the cover. I expected some dirt, but this was a surprise:
When I saw that, I was surprised there wasn't a dust fire when I turned it on! Fortunately, it cleaned well with an artist's brush and vacuum hose followed by wet washcloth for large areas and some electronics cleaner and q-tips for the small spots.
Cleaned the tuner and did and alignment. Definitely needed it, as I was not getting any medium strength stations, and the dial pointer was off by about .5 MHz FM. Reception improved substantially with the alignment. Not a very sophisticated tuner, but it does the job as well as other Sansui receiver tuners I've had.
After a number of hours of testing, it became very apparent there was a background hiss on the rear channels that only appeared when using one of the modes that passed through the quad boards. There was no hiss on the 4 channel direct setting, so I knew this was not in the amplifier section. This receiver uses the same Vario-Matrix board as the non-A versions (F-2048), but for the second processing board it has and updated board (F-2084), hence the "A" designation. The only receivers that had this combination of quad boards are the QRX-7500A, 5500A, and 5001. After looking at the SM, these two boards have multiple 2SA726 and 2SC1312 transistors.
Very likely one or more of these were the source of hiss. The F-2084 had three 2SC1312 transistors, whereas the F-2048 has 16 726/1312 transistors, so easiest path forward was to replace the transistors on F-2084 with KSC1845F.
One thing I have to say about the design of this receiver, everything is very accessible. The quad boards are plugin cards, and the bracket holding the cards can be removed with just three screws. Very nice!
Popped the board back in, check on DBT, then full main. The hiss was greatly reduced to the point where it's equal to what I'm getting on the front channels, which isn't much. I suspect changing the transistors on F-2048 will take care of the rest of that.
Connected everything up and put some of my "test music" through. No noticeable hiss on the speakers between CD tracks. However, I got a pleasant surprise: the channel separation made a noticeable improvement - much closer, if not equal to what I get with the QRX-8001!
I was hoping for a reduction in the hiss, but was not expecting any change or even improvement in separation. Very nice, indeed. Makes me wonder what replacing the transistors on the F-2048 will do? Wish I had a better understanding and there was more documentation of exactly what each board does, and which quad modes are processed through each circuit. At least there are good replacements for all of these old components, so we'll see what happens with further rehab.
Anyrate, this one arrived safely, and looked promising with all knobs there, lights working, good case, etc. (but needing an external cleaning.)
Everything appeared to work on first power up, first on DBT, then on mains power. Had the normal scratchy pots, but nothing unexpected. External was dirty but a good cleaning of the face/knobs would solve that. Reconditioning the wood case was also in order. Took off the cover. I expected some dirt, but this was a surprise:
When I saw that, I was surprised there wasn't a dust fire when I turned it on! Fortunately, it cleaned well with an artist's brush and vacuum hose followed by wet washcloth for large areas and some electronics cleaner and q-tips for the small spots.
Cleaned the tuner and did and alignment. Definitely needed it, as I was not getting any medium strength stations, and the dial pointer was off by about .5 MHz FM. Reception improved substantially with the alignment. Not a very sophisticated tuner, but it does the job as well as other Sansui receiver tuners I've had.
After a number of hours of testing, it became very apparent there was a background hiss on the rear channels that only appeared when using one of the modes that passed through the quad boards. There was no hiss on the 4 channel direct setting, so I knew this was not in the amplifier section. This receiver uses the same Vario-Matrix board as the non-A versions (F-2048), but for the second processing board it has and updated board (F-2084), hence the "A" designation. The only receivers that had this combination of quad boards are the QRX-7500A, 5500A, and 5001. After looking at the SM, these two boards have multiple 2SA726 and 2SC1312 transistors.
Very likely one or more of these were the source of hiss. The F-2084 had three 2SC1312 transistors, whereas the F-2048 has 16 726/1312 transistors, so easiest path forward was to replace the transistors on F-2084 with KSC1845F.One thing I have to say about the design of this receiver, everything is very accessible. The quad boards are plugin cards, and the bracket holding the cards can be removed with just three screws. Very nice!
Popped the board back in, check on DBT, then full main. The hiss was greatly reduced to the point where it's equal to what I'm getting on the front channels, which isn't much. I suspect changing the transistors on F-2048 will take care of the rest of that.
Connected everything up and put some of my "test music" through. No noticeable hiss on the speakers between CD tracks. However, I got a pleasant surprise: the channel separation made a noticeable improvement - much closer, if not equal to what I get with the QRX-8001!
I was hoping for a reduction in the hiss, but was not expecting any change or even improvement in separation. Very nice, indeed. Makes me wonder what replacing the transistors on the F-2048 will do? Wish I had a better understanding and there was more documentation of exactly what each board does, and which quad modes are processed through each circuit. At least there are good replacements for all of these old components, so we'll see what happens with further rehab.
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So, moved that to the 117V position. 
A setting between around 11 and 12 O'clock seems to fit most sources to use the range of the meters. As I mentioned, my background hiss is very low now - nearly inaudible, and none on either 2 channel or 4 channel discrete mode, so I'm sure the issue is old transistors on the F-2048 board.
(normal sound in 2 channel and 4 channel discreet settings - in those settings the signal does not pass through the quad boards). After a few hours trouble shooting narrowing down where the signal was getting lost, I came upon 
