Sansui QRX-7500A channel problem using CD-4/4-CH direct function

[Bladerun]

Hi!

I thought I had everything tested out and working correctly on my recently acquired Sansui QRX-7500A, but I noticed a problem with something I inadvertently overlooked. The reason I missed it is because I tested out all the front and rear speakers on both the A and B systems, but not while using CD-4/4-CH direct. When I choose this function, the left rear speaker and level indicator do not work on the A or B speaker system. This left rear speaker and level indicator work fine under OS Synthesizer, QS, and SQ - just not using the phono and this function.

I tried several different LP's and they all have the same issue with the missing left rear channel. It also doesn't help that I'm not familiar with the differences between CD-4 and 4-Channel direct. The CD-4 light was not lit while playing the LP's, so I'm guessing it defaults to 4-Channel direct. I'm just blown away by the sound difference between the 4-Channel direct sound as compared to OS Synthesizer, QS, and SQ. Not that those sound bad, but it's just better with the 4-Channel direct so I want to fix it.

Are there separate amplifiers for each channel in this mode? I'm just not sure where to start because I'm unfamiliar with how it operates exactly, and a little perplexed as to why the left rear speaker works in every other mode.

Any info/help would be greatly appreciated.

Thanks!

 

[kevzep]

4 channel direct is for using quadraphonic Tape Machines (RTR's), they have 4 channel outputs and are mixed as such.
When you are in this mode and you are playing a CD4 record, and you have a CD4 compatible cartridge, the decoder picks up the 30khz pilot tone and so then decodes the CD4 audio.

If you are having channel dropouts its most likely to be a dirty/oxidised selector switch, this is very common on these models....

 

[Bladerun]

4 channel direct is for using quadraphonic Tape Machines (RTR's), they have 4 channel outputs and are mixed as such.
When you are in this mode and you are playing a CD4 record, and you have a CD4 compatible cartridge, the decoder picks up the 30khz pilot tone and so then decodes the CD4 audio.

If you are having channel dropouts its most likely to be a dirty/oxidised selector switch, this is very common on these models....

Interesting....I never would have guessed it was for a tape machine. I wonder why the mode sounds so good compared to the others even though I'm not using an RTR. There must be some type of encoding going on with non CD4 records that improves the sound.

I never thought of the selector switch itself. So, it's possible that 1 out of 4 speakers not working in just this mode could be the selector switch in the CD4/4-channel direct position? I would have thought of this as being an on/off potentiometer type switch, not one that could control output to all 4 speakers.

Thanks for your help kevzep!

 

[tcdriver]

...I tried several different LP's and they all have the same issue with the missing left rear channel. It also doesn't help that I'm not familiar with the differences between CD-4 and 4-Channel direct. The CD-4 light was not lit while playing the LP's, so I'm guessing it defaults to 4-Channel direct. I'm just blown away by the sound difference between the 4-Channel direct sound as compared to OS Synthesizer, QS, and SQ. Not that those sound bad, but it's just better with the 4-Channel direct so I want to fix it....

If you have not done so already, read the owner's manual. It explains most things well. There is a PDF copy available online.

Quadraphonic records:
----------------------------------------------------------CD-4 ---------------------------------------------------------------------

To successfully play a CD-4 (aka Quadradisc) disc, you will need the following:

1. CD-4 encoded record. It should say CD-4 or Quadradisc on the album cover.

2. CD-4 demodulator, either stand alone or built into a receiver, preamp or integrated amplifier.

3. CD-4 capable phono cartridge. i.e. capable of frequency response to 45kHz. The carrier frequency, 30kHz., is frequency modulated from 20kHz. to 45kHz. Conical or elliptical shaped diamonds are not suitable for frequency response to 45kHz. The Shibata diamond tip was introduced specifically to play back the high frequencies found on CD-4 records. Other stylus shapes, “Quadrahedral”, “4 Dimensional”, “mico-ridge”, “line contact”, etc. were either developed for CD-4 disc use or can be used to play CD-4 discs.

4. Turntable with low capacitance arm wiring and low capacitance interconnects. The total capacitance from headshell to the end of the cable that plugs into the CD-4 capable phono inputs should be < 100pF.

Some examples of CD-4 (Quadradisc) records:

 

[kevzep]

There must be some type of encoding going on with non CD4 records that improves the sound.

No, direct 4ch is exactly that, Direct, No encoding, unless you have a CD4 compatible cartridge, the cart has to have enough bandwidth to pick up the 30khz pilot signal, most carts do not have this much bandwidth.
Unless that CD4 indicator is lit up, then there is no decoding going on.

 

[Bladerun]

If you are having channel dropouts its most likely to be a dirty/oxidised selector switch, this is very common on these models....

Kevzep, you were absolutely correct! I can't thank you enough for this info. I was staring at the schematics trying to figure out how the circuit worked and it never crossed my mind it could be something as simple as this. As soon as I blasted it with contact cleaner and did a few dozen movements with the dial, the left rear speaker came to life and the level indicator sprang into action.

I love this site, thanks everyone for the help and info!

 

[johnhoef]

A quick and easy question for you all...on this QRX-7500. I am currently working on one. The customer, in a feeble attempt to replace the CD-4 indicator lamp, broke the wires from the PC board, and I am having some difficulty figuring out where to connect it. Of course, one side of the bulb goes to ground. The other should go to the "function" switch, and eventually be switched in to 6vac, like all the other bulbs. So, my question - should this indicator light up in all positions of the function switch except 2 channel? I am guessing yes. Am I right?

 

[tcdriver]

The CD-4 indicator light should only illuminate when a CD-4 (aka Quadradisc) record is played and the high frequency carrier frequency is detected in the demodulator.

 

[johnhoef]

tcdrive, thanks....not what I expected, but it does make total sense. With your help, I finally found the indicator light on the schematic. It's driven from the CD-4 sub circuit, board #F2090.

 

[johnhoef]

There is one more issue with this unit. I did a level check injecting a signal into all four channels of the aux input. All four channels output at the same amplitude, and all four meters deflect to zero. This is as expected. However, upon switching to FM, and engaging any of the four channel modes using the function switch, I get much lower readings in the two rear channel meters, as well as much quieter sound coming out of the rear speakers. I expected equal. But, thinking about how sound would be in an auditorium, in a live musical situation, I would think sound coming from the rear of the auditorium would be weaker, indeed. So, perhaps the unit is working normally?