Sansui QR-4500 rear right channel out

[KeithD]

Just realized I've lost sound on the rear right channel of my QR-4500. This occurs when playing the unit set to "normal" 4 channel mode. When I rotate the sound (1/4 or 1/2 turn) the rear right comes in, but the lost channel follows the selection. So for example, 1/4 turn left brings sound to the RR channel, but now right front channel has no sound. Also, if I select "front + rear" or rear only, I get sound on the RR channel.

Moved front/rear and rear left/right balance controls to isolate the RR channel. When I turn the volume way up, thiere is faint sound on the channel. Also, slight scratchiness from the volume pot is at normal volumes on the channel.

Opened the unit up and since the driver board appears completely symmetrical, I flipped it around in the connector. This would flip which channel is going through which side of the board. However, the problem did not move with the flip, the RR channel still had no sound. So, I don't believe the problem is on the driver board. Also checked the fuses for both rear channels and those are good. Used Deoxit and Fader Lube on volume pot and all balance sliders to clear up scratchiness and get rid of any possible dead spots.

Problem is the same no matter if source is 4 channel AUX or synthesized quad from tuner.

Not really sure where to look next. Any ideas??

 

[kevzep]

I have found after working on a lot of different QRX models, they are possibly the worst Sansui models for dirty switches and pots, this is where I would be looking first.
Balance controls and Front/Rear controls are always very bad..

 

[KeithD]

I have found after working on a lot of different QRX models, they are possibly the worst Sansui models for dirty switches and pots, this is where I would be looking first.
Balance controls and Front/Rear controls are always very bad..

Yes, I did clean those with Deoxit followed by Fader Lube thinking that could be the issue, but heard no change. The rear channels do not have any tone controls, just the balance slider.

Just checked the rear "record out" on the 4 channel tape outputs. Both channels clear there. The volume on the channels did change with the volume knob. I thought tape out was supposed to have constant output (not controlled by the volume knob), is that not the case?

Only other thing I can think of is output transistors on that channel?

 

[kevzep]

If any output transistors were blown, there would be blown fuses.

Sounds like you have looked at all the obvious things, time to get the signal tracer or scope out and follow the signal through to find out where its going missing.
Since its the same on all functions, it should be fairly easy to eliminate the pathways that aren't involved.

The Record Outputs should be fixed, they should not be changing when you adjust the volume, perhaps this is telling you something...?

 

[sKiZo]

Bit confused here ... the QR4500 has TWO driver boards, and the connectors are one sided?

And yes, they are identical. I believe it's the one on the right (from the front) that handles the rear channels. The pots shown are for adjusting bias and current. I wouldn't be surprised if it isn't a PNP/NPN strapped pair that's the culprit - as you can see those don't hold up well over time.

Any external processors that could be the problem?

 

[kevzep]

Bit confused here ... the QR4500 has TWO driver boards, and the connectors are one sided?

And yes, they are identical. I believe it's the one on the right (from the front) that handles the rear channels. The pots shown are for adjusting bias and current. I wouldn't be surprised if it isn't a PNP/NPN strapped pair that's the culprit - as you can see those don't hold up well over time.

Any external processors that could be the problem?

The problem did not move with the driver board, so its not the driver board.

 

[KeithD]

Finally had time to get back to this problem. Upon further investigation, I'm getting no signal from the RR PRE-OUT, so the problem appears to be in the pre-amp. Does not matter if signal is two channel going through synthesized quad (tuner or CD) or a four channel source. Thoughts on what to check in the pre-amp section?

 

[KeithD]

I believe I have solved the issue. In the process of checking all inputs and configurations with the Mode switch, the RR channel suddenly came to life. Thus it appears this is a dirty switch/contacts issue, particularly on that channel. Will need to do a cleaning of the Mode and Selectors switches. Don't think I'd done that before.

 

[kevzep]

I believe I have solved the issue. In the process of checking all inputs and configurations with the Mode switch, the RR channel suddenly came to life. Thus it appears this is a dirty switch/contacts issue, particularly on that channel. Will need to do a cleaning of the Mode and Selectors switches. Don't think I'd done that before.

Yes I work on a lot of these Quad receivers, and the switches are always terribly dirty and scratchy with dropouts....
I would blow through and recap it, I find a lot of bad caps in these receivers...
There are some crappy transistors in these too, just a case of having a look and seeking them out...